Plug Power (PLUG) 10-K 13 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Mar. 12, 2019	Jun. 30, 2018
Document and Entity Information:
Entity Registrant Name	PLUG POWER INC
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Entity Central Index Key	0001093691
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Entity Common Stock, Shares Outstanding		233,835,808
Entity Public Float			$ 457,430,028

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 38,602	$ 24,828
Restricted cash	17,399	13,898
Accounts receivable	37,347	24,179
Inventory	47,910	48,776
Prepaid expenses and other current assets	14,357	7,926
Total current assets	155,615	119,607
Restricted cash	54,152	29,329
Property, plant, and equipment, net	12,869	10,414
Leased property, net	146,751	87,065
Goodwill	9,023	9,445
Intangible assets, net	3,890	3,785
Other assets	8,026	11,165
Total assets	390,326	270,810
Current liabilities:
Accounts payable	34,824	42,362
Accrued expenses	7,864	10,595
Deferred revenue	12,055	8,630
Finance obligations	74,264	34,506
Current portion of long-term debt	16,803	18,762
Other current liabilities	560	866
Total current liabilities	146,370	115,721
Deferred revenue	28,021	25,809
Common stock warrant liability	105	4,391
Finance obligations	118,076	37,069
Convertible senior notes, net	63,247
Long-term debt	133	13,371
Other liabilities	18	94
Total liabilities	355,970	196,455
Stockholders' equity:
Common stock, $0.01 par value per share; 750,000,000 shares authorized; Issued (including shares in treasury): 234,160,661 at December 31, 2018 and 229,073,517 at December 31, 2017	2,342	2,291
Additional paid-in capital	1,289,714	1,250,899
Accumulated other comprehensive income	1,584	2,194
Accumulated deficit	(1,260,290)	(1,178,636)
Less common stock in treasury: 15,002,663 at December 31, 2018 and 587,151 at December 31, 2017	(30,637)	(3,102)
Total stockholders' equity	2,713	73,646
Total liabilities, redeemable preferred stock, and stockholders' equity	390,326	270,810
Series C Redeemable Convertible Preferred Stock
Redeemable preferred stock:
Redeemable preferred stock	709	$ 709
Series E Redeemable Convertible Preferred Stock
Redeemable preferred stock:
Redeemable preferred stock	$ 30,934

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parentheticals) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Common Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common Stock, shares authorized	750,000,000	750,000,000
Common Stock, shares issued	234,160,661	229,073,517
Common stock in treasury, shares	15,002,663	587,151
Series C Redeemable Convertible Preferred Stock
Redeemable convertible Preferred Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Redeemable convertible Preferred Stock, shares authorized	10,431	10,431
Redeemable convertible Preferred Stock, shares issued	2,620	2,620
Redeemable convertible Preferred Stock, shares outstanding	2,620	2,620
Redeemable convertible Preferred Stock, aggregate involuntary liquidation preference (in dollars)	$ 16,664,000	$ 16,664,000
Series E Redeemable Convertible Preferred Stock
Redeemable convertible Preferred Stock, par value (in dollars per share)	$ 0.01	$ 0.01
Redeemable convertible Preferred Stock, shares authorized	35,000	0
Redeemable convertible Preferred Stock, shares issued	35,000	0
Redeemable convertible Preferred Stock, shares outstanding	35,000	0
Redeemable convertible Preferred Stock, aggregate involuntary liquidation preference (in dollars)	$ 35,000,000	$ 0

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net revenue:
Net revenue	$ 174,632	$ 100,153	$ 82,819
Cost of revenue:
Total cost of revenue	172,010	128,242	78,873
Provision for loss contracts related to service			(1,071)
Gross profit (loss)	2,622	(28,089)	3,946
Operating expenses:
Research and development	33,907	28,693	21,177
Selling, general and administrative	38,198	45,010	34,288
Total operating expenses	72,105	73,703	55,465
Operating loss	(69,483)	(101,792)	(51,519)
Interest and other expense, net	(22,135)	(10,100)	(10,704)
Change in fair value of common stock warrant liability	4,286	(15,188)	4,344
Loss before income taxes	(87,332)	(127,080)	(57,879)
Income tax benefit	9,217		392
Net loss attributable to the Company	(78,115)	(127,080)	(57,487)
Preferred stock dividends declared and accretion of discount	(52)	(3,098)	(104)
Net loss attributable to common shareholders	$ (78,167)	$ (130,178)	$ (57,591)
Net loss per share:
Basic and diluted (in dollars per share)	$ (0.36)	$ (0.60)	$ (0.32)
Weighted average number of common shares outstanding	218,882,337	216,343,985	180,619,860
Sales of fuel cell systems and related infrastructure
Net revenue:
Net revenue	$ 107,292	$ 62,631	$ 39,985
Cost of revenue:
Total cost of revenue	84,439	54,815	29,543
Services performed on fuel cell systems and related infrastructure
Net revenue:
Net revenue	22,002	16,202	17,347
Cost of revenue:
Total cost of revenue	23,698	19,814	19,071
Provision for loss contracts related to service
Cost of revenue:
Provision for loss contracts related to service			(1,071)
Power Purchase Agreements
Net revenue:
Net revenue	22,869	12,869	13,687
Cost of revenue:
Total cost of revenue	36,161	31,292	16,601
Fuel delivered to customers
Net revenue:
Net revenue	22,469	8,167	10,916
Cost of revenue:
Total cost of revenue	$ 27,712	22,013	13,864
Other
Net revenue:
Net revenue		284	884
Cost of revenue:
Total cost of revenue		$ 308	$ 865

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Consolidated Statements of Comprehensive Loss
Net loss attributable to the Company	$ (78,115)	$ (127,080)	$ (57,487)
Other comprehensive (loss) income - foreign currency translation adjustment	(610)	1,947	(551)
Comprehensive loss	$ (78,725)	$ (125,133)	$ (58,038)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Common StockSeries C Redeemable Convertible Preferred Stock	Common StockSeries D Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-in-CapitalSeries C Redeemable Convertible Preferred Stock	Additional Paid-in-CapitalSeries D Redeemable Convertible Preferred Stock	Additional Paid-in-Capital	Accumulated Other Comprehensive Income	Treasury Stock	Accumulated Deficit	Series C Redeemable Convertible Preferred Stock	Series D Redeemable Convertible Preferred Stock	Total
Balance at Dec. 31, 2015			$ 1,806			$ 1,118,917	$ 798	$ (2,909)	$ (993,876)			$ 124,736
Balance (in shares) at Dec. 31, 2015			180,567,444					479,953
Increase (Decrease) in Stockholders' Equity
Net loss attributable to the Company									(57,487)			(57,487)
Other comprehensive loss							(551)					(551)
Stock-based compensation			$ 1			9,289						9,290
Stock-based compensation (in shares)			105,479
Stock dividend			$ 1			103			(104)
Stock dividend (in shares)			66,061
Public offerings, common stock, net			$ 104			8,824						8,928
Public offerings, common stock, net (in shares)			10,400,000
Stock option exercises			$ 4			98		$ (182)				(80)
Stock option exercises (in shares)			465,111					102,375
Exercise of warrants			$ 1			251						252
Exercise of warrants (in shares)			119,879
Balance at Dec. 31, 2016			$ 1,917			1,137,482	247	$ (3,091)	(1,051,467)			85,088
Balance (in shares) at Dec. 31, 2016			191,723,974					582,328
Increase (Decrease) in Stockholders' Equity
Net loss attributable to the Company									(127,080)			(127,080)
Other comprehensive loss							1,947					1,947
Stock-based compensation			$ 1			9,208						9,209
Stock-based compensation (in shares)			148,077
Stock dividend			$ 1			88			(89)
Stock dividend (in shares)			54,130
Public offerings, common stock, net			$ 102			22,890						22,992
Public offerings, common stock, net (in shares)			10,170,759
Conversion of preferred stock	$ 28	$ 95		$ 416	$ 7,683					$ 444	$ 7,778
Conversion of preferred stock (in shares)	2,772,518	9,548,393
Stock option exercises			$ 2			106		$ (11)				97
Stock option exercises (in shares)			154,166					4,823
Exercise of warrants			$ 145			39,713						39,858
Exercise of warrants (in shares)			14,501,500
Provision for common stock warrants						36,322						36,322
Accretion of discount						(3,009)						(3,009)
Balance at Dec. 31, 2017			$ 2,291			1,250,899	2,194	$ (3,102)	(1,178,636)			$ 73,646
Balance (in shares) at Dec. 31, 2017			229,073,517					587,151				229,073,517
Increase (Decrease) in Stockholders' Equity
Net loss attributable to the Company									(78,115)			$ (78,115)
Other comprehensive loss							(610)					(610)
Stock-based compensation			$ 8			8,763						8,771
Stock-based compensation (in shares)			741,216
Stock dividend						52			(52)
Stock dividend (in shares)			29,762
Public offerings, common stock, net			$ 38			6,978						7,016
Public offerings, common stock, net (in shares)			3,804,654
Stock option exercises			$ 5			168		$ (35)				138
Stock option exercises (in shares)			511,412					17,606
Equity component of convertible senior notes, net of issuance costs and income tax benefit						28,664						28,664
Purchase of capped call						(16,000)						(16,000)
Purchase of common stock forward								$ (27,500)				(27,500)
Purchase of common stock forward (in shares)								14,397,906
Exercise of warrants (in shares)			100
Provision for common stock warrants						10,190						10,190
Balance at Dec. 31, 2018			$ 2,342			$ 1,289,714	$ 1,584	$ (30,637)	(1,260,290)			$ 2,713
Balance (in shares) at Dec. 31, 2018			234,160,661					15,002,663				234,160,661
Increase (Decrease) in Stockholders' Equity
Cumulative effect from adoption of ASC 842									$ (3,487)			$ (3,487)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows From Operating Activities:
Net loss attributable to the Company	$ (78,115)	$ (127,080)	$ (57,487)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation of property, plant and equipment, and leased property	11,014	9,190	4,650
Amortization of intangible assets	693	593	590
Stock-based compensation	8,771	9,209	9,290
Provision for bad debts and other assets	1,626	250
Amortization of debt issuance costs and discount on convertible senior notes	6,347	770	1,760
Provision for common stock warrants	10,190	36,360
Loss on disposal of leased property			41
Provision for loss contracts related to service			(1,071)
Change in fair value of common stock warrant liability	(4,286)	15,188	(4,344)
Income tax benefit	(9,217)		(392)
Changes in operating assets and liabilities that provide (use) cash:
Accounts receivable	(14,398)	(9,951)	9,280
Inventory	19,041	(18,836)	2,812
Prepaid expenses and other assets	(4,654)	2,157	(2,386)
Accounts payable, accrued expenses, and other liabilities	(10,266)	11,430	11,164
Accrual for loss contracts related to service		(752)	(8,227)
Deferred revenue	5,637	11,290	4,684
Net cash used in operating activities	(57,617)	(60,182)	(29,636)
Cash Flows From Investing Activities:
Purchases of property, plant and equipment	(5,142)	(4,090)	(2,743)
Purchase of intangible asset	(929)
Purchases for construction of leased property	(13,501)	(40,273)	(55,332)
Net cash used in investing activities	(19,572)	(44,363)	(58,075)
Cash Flows From Financing Activities:
Proceeds from exercise of warrants, net of transaction costs		17,636	111
Proceeds from issuance of preferred stock and warrant, net of transaction costs	30,934		15,594
Purchase of treasury stock			(182)
Proceeds from public offerings, net of transaction costs	7,195	22,992	11,940
Proceeds from exercise of stock options	138	97	102
Payments for redemption of preferred stock		(3,700)
Proceeds from issuance of convertible senior notes, net	95,856
Purchase of capped call and common stock forward	(43,500)
Proceeds from short-term borrowing, net of transaction costs			23,673
Principal payments on short-term borrowings			(25,000)
Proceeds from borrowing of long-term debt, net of transaction costs		20,147	47,400
Principal payments on long-term debt	(16,190)	(12,292)	(25,000)
Proceeds from sale/leaseback transactions accounted for as finance obligations	76,175	45,368	32,715
Repayments of finance obligations	(31,264)	(18,632)	(4,681)
Net cash provided by financing activities	119,344	71,616	76,672
Effect of exchange rate changes on cash	(57)	348	(121)
Increase (decrease) in cash, cash equivalents and restricted cash	42,098	(32,581)	(11,160)
Cash, cash equivalents, and restricted cash beginning of period	68,055	100,636	111,796
Cash, cash equivalents, and restricted cash end of period	110,153	68,055	100,636
Supplemental disclosure of cash flow information
Cash paid for interest	13,057	8,791	8,263
Summary of non-cash investing and financing activity:
Recognition of right of use asset	79,057
Net transfers between inventory, leased assets and property, plant and equipment	$ 18,175
Conversion of preferred stock to common stock		$ 8,222
Issuance of common stock for acquisitions			$ 14,459

Nature of Operations

Nature of Operations	12 Months Ended
	Dec. 31, 2018
Nature of Operations.
Nature of Operations	1. Nature of Operations Description of Business Plug Power Inc., or the Company, is a leading provider of alternative energy technology focused on the design, development, commercialization and manufacture of hydrogen and fuel cell systems used primarily for the electric mobility and stationary power markets. As part of the global drive to electrification, the Company has recently leveraged product proven in the material handling vehicle space to enter new, adjacent, electric vehicle markets, specifically electric delivery vans. We are focused on proton exchange membrane, or PEM, fuel cell and fuel processing technologies, fuel cell/battery hybrid technologies, and associated hydrogen storage and dispensing infrastructure from which multiple products are available. A fuel cell is an electrochemical device that combines hydrogen and oxygen to produce electricity and heat without combustion. Hydrogen is derived from hydrocarbon fuels such as liquid petroleum gas (LPG), propane, methanol, ethanol, gasoline or biofuels. The Company develops complete hydrogen generation, delivery, storage and refueling solutions for customer locations. Currently the Company obtains the majority of its hydrogen by purchasing it from fuel suppliers for resale to customers. In our core business, we provide and continue to develop commercially-viable hydrogen and fuel cell product solutions to replace lead‑acid batteries in electric material handling vehicles and industrial trucks for some of the world’s largest retail-distribution and manufacturing businesses. We are focusing our efforts on industrial mobility applications (electric forklifts and electric industrial vehicles) at multi‑shift high volume manufacturing and high throughput distribution sites where our products and services provide a unique combination of productivity, flexibility and environmental benefits. Additionally, we manufacture and sell fuel cell products to replace batteries and diesel generators in stationary backup power applications. These products prove valuable with telecommunications, transportation and utility customers as robust, reliable and sustainable power solutions. Our current products and services include: GenDrive: GenDrive is our hydrogen fueled PEM fuel cell system providing power to material handling electric vehicles, including class 1, 2, 3 and 6 electric forklifts and ground support equipment; GenFuel: GenFuel is our hydrogen fueling delivery, generation, storage and dispensing system; GenCare: GenCare is our ongoing ‘internet of things’-based maintenance and on-site service program for GenDrive fuel cells, GenSure products, GenFuel products and ProGen engines; GenSure: GenSure is our stationary fuel cell solution providing scalable, modular PEM fuel cell power to support the backup and grid-support power requirements of the telecommunications, transportation, and utility sectors; GenKey: GenKey is our turn-key solution combining either GenDrive or GenSure power with GenFuel fuel and GenCare aftermarket service, offering complete simplicity to customers transitioning to fuel cell power; and ProGen: ProGen is our fuel cell stack and engine technology currently used globally in mobility and stationary fuel cell systems, and as engines in electric delivery vans; We provide our products worldwide through our direct product sales force, and by leveraging relationships with original equipment manufacturers, or OEMs, and their dealer networks. We manufacture our commercially-viable products in Latham, NY. We were organized as a corporation in the State of Delaware on June 27, 1997. Unless the context indicates otherwise, the terms “Company,” “Plug Power,” “we,” “our” or “us” as used herein refers to Plug Power Inc. and its subsidiaries. Liquidity Our cash requirements relate primarily to working capital needed to operate and grow our business, including funding operating expenses, growth in inventory to support both shipments of new units and servicing the installed base, growth in equipment leased to customers under long-term arrangements, funding the growth in our GenKey “turn-key” solution, which includes the installation of our customers’ hydrogen infrastructure as well as delivery of the hydrogen fuel, continued development and expansion of our products, payment of lease/financing obligations under sale/leaseback financings, and the repayment or refinancing of our long-term debt. Our ability to achieve profitability and meet future liquidity needs and capital requirements will depend upon numerous factors, including the timing and quantity of product orders and shipments; attaining and expanding positive gross margins across all product lines; the timing and amount of our operating expenses; the timing and costs of working capital needs; the timing and costs of developing marketing and distribution channels; the ability of our customers to obtain financing to support commercial transactions; our ability to obtain financing arrangements to support the sale or leasing of our products and services to customers and to repay or refinance our long-term debt, and the terms of such agreements that may require us to pledge or restrict substantial amounts of our cash to support these financing arrangements; the timing and costs of developing marketing and distribution channels; the timing and costs of product service requirements; the timing and costs of hiring and training product staff; the timing and costs of product development and introductions; the extent of our ongoing and new research and development programs; and changes in our strategy or our planned activities. If we are unable to fund our operations with positive cash flows and cannot obtain external financing, we may not be able to sustain future operations. As a result, we may be required to delay, reduce and/or cease our operations and/or seek bankruptcy protection. We have experienced and continue to experience negative cash flows from operations and net losses. The Company incurred net losses attributable to common shareholders of $78.2 million, $130.2 million and $57.6 million for the years ended December 31, 2018, 2017, and 2016, respectively, and has an accumulated deficit of $1.3 billion at December 31, 2018. During the year ended December 31, 2018, cash used in operating activities was $57.6 million, consisting primarily of a net loss attributable to the Company of $78.1 million, offset by the impact of noncash charges of $25.1 million, and net outflows from fluctuations in working capital and other assets and liabilities of $4.6 million. The changes in working capital primarily were related to reduction of inventory and an increase in deferred revenue offset by increases in accounts receivable and prepaid expenses, and a decrease in accounts payable, accrued expenses, and other liabilities. As of December 31, 2018, we had cash and cash equivalents of $38.6 million and net working capital of $9.2 million. By comparison, at December 31, 2017, we had cash and cash equivalents of $24.8 million and net working capital of $3.9 million. Net cash used in investing activities for the year ended December 31, 2018, totaled $19.6 million and included purchases of property, plant and equipment and outflows associated with materials, labor, and overhead necessary to construct new leased property. Cash outflows related to equipment that we sell and equipment we lease directly to customers are included in net cash used in operating activities and net cash used in investing activities, respectively. Net cash provided by financing activities for the year ended December 31, 2018 totaled $119.3 million and primarily resulted from net proceeds of $52.4 million from the issuance of Convertible Senior Notes, net of purchases of a capped call and a common stock forward, $30.9 million from the issuance of preferred stock and a $44.9 million net increase in finance obligations, offset by $16.2 million of principal payments on long-term debt. In March 2018, we issued $100.0 million in aggregate principal amount of 5.5% Convertible Senior Notes due in 2023 (Convertible Senior Notes). The total net proceeds from this offering, after considering costs of the issuance, were approximately $95.9 million. Approximately $43.5 million of the proceeds were used for the cost of a capped call and a common stock forward, both of which are hedges related to the Convertible Senior Notes. The remaining net proceeds from the Convertible Senior Notes were or will be used for general corporate purposes, including working capital. The Company enters into sale/leaseback agreements with various financial institutions to facilitate the Company’s commercial transactions with key customers. The Company sells certain fuel cell systems and hydrogen infrastructure to the financial institutions and leases the equipment back to support certain customer locations and to fulfill its varied Power Purchase Agreements (PPAs). In connection with certain operating leases, the financial institutions required the Company to maintain cash balances in restricted accounts securing the Company’s finance obligations. Cash received from customers under the PPAs is used to make payments against our finance obligations. As the Company performs under these agreements, the required restricted cash balances are released, according to a set schedule. The total remaining lease payments to financial institutions under these agreements at December 31, 2018 is $82.2 million, which have been partially secured with restricted cash, security deposits and pledged service escrows of $80.4 million. The Company has a master lease agreement with Wells Fargo (Wells Fargo MLA) to finance the Company’s commercial transactions with Walmart. The Wells Fargo MLA was entered into in 2017 and amended in 2018. Pursuant to the Wells Fargo MLA, the Company sells fuel cell systems and hydrogen infrastructure to Wells Fargo and then leases them back and operates them at Walmart sites under lease agreements with Walmart. The total remaining lease payments to Wells Fargo was $67.5 million at December 31, 2018. Transactions completed under the Wells Fargo MLA in 2018 were accounted for as operating leases and therefore recognized as revenue for the year ended December 31, 2018. Transactions completed under the Wells Fargo MLA in 2017 were accounted for as capital leases. The difference in lease classification is due to changes in financing terms and their bearing on lease assessment criteria. Also included in the remaining lease payments to Wells Fargo is a sale/leaseback transaction in 2015 that was accounted for as an operating lease. In connection with the Wells Fargo MLA, the Company has a customer guarantee for a majority of the transactions. The Wells Fargo transactions in 2018 required a letter of credit for the unguaranteed portion totaling $20.1 million. In November 2018, the Company completed a private placement of an aggregate of 35,000 shares of the Company’s Series E Convertible Preferred Stock, par value $0.01 per share (Series E Preferred Stock), resulting in net proceeds of approximately $30.9 million. We have historically funded our operations primarily through public and private offerings of common and preferred stock, as well as short-term borrowings, long-term debt and project financing, and recently with Convertible Senior Notes. The Company believes that its current working capital and cash anticipated to be generated from future operations, as well as borrowings from lending and project financing sources and proceeds from equity offerings, will provide sufficient liquidity to fund operations for at least one year after the date the financial statements are issued. There is no guarantee that future funding will be available if and when required or at terms acceptable to the Company. This projection is based on our current expectations regarding new project financing and product sales and service, cost structure, cash burn rate and other operating assumptions. Additionally, the Company has other capital sources available, including the At Market Issuance Sales Agreement discussed further below. As of December 31, 2018, the Company has approximately $91.1 million of finance obligations and long-term debt classified as current liabilities, approximately $50.0 million of which is due in the second quarter of 2019. The Company is in the process of negotiating a refinance of a majority of these current liabilities with its primary lender in order to consolidate some of these facilities and extend the repayment schedule. Should the Company not come to an agreement with this lender, it will need to seek an extension and other capital sources in the near term, such as debt facilities with other lenders and/or equity solutions, which may include the At Market Issuance Sales Agreement.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Earnings Per Share
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Principles of Consolidation The consolidated financial statements include the financial statements of the Company and its wholly-owned subsidiaries. All significant intercompany balances and transactions have been eliminated in consolidation. Leases The Company is a lessee in several noncancelable (1) operating leases, primarily related to sale/leaseback transactions with financial institutions for deployment of the Company’s products at certain customer sites, and (2) finance leases, also primarily related to sale/leaseback transactions with financial institutions for similar commercial purposes. The Company accounts for leases in accordance with ASC Topic 842, Leases (ASC Topic 842). See Recently Adopted Accounting Standards, as the Company has early adopted ASC Topic 842 in 2018. The Company determines if an arrangement is or contains a lease at contract inception. The Company recognizes a right of use (ROU) asset and a lease liability (i.e. finance obligation) at the lease commencement date. For operating leases, the lease liability is initially measured at the present value of the unpaid lease payments at the lease commencement date. For finance leases, the lease liability is initially measured in the same manner and date as for operating leases, and is subsequently measured at amortized cost using the effective interest method. Key estimates and judgments include how the Company determines (1) the discount rate it uses to discount the unpaid lease payments to present value, (2) lease term and (3) lease payments. · ASC Topic 842 requires a lessee to discount its unpaid lease payments using the interest rate implicit in the lease or, if that rate cannot be readily determined, its incremental borrowing rate. Generally, the Company cannot determine the interest rate implicit in the lease because it does not have access to the lessor’s estimated residual value or the amount of the lessor’s deferred initial direct costs. Therefore, the Company generally uses its incremental borrowing rate as the discount rate for the lease. The Company’s incremental borrowing rate for a lease is the rate of interest it would have to pay on a collateralized basis to borrow an amount equal to the lease payments under similar terms. · The lease term for all of the Company’s leases includes the noncancelable period of the lease, plus any additional periods covered by either a Company option to extend (or not to terminate) the lease that the Company is reasonably certain to exercise, or an option to extend (or not to terminate) the lease controlled by the lessor. · Lease payments included in the measurement of the lease liability comprise fixed payments, and the exercise price of a Company option to purchase the underlying asset if the Company is reasonably certain to exercise the option. The ROU asset is initially measured at cost, which comprises the initial amount of the lease liability adjusted for lease payments made at or before the lease commencement date, plus any initial direct costs incurred less any lease incentives received. For operating leases, the ROU asset is subsequently measured throughout the lease term at the carrying amount of the lease liability, plus initial direct costs, plus (minus) any prepaid (accrued) lease payments, less the unamortized balance of lease incentives received. Lease expense for lease payments is recognized on a straight-line basis over the lease term. For finance leases, the ROU asset is subsequently amortized using the straight-line method from the lease commencement date to the earlier of the end of its useful life or the end of the lease term unless the lease transfers ownership of the underlying asset to the Company or the Company is reasonably certain to exercise an option to purchase the underlying asset. In those cases, the ROU asset is amortized over the useful life of the underlying asset. Amortization of the ROU asset is recognized and presented separately from interest expense on the lease liability. The Company’s leases do not contain variable lease payments. ROU assets for operating and finance leases are periodically reviewed for impairment losses. The Company uses the long-lived assets impairment guidance in ASC Subtopic 360-10, Property, Plant, and Equipment – Overall , to determine whether an ROU asset is impaired, and if so, the amount of the impairment loss to recognize. No impairment losses have been recognized to date. The Company monitors for events or changes in circumstances that require a reassessment of one of its leases. When a reassessment results in the remeasurement of a lease liability, a corresponding adjustment is made to the carrying amount of the corresponding ROU asset. Operating and finance lease ROU assets are presented within leased property, net on the consolidated balance sheet. The current portion of operating and finance lease liabilities is included in finance obligations within current liabilities and the long-term portion is presented in finance obligations within noncurrent liabilities on the consolidated balance sheet. The Company has elected not to recognize ROU assets and lease liabilities for short-term leases that have a lease term of 12 months or less. The Company has elected to apply the short-term lease recognition and measurement exemption for other classes of leased assets. The Company recognizes the lease payments associated with its short-term leases as an expense on a straight-line basis over the lease term. Adoption of ASC Topic 842 - Transition Approach As discussed in Recent Accounting Pronouncements, effective January 1, 2018, the Company early adopted ASC Topic 842. Further, ASU 2018-11 was issued in July 2018, which allowed for a modified retrospective basis of accounting for the transition. The Company selected this transition method and, therefore, restatement of prior period consolidated financial statements or presentation of comparative disclosures is not necessary. While determining the impact from the transition, the Company elected the practical expedients allowed under ASC 842. The Company did not reassess (1) whether any expired or existing contracts are or contain leases, (2) the lease classification for any expired or existing leases, or (3) initial direct costs for any existing leases. No modifications to leases effective upon January 1, 2018 were noted where hindsight expedients were necessary to apply. Revenue Recognition The Company enters into contracts that may contain one or a combination of fuel cell systems and infrastructure, installation, maintenance, spare parts, fuel delivery and other support services. Contracts containing fuel cell systems and related infrastructure may be sold, or provided to customers under a PPA. The Company does not include a right of return on its products other than rights related to standard warranty provisions that permit repair or replacement of defective goods. The Company accrues for anticipated standard warranty costs at the same time that revenue is recognized for the related product, or when circumstances indicate that warranty costs will be incurred, as applicable. Only a limited number of fuel cell units are under standard warranty. Revenue is measured based on the transaction price specified in a contract with a customer, subject to the allocation of the transaction price to distinct performance obligations as discussed below. The Company recognizes revenue when it satisfies a performance obligation by transferring control over a product or service to a customer. The Company accounts for each distinct performance obligation of within its arrangements as a distinct unit of accounting if the items under the performance obligation have value to the customer on a standalone basis. The Company considers a performance obligation to be distinct and have a standalone value if the customer can benefit from the good or service either on its own or together with other resources readily available to the customer and the Company’s promise to transfer the goods or service to the customer is separately identifiable from other promises in the contract. The Company allocates revenue to each distinct performance obligation based on relative standalone selling prices. Payment terms for fuel cells, infrastructure and service are invoiced with terms ranging from 30 to 90 days. Service is prepaid upfront in a majority of the arrangements. The Company does not adjust the transaction price for a significant financing component when the performance obligation is expected to be fulfilled within a year. The Company presents the provision for common stock warrants within each revenue-related line item on the consolidated statements of operations. This presentation reflects a discount that those common stock warrants represent, and therefore revenue is net of these non-cash charges. The provision of common stock warrants is allocated to the relevant revenue-related line items based upon the expected mix of the revenue for each respective contract. Nature of goods and services The following is a description of principal activities from which the Company generates its revenue. (i) Revenue from sales of fuel cell systems and related infrastructure represents sales of our GenDrive units, GenSure stationary backup power units, as well as hydrogen fueling infrastructure. The Company considers comparable list prices, as well as historical average pricing approaches to determine stand-alone selling prices. Once relative standalone selling prices are determined, the Company proportionately allocates the transaction price to each performance obligation within the customer arrangement. The allocated transaction price related to fuel cell systems, spare parts and hydrogen infrastructure is recognized as revenue at a point in time which usually occurs at shipment (and occasionally upon delivery) for fuel cells and spare parts and upon customer acceptance for hydrogen infrastructure, depending on the terms of the arrangement, the point at which transfer of control passes to the customer and the performance obligation has been satisfied. (ii) Revenue from services performed on fuel cell systems and related infrastructure represents revenue earned on our service and maintenance contracts and sales of spare parts. The transaction price allocated to services as discussed above is generally recognized as revenue over time on a straight-line basis over the expected service period. In substantially all of its commercial transactions, the Company sells extended maintenance contracts that generally provide for a five to ten year service period from the date of product installation. Services include monitoring, technical support, maintenance and services that provide for 97-98% uptime of the fleet. These services are accounted for as a separate performance obligation, and accordingly, revenue generated from these transactions, subject to the proportional allocation of transaction price, is deferred and recognized in income over the term of the contract, generally on a straight-line basis. Additionally, the Company may enter into annual service and extended maintenance contracts that are billed monthly. Revenue generated from these transactions is recognized in income on a straight-line basis over the term of the contract. Costs are recognized as incurred over the term of the contract. Sales of spare parts are included within service revenue on the accompanying consolidated statements of operations. When costs are projected to exceed revenues over the life of the contract, an accrual for loss contracts is recorded. Costs are estimated based upon historical experience and consider the estimated impact of the Company’s cost reduction initiatives. The actual results may differ from these estimates. When costs are projected to exceed revenues over the life of an extended maintenance contract, an accrual for loss contracts is recorded. Costs are estimated based upon historical experience and consider the estimated impact of the Company’s cost reduction initiatives. The actual results may differ from these estimates. Upon expiration of the extended maintenance contracts, customers either choose to extend the contract or switch to purchasing spare parts and maintaining the fuel cell systems on their own. (iii) Revenue from PPAs primarily represents payments received from customers who make monthly payments to access the Company’s GenKey solution. When fuel cell systems and related infrastructure are provided to customers through a PPA, revenues associated with these agreements are treated as rental income and recognized on a straight-line basis over the life of the agreements. In conjunction with entering into a PPA with a customer, the Company may enter into sale/leaseback transactions with third-party financial institutions, whereby the fuel cells, a majority of the related infrastructure, and, in some cases service are sold to the third-party financial institution and leased back to the Company through either an operating or finance lease. Certain of the Company’s sale/leaseback transactions with third-party financial institutions are required to be accounted for as financing leases. As a result, no upfront revenue was recognized at the closing of these transactions and a finance obligation for each lease was established. The fuel cell systems and related infrastructure that are provided to customers through these PPAs are considered leased property on the accompanying consolidated balance sheet. Costs to service the leased property, depreciation of the leased property, and other related costs are considered cost of PPA revenue on the accompanying consolidated statements of operations. Interest cost associated with finance leases is presented within interest and other expense, net on the accompanying consolidated statement of operations. The Company also has sale/leaseback transactions with financial institutions, which were required to be accounted for as an operating lease. The Company has rental expense associated with these sale/leaseback agreements with financial institutions. Rental expense is recognized on a straight-line basis over the life of the agreements and is characterized as cost of PPA revenue on the accompanying consolidated statements of operations. The Company adopted ASC Topic 842, effective January 1, 2018. As part of the adoption, the Company elected the practical expedient to not separate lease and non-lease components (i.e. maintenance services) within its rental income related to all PPA-related assets. (iv) Revenue associated with fuel delivered to customers represents the sale of hydrogen to customers that has been purchased by the Company from a third party or generated on site. The Company purchases hydrogen fuel from suppliers in certain cases (and produces hydrogen onsite) and sells to its customers upon delivery. Revenue and cost of revenue related to this fuel is recorded as dispensed, and is included in the respective “Fuel delivered to customers” lines on the consolidated statements of operations. Contract costs The Company expects that incremental commission fees paid to employees as a result of obtaining sales contracts are recoverable and therefore the Company capitalizes them as contract costs. Capitalized commission fees are amortized on a straight-line basis over the period of time over which the transfer of goods or services to which the assets relate occur, typically ranging from 5 to 10 years. Amortization of the capitalized commission fees is included in selling, general, and administrative expenses. The Company recognizes the incremental costs of obtaining contracts as an expense when incurred if the amortization period of the assets that the Company otherwise would have recognized is one year or less. These costs are included in selling, general, and administrative expenses. Adoption of ASC Topic 606 - Transition Approach As discussed in Recent Accounting Pronouncements, on January 1, 2018, the Company adopted ASC Topic 606, Revenue from Contracts with Customers (ASC Topic 606), which offers two transition approaches: full retrospective and modified retrospective. The Company chose the modified retrospective approach as its transition method and will not experience a significant effect on the timing and amount of revenue recognized or the amount of revenue allocated to the identified performance obligations. There was an insignificant amount of historical contract acquisition costs that were expensed and were not capitalized upon adoption of ASC Topic 606. However, upon adoption, contract acquisition costs of $0.1 million were capitalized and are being amortized as described above. Cash Equivalents For purposes of the consolidated statements of cash flows, the Company considers all highly-liquid debt instruments with original maturities of three months or less to be cash equivalents. At December 31, 2018 and 2017, cash equivalents consist of money market accounts. The Company’s cash and cash equivalents are deposited with financial institutions located in the U.S. and may at times exceed insured limits. Common Stock Warrant Accounting The Company accounts for common stock warrants as either derivative liabilities or as equity instruments depending on the specific terms of the respective warrant agreements. Derivative Liabilities Registered common stock warrants that require the issuance of registered shares upon exercise and do not sufficiently preclude an implied right to cash settlement are accounted for as derivative liabilities. We currently classify these derivative warrant liabilities on the consolidated balance sheet as a long-term liability, which are revalued at each balance sheet date subsequent to the initial issuance, using the Black-Scholes pricing model. This pricing model, which is based, in part, upon unobservable inputs for which there is little or no market data, requires the Company to develop its own assumptions. Changes in the fair value of the warrants are reflected in the consolidated statements of operations as change in fair value of common stock warrant liability. Accounts Receivable Accounts receivable are stated at the amount billed or billable to customers and are ordinarily due between 30 and 60 days after the issuance of the invoice. Receivables are reserved or written off based on individual credit evaluation and specific circumstances of the customer. The allowance for doubtful accounts and related receivable are reduced when the amount is deemed uncollectible. As of December 31, 2018, and 2017, the allowance for doubtful accounts was $249 thousand. Inventory Inventories are valued at the lower of cost, determined on a first-in, first-out basis, and net realizable value. All inventory, including spare parts inventory held at service locations, is not relieved until the customer has received the product, at which time the risks and rewards of ownership have transferred. Property, Plant and Equipment Property, plant and equipment are originally recorded at cost or, if acquired as part of business combination, at fair value. Maintenance and repairs are expensed as costs are incurred. Depreciation on plant and equipment, which includes depreciation on the Company’s primary manufacturing facility, which is accounted for as a financing obligation, is calculated on the straight-line method over the estimated useful lives of the assets. The Company records depreciation and amortization over the following estimated useful lives: Buildings 20 years Building improvements 5 ‑ 20 years Software, machinery and equipment 1 ‑ 15 years Gains and losses resulting from the sale of property and equipment are recorded in current operations. Leased Property Leased property primarily consists of the cost of assets deployed related to finance leases. Depreciation expense is recorded on a straight-line basis over the shorter of the lease term or the estimated useful life of the asset, generally six to seven years, and is included in cost of revenue for PPAs in the accompanying consolidated statements of operations. Impairment of Long-Lived Assets Long-lived assets, such as property, plant, and equipment, leased property and purchased intangibles subject to amortization, are reviewed for potential impairment whenever events or changes in circumstances indicate that the carrying amount of an asset or asset group may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset or asset group to estimated undiscounted future cash flows expected to be generated by the asset or asset group. If the carrying amount of an asset or asset group exceeds its estimated undiscounted future cash flows, an impairment charge is recognized by the amount by which the carrying amount of the asset or asset group exceeds the fair value of the asset or asset group. Fair value is determined through various valuation techniques, including discounted cash flow models, quoted market values and third party independent appraisals, as considered necessary. Assets to be disposed of and considered held for sale would be separately presented in the consolidated balance sheet and reported at the lower of the carrying amount or fair value less costs to sell, and are no longer depreciated. The assets and liabilities of a disposal group classified as held for sale would be presented separately in the appropriate asset and liability sections of the consolidated balance sheet. Goodwill Goodwill is an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. Goodwill is reviewed for impairment at least annually. The Company has the option to perform a qualitative assessment to determine whether it is more-likely-than-not that the fair value of a reporting unit is less than its carrying amount prior to performing the two-step goodwill impairment test. If this is the case, the two-step goodwill impairment test is required. If it is more-likely-than-not that the fair value of a reporting unit is greater than its carrying amount, the two-step goodwill impairment test is not required. The Company performs an impairment review of goodwill on an annual basis at December 1, and when a triggering event is determined to have occurred between annual impairment tests. For the years ended December 31, 2018, 2017, and 2016, the Company performed a qualitative assessment of goodwill for its single reporting unit based on multiple factors including market capitalization, and determined that it is not more likely than not that the fair value of its reporting unit is less than the carrying amount. Intangible Assets Intangible assets consist of acquired technology, customer relationships and trademarks, and are amortized using a straight-line method over their useful lives of 5 - 10 years. Additionally, the intangible assets are reviewed for impairment when certain triggering events occur. Product Warranty Reserve Aside from when included in the sale of an extended maintenance contract, the Company provides a one to two year standard product warranty to customers from date of installation of GenDrive units, and the GenSure sales generally include a two year standard product warranty. We currently estimate the costs of satisfying warranty claims based on an analysis of past experience and provide for future claims in the period the revenue is recognized. Factors that affect our warranty liability include the number of installed units, estimated material costs, estimated travel, and labor costs. The warranty reserve is included within the other current liabilities on the accompanying consolidated balance sheet. Equity Instruments Common stock warrants that meet certain applicable requirements of ASC Subtopic 815-40, Derivatives and Hedging – Contracts in Entity’s Own Equity , and other related guidance, including the ability of the Company to settle the warrants without the issuance of registered shares or the absence of rights of the grantee to require cash settlement, are accounted for as equity instruments. The Company classifies these equity instruments within additional paid-in capital on the accompanying consolidated balance sheets. Common stock warrants accounted for as equity instruments represent the warrants issued to Amazon and Walmart as discussed in Note 13, Warrant Transaction Agreements. These warrants are remeasured at each financial reporting date prior to vesting, using the Monte Carlo pricing model. Once these warrants vest, they are no longer remeasured. This pricing model, which is based, in part, upon unobservable inputs for which there is little or no market data, requires the Company to develop its own assumptions. Changes in fair value resulting from remeasurement of common stock warrants issued in connection with the Amazon Transaction Agreement and the Walmart Transaction Agreement, as described in Note 13, Warrant Transaction Agreements and are recorded as cumulative catch up adjustments as a reduction of revenue. Redeemable Preferred Stock We account for redeemable preferred stock as temporary equity in accordance with applicable accounting guidance in FASB ASC Topic 480, Distinguishing Liabilities from Equity . Dividends on the redeemable preferred stock are accounted for as an increase in the net loss attributable to common shareholders. Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax basis and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the period that includes the enactment date. A valuation allowance is recorded to reduce the carrying amounts of deferred tax assets if it is more likely than not that such assets will not be realized. The Company accounts for uncertain tax positions in accordance with FASB ASC No. 740-10-25, Income Taxes-Overall-Recognition . The Company recognizes in its consolidated financial statements the impact of a tax position only if that position is more likely than not to be sustained on audit, based on the technical merits of the position. Foreign Currency Translation Foreign currency translation adjustments arising from conversion of the Company’s foreign subsidiary’s financial statements to U.S. dollars for reporting purposes are included in accumulated other comprehensive income in stockholders’ equity on the accompanying consolidated balance sheets. Transaction gains and losses resulting from the effect of exchange rate changes on transactions denominated in currencies other than the functional currency of the Company’s operations give rise to realized foreign currency transaction gains and losses, and are included in interest and other income and interest and other expense, respectively, in the accompanying consolidated statements of operations. Research and Development Costs related to research and development activities by the Company are expensed as incurred. Stock-Based Compensation The Company maintains employee stock-based compensation plans, which are described more fully in Note 15, Employee Benefit Plans. Stock-based compensation represents the cost related to stock-based awards granted to employees and directors. The Company measures stock-based compensation cost at grant date, based on the fair value of the award, and recognizes the cost as expense on a straight-line basis over the option’s requisite service period. The Company estimates the fair value of stock-based awards using a Black-Scholes valuation model. Stock-based compensation expense is recorded in cost of revenue associated with sales of fuel cell systems and related infrastructure, cost of revenue for services performed on fuel cell systems and related infrastructure, research and development expense and selling, general and administrative expenses in the accompanying consolidated statements of operations based on the employees’ respective function. The Company records deferred tax assets for awards that result in deductions on the Company’s income tax returns, based upon the amount of compensation cost recognized and the Company's statutory tax rate. Differences between the deferred tax assets recognized for financial reporting purposes and the actual tax deduction reported on the Company's income tax return are recorded in the income statement. No tax benefit or expense for stock-based compensation has been recorded during the years ended December 31, 2018, 2017 and 2016 since the Company remains in a net operating loss (NOL) position. Convertible Senior Notes The Company accounts for the issued Convertible Senior Notes with separate liability and equity components. The carrying amount of the liability component was initially determined by estimating the fair value of a similar debt instrument that does not have an associated convertible feature. The carrying amount of the equity component representing the conversion option was determined by deducting the estimated fair value of the liability component from the par value of the Convertible Senior Notes as a whole as of the date of issuance. This difference represents a debt discount that is amortized to interest expense, with a corresponding increase to the carrying amount of the liability component, over the term of the Convertible Senior Notes using the effective interest rate method. The equity component is not remeasured as long as it continues to meet the conditions for equity classification. The Company has allocated issuance costs incurred to the liability and equity components. Issuance costs attributable to the liability component are being amortized to expense over the respective term of the Convertible Senior Notes, and issuance costs attributable to the equity components were netted with the respective equity component in additional paid-in capital. Use of Estimates The consolidated financial statements of the Company have been prepared in conformity with U.S. generally accepted accounting principles, which require management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Reclassifications and Correction of Immaterial Errors Reclassifications are made, whenever necessary, to prior period financial statements to conform to the current period presentation. The provision for common stock warrants presented historically as one line item on the consolidated statements of operations has been allocated to each of the relevant revenue line items. This reclassification did not have an impact on gross profit (loss) or net loss within the consolidated statements of operations or major categories within the consolidated statements of cash flows in the periods presented. In the third quarter of 2018, it was determined that the presentation in our consolidated statements of operations of certain service arrangements and the amortization of the associated finance obligations had not been appropriately accounted for resulting in an overstatement of our revenue and cost of revenue. This previous presentation resulted in a gross up of these line items and had no impact on our gross profit (loss) or net loss. The Company corrected the 2017 and 2016 annual financial statements to be consistent with the current period presentation and will correct comparable financial information in future filings. The amount reclassified from revenue on service performed on fuel cell systems and related infrastructure to cost of revenue on PPAs for years ended December 31, 2017 and 2016 was $3.8 million and $3.6 million, respectively. The amount reclassified from cost of revenue on service performed on fuel cell systems and related infrastructure to cost of revenue on PPAs for the years ended December 31, 2017 and 2016 was $3.1 million and $3.1 million, respectively. The Company does not consider the impact of the prior period correction to be mater

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share.
Earnings Per Share	3. Earnings Per Share Basic earnings per common share are computed by dividing net loss attributable to common shareholders by the weighted average number of common shares outstanding during the reporting period. Diluted earnings per share reflects the potential dilution that could occur if securities or other contracts to issue common stock (such as stock options, unvested restricted stock, common stock warrants, and preferred stock) were exercised or converted into common stock or resulted in the issuance of common stock (net of any assumed repurchases) that then shared in the earnings of the Company, if any. This is computed by dividing net earnings by the combination of dilutive common share equivalents, which is comprised of shares issuable under outstanding warrants, the conversion of preferred stock, and the Company’s share-based compensation plans, and the weighted average number of common shares outstanding during the reporting period. Since the Company is in a net loss position, all common stock equivalents would be considered to be anti-dilutive and are, therefore, not included in the determination of diluted earnings per share. Accordingly, basic and diluted loss per share are the same. The following table provides the components of the calculations of basic and diluted earnings per share (in thousands, except share amounts): Year ended December 31, 2018 2017 2016 Numerator: Net loss attributable to common shareholders $ (78,167) $ (130,178) $ (57,591) Denominator: Weighted average number of common shares outstanding 218,882,337 216,343,985 180,619,860 The dilutive potential common shares are summarized as follows: At December 31, 2018 2017 2016 Stock options outstanding (1) 21,957,150 19,872,029 14,760,054 Restricted stock outstanding (2) 2,347,347 234,744 13,333 Common stock warrants (3) 115,824,142 115,824,242 14,501,600 Preferred stock (4) 17,933,591 2,782,075 17,490,078 Convertible Senior Notes (5) 43,630,020 - - Number of dilutive potential common shares 201,692,250 138,713,090 46,765,065 (1) During the years ended December 31, 2018, 2017, and 2016, the Company granted 2,679,667, 5,485,863, and 3,702,500 stock options, respectively. (2) During the years ended December 31, 2018, 2017, and 2016, the Company granted 2,347,347, 234,744, and zero shares of restricted stock, respectively. (3) In February 2013, the Company issued 23,637,500 warrants as part of an underwritten public offering with an exercise price of $0.15 per warrant. Of these warrants issued in February 2013, zero, 100 and 100 were unexercised as of December 31, 2018, 2017 and 2016. In January 2014, the Company issued 4,000,000 warrants as part of an underwritten public offering with an exercise price of $4.00 per warrant. In December 2016, as a result of additional public offerings, and pursuant to the effect of the anti-dilution provisions of these warrants, the exercise price of the $4.00 warrants was reduced to $0.65. Of these warrants issued in January 2014, all 4,000,000 warrants were exercised during the year ended December 31, 2017, as described in Note 11, Stockholders’ Equity. In December 2016, the Company issued 10,501,500 warrants as part of two concurrent underwritten public offerings with an exercise price of $1.50 per warrant. Of these warrants issued in December 2016, all 10,501,500 warrants were exercised during the year ended December 31, 2017, respectively, as described in Note 11, Stockholders’ Equity. In April 2017, the Company issued 5,250,750 warrants with an exercise price of $2.69 per warrant, as described in Note 11, Stockholders’ Equity. Of these warrants issued in April 2017, none have been exercised as of December 31, 2018. In April 2017, the Company issued warrants to acquire up to 55,286,696 of the Company’s common stock as part of a transaction agreement, subject to certain vesting events, as described in Note 13, Warrant Transaction Agreements. Of these warrants issued, none have been exercised as of December 31, 2018. In July 2017, the Company issued warrants to acquire up to 55,286,696 of the Company’s common stock as part of a transaction agreement, subject to certain vesting events, as described in Note 13, Warrant Transaction Agreements. Of these warrants issued, none have been exercised as of December 31, 2018. (4) The preferred stock amount represents the dilutive potential common shares of the Series C, D and E redeemable convertible preferred stock, based on the conversion price of the preferred stock as of December 31, 2018, 2017 and 2016, respectively. Of the 10,431 Series C redeemable preferred stock issued on May 16, 2013, 7,811 had been converted to common stock through December 31, 2017, with the remainder still outstanding. Of the 18,500 Series D redeemable convertible preferred stock issued on December 22, 2016, 3,700 shares have been redeemed and the remaining 14,800 have been converted to common stock during the year ended December 31, 2017. On November 1, 2018, the Company issued 35,000 shares of Series E redeemable convertible preferred stock. As of December 31, 2018, all Series E redeemable convertible preferred stock are outstanding. (5) In March 2018, the Company issued $100.0 million in aggregate principal amount of 5.5% Convertible Senior Notes. See Note 10, Convertible Senior Notes.

Inventory

Inventory	12 Months Ended
	Dec. 31, 2018
Inventory
Inventory	4. Inventory Inventory as of December 31, 2018 and 2017 consists of the following (in thousands): December 31, 2018 December 31, 2017 Raw materials and supplies - production locations $ 32,941 $ 37,369 Raw materials and supplies - customer locations 6,755 5,482 Work-in-process 5,589 3,492 Finished goods 2,625 2,433 Inventory $ 47,910 $ 48,776

Property, Plant and Equipment

Property, Plant and Equipment	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment
Property, Plant and Equipment	5. Property, Plant and Equipment Property, plant and equipment at December 31, 2018 and 2017 consists of the following (in thousands): December 31, December 31, 2018 2017 Land $ — $ 90 Buildings — 15,332 Building improvements 214 5,230 Software, machinery and equipment 27,058 21,350 Property, plant, and equipment 27,272 42,002 Less: accumulated depreciation (14,403) (31,588) Property, plant, and equipment, net $ 12,869 $ 10,414 Depreciation expense related to property, plant and equipment was $2.6 million, $1.9 million, and $1.8 million for the years ended December 31, 2018, 2017 and 2016, respectively. Upon the adoption of ASC 842, the Company reclassified property and equipment, subject to a finance lease, to right of use assets. See Note 18, Commitments and Contingencies – Finance Leases as Lessee.

Leased Property

Leased Property	12 Months Ended
	Dec. 31, 2018
Leased Property
Leased Property	6. Leased Property Leased property at December 31, 2018 and 2017 consists of the following (in thousands): December 31, December 31, 2018 2017 Right of use assets - operating $ 76,747 $ — Right of use assets - finance 39,905 — Capitalized costs of lessor assets 41,040 98,877 Less: accumulated depreciation (10,941) (11,812) Leased property, net $ 146,751 $ 87,065 Capitalized costs of lessor assets associated with finance leases have been reclassified to right of use assets upon the adoption of ASC 842. Depreciation expense related to leased property was $8.4 million, $7.3 million, and $2.9 million the years ended December 31, 2018, 2017 and 2016, respectively.

Intangible Assets

Intangible Assets	12 Months Ended
	Dec. 31, 2018
Intangible Assets
Intangible Assets	7. Intangible Assets The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of December 31, 2018 are as follows (in thousands): Weighted Average Gross Carrying Accumulated Amortization Period Amount Amortization Total Acquired technology 9 years $ 5,926 $ (2,176) $ 3,750 Customer relationships 10 years 260 (123) 137 Trademark 5 years 60 (57) 3 $ 6,246 $ (2,356) $ 3,890 The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of December 31, 2017 are as follows (in thousands): Weighted Average Gross Carrying Accumulated Amortization Period Amount Amortization Total Acquired technology 9 years $ 5,200 $ (1,593) $ 3,607 Customer relationships 10 years 260 (97) 163 Trademark 5 years 60 (45) 15 $ 5,520 $ (1,735) $ 3,785 The change in the gross carrying amount and accumulated amortization of the acquired technology from December 31, 2017 to December 31, 2018 is due to the acquisition of intellectual property from American Fuel Cell LLC in April 2018, as well as changes attributed to foreign currency translation. As part of the agreement to acquire the intellectual property, the Company shall pay American Fuel Cell LLC milestone payments not to exceed $2.9 million in total, if certain milestones Amortization expense for acquired identifiable intangible assets for the years ended December 31, 2018, 2017, and 2016 was $0.7 million, $0.6 million, and $0.6 million, respectively. Estimated amortization expense for subsequent years is as follows (in thousands): 2019 $ 534 2020 498 2021 498 2022 498 2023 498 2024 and thereafter 1,364 Total $ 3,890

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2018
Accrued Expenses
Accrued Expenses	8. Accrued Expenses Accrued expenses at December 31, 2018 and 2017 consist of (in thousands): 2018 2017 Accrued payroll and compensation related costs $ 2,150 $ 1,473 Accrued accounts payable 1,790 5,821 Accrued sales and other taxes 1,478 1,278 Accrued interest 1,605 — Accrued litigation — 1,076 Accrued other 841 947 Total $ 7,864 $ 10,595

Long-Term Debt

Long-Term Debt	12 Months Ended
	Dec. 31, 2018
Long-Term Debt
Long-Term Debt	9. Long-Term Debt NY Green Bank Loan The Company, and its subsidiaries Emerging Power Inc. and Emergent Power Inc., are parties to a loan and security agreement with NY Green Bank, a Division of the New York State Energy Research & Development Authority (NY Green Bank), providing for a secured term loan facility in the amount of $45.0 million (Term Loan Facility). The Term Loan Facility was initially entered in December 2016 and provided for a term loan of $25 million. The Term Loan Facility was amended in July 2017 to provide for an additional $20.0 million term loan, increasing the size of the total commitment to $45.0 million. The new Term Loan Facility will be repaid primarily as the Company’s various restricted cash reserves are released over the term of the facility. In June 2018, the timing and amount of the remaining principal payments were modified. On December 31, 2018, the outstanding principal balance under the Term Loan Facility was $17.2 million. Advances under the Term Loan Facility bear interest at a rate equal to the sum of the LIBOR rate for the applicable interest period, plus applicable margin of 9.5%. The interest rate at December 31, 2018 was approximately 11.9%. The term of the loan is three years, with a maturity date of December 23, 2019. As of December 31, 2018, the remaining balance of $17.2 million is scheduled to be repaid during the year ending December 31, 2019. These payments will be funded in part by restricted cash released, as described in Note 18, Commitments and Contingencies. Interest and a varying portion of the principal amount is payable on a quarterly basis and the entire then outstanding principal balance of the Term Loan Facility, together with all accrued and unpaid interest, is due and payable on the maturity date. On the maturity date, the Company may also be required to pay additional fees of up to $1.8 million if the Company is unable to meet certain goals related to the deployment of fuel cell systems in the State of New York and increasing the Company’s number of full-time employees in the State of New York. The Company is currently on track to meet those goals. The Term Loan Facility is secured by substantially all of the Company’s and the guarantor subsidiaries’ assets, including, among other assets, all intellectual property, all securities in domestic subsidiaries and 65% of the securities in foreign subsidiaries, subject to certain exceptions and exclusions. The Term Loan Facility contains covenants, including, among others, (i) the provision of annual and quarterly financial statements, management rights and insurance policies and (ii) restrictions on incurring debt, granting liens, making acquisitions, making loans, paying dividends, dissolving, and entering into leases and asset sales. The Term Loan Facility also provides for events of default, including, among others, payment, bankruptcy, covenant, representation and warranty, change of control, judgment and material adverse effect defaults at the discretion of the lender. The Term Loan Facility provides that if there is an event of default due to the Company’s insolvency or if the Company fails to perform, in any material respect, the servicing requirements for fuel cell systems under certain customer agreements, which failure would entitle the customer to terminate such customer agreement, replace the Company or withhold the payment of any material amount to the Company under such customer agreement, then the NY Green Bank has the right to cause a wholly owned subsidiary of the Company to replace the Company in performing the maintenance services under such customer agreement.

Convertible Senior Notes

Convertible Senior Notes	12 Months Ended
	Dec. 31, 2018
Convertible Senior Notes
Convertible Senior Notes	10. Convertible Senior Notes In March 2018, the Company issued $100.0 million in aggregate principal amount of 5.5% Convertible Senior Notes due on March 15, 2023, in a private placement to qualified institutional buyers pursuant to Rule 144A under the Securities Act of 1933, as amended. There are no required principal payments prior to maturity of the Convertible Senior Notes. The total net proceeds from the Convertible Senior Notes were as follows: Amount (in thousands) Principal amount $ 100,000 Less initial purchasers' discount (3,250) Less cost of related capped call and common stock forward (43,500) Less other issuance costs (894) Net proceeds $ 52,356 The Convertible Senior Notes bear interest at 5.5%, payable semi-annually in cash on March 15 and September 15 of each year. The Convertible Senior Notes will mature on March 15, 2023, unless earlier converted or repurchased in accordance with their terms. The Convertible Senior Notes are unsecured and do not contain any financial covenants or any restrictions on the payment of dividends, or the issuance or repurchase of common stock by the Company. Each $1,000 of principal of the Convertible Senior Notes will initially be convertible into 436.3002 shares of the Company’s common stock, which is equivalent to an initial conversion price of approximately $2.29 per share, subject to adjustment upon the occurrence of specified events. Holders of these Convertible Senior Notes may convert their Convertible Senior Notes at their option at any time prior to the close of the last business day immediately preceding September 15, 2022, only under the following circumstances: 1) during any calendar quarter (and only during such calendar quarter), if the last reported sale price of the Company’s common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; 2) during the five business day period after any five consecutive trading day period (the “measurement period”) in which the trading price (as defined in the indenture governing the Convertible Senior Notes) per $1,000 principal amount of notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price of the Company’s common stock and the conversion rate for the notes on each such trading day; 3) if the Company calls any or all of the Convertible Senior Notes for redemption, at any time prior to the close of business on the second scheduled trading day immediately preceding the redemption date; or 4) upon the occurrence of certain specified corporate events, such as a beneficial owner acquiring more than 50% of the total voting power of the Company’s common stock, recapitalization of the Company, dissolution or liquidation of the Company, or the Company’s common stock ceases to be listed on an active market exchange. On or after September 15, 2022, holders may convert all or any portion of their Convertible Senior Notes at any time prior to the close of business on the second scheduled trading day immediately preceding the maturity date regardless of the foregoing conditions. Upon conversion of the Convertible Senior Notes, the Company will pay or deliver, as the case may be, cash, shares of the Company’s common stock or a combination of cash and shares of the Company’s common stock, at the Company’s election. While the Company plans to settle the principal amount of the Convertible Senior Notes in cash subject to available funding at time of settlement, we currently use the if-converted method for calculating any potential dilutive effect of the conversion option on diluted net income per share, subject to meeting the criteria for using the treasury stock method in future periods. The conversion rate will be subject to adjustment in some events but will not be adjusted for any accrued or unpaid interest. Holders who convert their Convertible Senior Notes in connection with certain corporate events that constitute a “make-whole fundamental change” per the indenture governing the Convertible Senior Notes or in connection with a redemption will be, under certain circumstances, entitled to an increase in the conversion rate. In addition, if the Company undergoes a fundamental change prior to the maturity date, holders may require the Company to repurchase for cash all or a portion of its Convertible Senior Notes at a repurchase price equal to 100% of the principal amount of the repurchased Convertible Senior Notes, plus accrued and unpaid interest. The Company may not redeem the Convertible Senior Notes prior to March 20, 2021. The Company may redeem for cash all or any portion of the Convertible Senior Notes, at the Company’s option, on or after March 20, 2021 if the last reported sale price of the Company’s common stock has been at least 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including at least one of the three trading days immediately preceding the date on which the Company provides notice of redemption, during any 30 consecutive trading day period ending on, and including, the trading day immediately preceding the date on which the Company provides notice of redemption at a redemption price equal to 100% of the principal amount of the Convertible Senior Notes to be redeemed, plus accrued and unpaid interest to, but excluding, the redemption date. In accounting for the issuance of the Convertible Senior Notes, the Company separated the Convertible Senior Notes into liability and equity components. The initial carrying amount of the liability component of approximately $58.2 million, net of costs incurred, was calculated by measuring the fair value of a similar liability that does not have an associated convertible feature. The carrying amount of the equity component of approximately $37.7 million, net of costs incurred, representing the conversion option, was determined by deducting the fair value of the liability component from the par value of the Convertible Senior Notes. The difference between the principal amount of the Convertible Senior Notes and the liability component (the “debt discount”) is amortized to interest expense using the effective interest method over the term of the Convertible Senior Notes. The effective interest rate is approximately 16.0%. The equity component of the Convertible Senior Notes is included in additional paid-in capital in the consolidated balance sheet and is not remeasured as long as it continues to meet the conditions for equity classification. We incurred transaction costs related to the issuance of the Convertible Senior Notes of approximately $4.1 million, consisting of initial purchasers' discount of approximately $3.2 million and other issuance costs of $0.9 million. In accounting for the transaction costs, we allocated the total amount incurred to the liability and equity components using the same proportions as the proceeds from the Convertible Senior Notes. Transaction costs attributable to the liability component were approximately $2.4 million, were recorded as debt issuance cost (presented as contra debt in the consolidated balance sheet) and are being amortized to interest expense over the term of the Convertible Senior Notes. The transaction costs attributable to the equity component were approximately $1.7 million and were netted with the equity component in stockholders’ equity. The Convertible Senior Notes consist of the following at December 31, 2018 (in thousands): Principal amounts: Principal $ 100,000 Unamortized debt discount (1) (34,706) Unamortized debt issuance costs (1) (2,047) Net carrying amount $ 63,247 Carrying amount of the equity component (2) $ 37,702 1) Included in the consolidated balance sheet within Convertible senior notes, net and amortized over the remaining life of the Convertible senior notes using the effective interest rate method. 2) Included in the consolidated balance sheet within additional paid-in capital, net of $1.7 million in equity issuance costs and associated income tax benefit of $9.2 million. As of December 31, 2018, the remaining life of the Convertible Senior Notes is approximately 51 months. Based on the closing price of the Company’s common stock of $1.24 on December 31, 2018, the if-converted value of the Convertible Senior Notes was less than the principal amount. Capped Call In conjunction with the issuance of the Convertible Senior Notes, the Company entered into capped call options (Capped Call) on the Company’s common stock with certain counterparties at a price of $16.0 million. The net cost incurred in connection with the Capped Call has been recorded as a reduction to additional paid-in capital in the consolidated balance sheet. The Capped Call is generally expected to reduce or offset the potential dilution to the Company’s common stock upon any conversion of the Convertible Senior Notes and/or offset any cash payments the Company is required to make in excess of the principal amount of converted Convertible Senior Notes, as the case may be, with such reduction and/or offset subject to a cap based on the cap price. The cap price of the Capped Call transactions will initially be $3.82 per share, which represents a premium of 100% over the last reported sale price of the Company’s common stock of $1.91 per share on the date of the transaction, and is subject to certain adjustments under the terms of the Capped Call. The Capped Call becomes exercisable if the conversion option is exercised. By entering into the Capped Call, the Company expects to reduce the potential dilution to its common stock (or, in the event the conversion is settled in cash, to provide a source of cash to settle a portion of its cash payment obligation) in the event that at the time of conversion its stock price exceeds the conversion price under the Convertible Senior Notes. Common Stock Forward In connection with the sale of the Convertible Senior Notes, the Company also entered into a forward stock purchase transaction (Common Stock Forward), pursuant to which the Company agreed to purchase 14,397,906 shares of its common stock for settlement on or about March 15, 2023. The number of shares of common stock that the Company will ultimately repurchase under the Common Stock Forward is subject to customary anti-dilution adjustments. The Common Stock Forward is subject to early settlement or settlement with alternative consideration in the event of certain corporate transactions. The net cost incurred in connection with the Common Stock Forward of $27.5 million has been recorded as an increase in treasury stock in the consolidated balance sheet. The related shares were accounted for as a repurchase of common stock. The fair values of the Capped Call and Common Stock Forward are not remeasured each reporting period.

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2018
Stockholders? Equity
Stockholders' Equity	11. Stockholders’ Equity Preferred Stock The Company has authorized 5.0 million shares of preferred stock, par value $0.01 per share. The Company’s certificate of incorporation provides that shares of preferred stock may be issued from time to time in one or more series. The Company’s Board of Directors is authorized to fix the voting rights, if any, designations, powers, preferences, qualifications, limitations and restrictions thereof, applicable to the shares of each series. The Company has authorized Series A Junior Participating Cumulative Preferred Stock, par value $.01 per share. As of December 31, 2018 and 2017, there were no shares of Series A Junior Participating Cumulative Preferred Stock issued and outstanding. See Note 12, Redeemable Preferred Stock, for a description of the Company’s issued and outstanding Series C and E redeemable preferred stock. Common Stock and Warrants The Company has one class of common stock, par value $0.01 per share. Each share of the Company’s common stock is entitled to one vote on all matters submitted to stockholders. There were 219,157,998 and 228,486,366 shares of common stock outstanding as of December 31, 2018 and 2017, respectively. On December 22, 2016, the Company issued warrants to purchase 10,501,500 shares of common stock in connection with offerings of common stock and Series D Redeemable Preferred Stock at an exercise price of $1.50 per share. On April 12, 2017, the Company and Tech Opportunities LLC (“Tech Opps”) entered into an agreement, pursuant to which Tech Opps exercised in full its warrants to purchase an aggregate of 10,501,500 shares of common stock. The net proceeds received by the Company pursuant to the exercise of the existing warrants was $15.1 million and the Company issued to Tech Opps warrants to acquire up to 5,250,750 shares of common stock at an exercise price of $2.69 per share. The warrants were exercisable as of October 12, 2017 and will expire on October 12, 2019. The warrants are subject to anti-dilution provisions in the event of issuance of additional shares of common stock and certain other conditions, as further described in the warrant agreement. During April 2017, warrants issued in January 2014 as part of an underwritten public offering with Heights Capital Management Inc., were exercised in full to purchase an aggregate of 4,000,000 shares of the Company’s common stock, at an exercise price of $0.65 per share. The aggregate cash exercise price paid to the Company pursuant to the exercise of the warrants was $2.6 million. Pursuant to the exercised of the above warrants in 2017, additional paid-in capital was increased and warrant liability reduced by $27.1 million. During 2013, the Company completed a series of underwritten public offerings. One of the underwritten public offerings included accompanying warrants to purchase common stock. As of December 31, 2017 and 2016, 100 warrants with an exercise price of $0.15 per share, remained outstanding. During February 2018, the remaining 100 warrants were exercised. During 2017, additional warrants to purchase up to 110,573,392 shares of common stock were issued in connection with transaction agreements with Amazon and Walmart, as discussed in Note 13, Warrant Transaction Agreements. At December 31, 2018 and 2017, in connection with these agreements, warrants to acquire 26,188,434 and 18,913,869 shares of common stock, respectively, have vested and are therefore exercisable. These warrants are measured at fair value and are classified as equity instruments on the consolidated balance sheet. At Market Issuance Sales Agreement On April 3, 2017, the Company entered into an At Market Issuance Sales Agreement (the “Sales Agreement”) with FBR Capital Markets & Co., as sales agent (“FBR”), pursuant to which the Company may offer and sell, from time to time through FBR, shares of common stock par value $0.01 per share having an aggregate offering price of up to $75.0 million. Under the Sales Agreement, in no event shall the Company issue or sell through FBR such a number of shares that exceeds the number of shares or dollar amount of common stock registered. During 2018, the Company issued 3,804,654 shares of common stock and raised net proceeds of 7.0 million, after accruals, underwriting discounts and commissions and other fees and expenses, pursuant to the Sales Agreement. The Company has raised $30.2 million to date during the term of the Sales Agreement.

Redeemable Preferred Stock

Redeemable Preferred Stock	12 Months Ended
	Dec. 31, 2018
Redeemable Preferred Stock
Redeemable Preferred Stock	12. Redeemable Preferred Stock In November 2018, the Company completed an offering of an aggregate of 35,000 shares of the Company’s Series E Redeemable Preferred Stock, par value $0.01 per share (Series E Stock), resulting in aggregate net proceeds of approximately $30.9 million, after deducting underwriting discounts and commissions and expenses payable by the Company. The Company is required to redeem the Series E Stock in thirteen monthly installments in the amount of $2.7 million each from May 2019 through May 2020. Each share of Series E Stock was issued with an initial stated value of $1,000 per share. The Company is required to elect, on a monthly basis, whether it will redeem or convert the installment. Should the Company elect to redeem, the shares are valued at the stated value. Should the Company elect to convert, the holder of the shares will receive common stock, with a conversion price discounted by 15% from the then current market value. The holders of the shares may elect to convert all or any whole amount of shares, at any time at a conversion price of $2.31 per share. Conversion prices are discounted upon a change in control, certain triggering events, or failure to make a redemption payment. Except for our Series C Redeemable Convertible Preferred Stock (Series C Stock), which shall rank senior to the Series E Stock as to dividends, distributions and payments upon liquidation, dissolution and winding up, all shares of the Company’s capital stock, including common stock, rank junior in rank to the Series E Stock with respect to dividends, distributions and payments upon liquidation, dissolution and winding up. Holders of the Series E Stock are not entitled to receive dividends except in connection with certain purchase rights and other corporate events, as described in the certificate of designations, or in connection with certain distributions of assets, as described in the certificate of designations, or as, when and if declared by the Company’s Board of Directors acting in its sole and absolute discretion. Holders of the Series E Stock have no voting rights, except on matters required by law or under the certificate of designations to be submitted to a class vote of the Series E Stock. In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company or other deemed liquidation event, the holders of the Series E Stock are entitled to receive, after any amount that is required to be paid to the Series C Stock and before any amount is paid to the holders of any of capital stock ranking junior to the Series E Stock, an amount per share equal to the greater of (i) 125% of the sum of the stated value plus any declared and unpaid dividends and late charges as provided in the certificate of designations, on the date of such payment and (ii) the amount per share such holder would receive if such holder converted such Series E Stock into common stock immediately prior to the date of such payment. The Company had 2,620 shares of Series C Stock outstanding at December 31, 2018 and 2017. The holder of the Series C Stock is entitled to receive dividends at a rate of 8% per annum, based on the original issue price per share of $248.794, payable in equal quarterly installments in cash or in shares of common stock, at the Company’s option. During the years ended December 31, 2018 and 2017, respectively, all dividends have been paid in shares of common stock. Each share of Series C Stock is convertible into shares of common stock with the number of shares of common stock issuable upon conversion determined by dividing the original issue price per share of $248.794 by the conversion price in effect at the time the shares are converted. The conversion price of the Series C Stock as of December 31, 2018 and December 31, 2017 was $0.2343. The Series C Stock votes together with the common stock on an as-converted basis on all matters. In the event of any voluntary or involuntary liquidation, dissolution or winding up of the Company, or other deemed liquidation event, the holder of the Series C Stock will be entitled to be paid an amount per share equal to the greater of (i) the original issue price, plus any accrued but unpaid dividends or (ii) the amount per share that would have been payable had all shares of the Series C Stock been converted to shares of common stock immediately prior to such liquidation event. The Series C Stock is redeemable at the election of the holder of the Series C Stock or the Company.

Warrant Transaction Agreements

Warrant Transaction Agreements	12 Months Ended
	Dec. 31, 2018
WarrantsAndRightsWalmartStoresIncTransactionAgreementNoteDisclosureAbstract
Warrant Transaction Agreements	13. Warrant Transaction Agreements Amazon Transaction Agreement On April 4, 2017, the Company and Amazon entered into a Transaction Agreement (the “Amazon Transaction Agreement”), pursuant to which the Company agreed to issue to Amazon.com NV Investment Holdings LLC, a wholly owned subsidiary of Amazon, warrants to acquire up to 55,286,696 shares of the Company’s common stock (the “Amazon Warrant Shares”), subject to certain vesting events described below. The Company and Amazon entered into the Amazon Transaction Agreement in connection with existing commercial agreements between the Company and Amazon with respect to the deployment of the Company’s GenKey fuel cell technology at Amazon distribution centers. The existing commercial agreements contemplate, but do not guarantee, future purchase orders for the Company’s fuel cell technology. Additionally, Amazon and the Company will begin working together on technology collaboration, exploring the expansion of applications for the Company’s line of ProGen fuel cell engines. The vesting of the Amazon Warrant Shares is linked to payments made by Amazon or its affiliates (directly or indirectly through third parties) pursuant to the existing commercial agreements. The majority of the Amazon Warrant Shares will vest based on Amazon’s payment of up to $600.0 million to the Company in connection with Amazon’s purchase of goods and services from the Company. The first tranche of 5,819,652 Amazon Warrant Shares vested upon the execution of the Amazon Transaction Agreement. Accordingly, $6.7 million, the fair value of the first tranche of Amazon Warrant Shares, was recognized as selling, general and administrative expense on the consolidated statements of operations during 2017. The second tranche of 29,098,260 Amazon Warrant Shares will vest in four installments of 7,274,565 Amazon Warrant Shares each time Amazon or its affiliates, directly or indirectly through third parties, make an aggregate of $50.0 million in payments for goods and services to the Company, up to payments totaling $200.0 million in the aggregate. The exercise price for the first and second tranches of Amazon Warrant Shares will be $1.1893 per share. After Amazon has made payments to the Company totaling $200.0 million, the third tranche of 20,368,784 Amazon Warrant Shares will vest in eight installments of 2,546,098 Amazon Warrant Shares each time Amazon or its affiliates, directly or indirectly through third parties, make an aggregate of $50.0 million in payments for goods and services to the Company, up to payments totaling $400.0 million in the aggregate. The exercise price of the third tranche of Amazon Warrant Shares will be an amount per share equal to ninety percent (90%) of the 30-day volume weighted average share price of the common stock as of the final vesting date of the second tranche of Amazon Warrant Shares. The Amazon Warrant Shares are exercisable through April 4, 2027. The Amazon Warrant Shares provide for net share settlement that, if elected by the holders, will reduce the number of shares issued upon exercise to reflect net settlement of the exercise price. The Amazon Warrant Shares provide for certain adjustments that may be made to the exercise price and the number of shares of common stock issuable upon exercise due to customary anti-dilution provisions based on future events. These warrants are classified as equity instruments. Because the Amazon Warrant Shares contain performance criteria (i.e. aggregate purchase levels), which Amazon must achieve for the Amazon Warrant Shares to vest, as detailed above, the final measurement date for the Amazon Warrant Shares is the date on which the Amazon Warrant Shares vest. Prior to the final measurement, when achievement of the performance criteria has been deemed probable, the estimated fair value of Amazon Warrant Shares is being recorded as a reduction to revenue and an addition to additional paid-in capital based on the projected number of Amazon Warrant Shares expected to vest, the proportion of purchases by Amazon and its affiliates within the period relative to the aggregate purchase levels required for the Amazon Warrant Shares to vest and the then-current fair value of the related Amazon Warrant Shares. To the extent that projections change in the future as to the number of Amazon Warrant Shares that will vest, as well as changes in the fair value of the Amazon Warrant Shares, a cumulative catch-up adjustment will be recorded in the period in which the estimates change. At December 31, 2018 and 2017, 20,368,782 and 13,094,217 of the Amazon Warrant Shares had vested, respectively. The amount of provision for common stock warrants recorded as a reduction of revenue for the Amazon Warrant during the years ended December 31, 2018 and 2017 was $9.8 million and $17.3 million, respectively. Walmart Transaction Agreement On July 20, 2017, the Company and Walmart entered into a Transaction Agreement (the “Walmart Transaction Agreement”), pursuant to which the Company agreed to issue to Walmart a warrant to acquire up to 55,286,696 shares of the Company’s common stock, subject to certain vesting events (the “Walmart Warrant Shares”). The Company and Walmart entered into the Walmart Transaction Agreement in connection with existing commercial agreements between the Company and Walmart with respect to the deployment of the Company’s GenKey fuel cell technology across various Walmart distribution centers. The existing commercial agreements contemplate, but do not guarantee, future purchase orders for the Company’s fuel cell technology. The vesting of the warrant shares, is linked to payments made by Walmart or its affiliates (directly or indirectly through third parties) pursuant to transactions entered into after January 1, 2017 under existing commercial agreements. The majority of the Walmart Warrant Shares will vest based on Walmart’s payment of up to $600.0 million to the Company in connection with Walmart’s purchase of goods and services from the Company. The first tranche of 5,819,652 Walmart Warrant Shares vested upon the execution of the Walmart Transaction Agreement. Accordingly, $10.9 million, the fair value of the first tranche of Walmart Warrant Shares, was recorded as a provision for common stock warrants and presented as a reduction to revenue on the consolidated statements of operations during 2017. The second tranche of 29,098,260 Walmart Warrant Shares will vest in four installments of 7,274,565 Walmart Warrant Shares each time Walmart or its affiliates, directly or indirectly through third parties, make an aggregate of $50.0 million in payments for goods and services to the Company, up to payments totaling $200.0 million in the aggregate. The exercise price for the first and second tranches of Walmart Warrant Shares will be $2.1231 per share. After Walmart has made payments to the Company totaling $200.0 million, the third tranche of 20,368,784 Walmart Warrant Shares will vest in eight installments of 2,546,098 Walmart Warrant Shares each time Walmart or its affiliates, directly or indirectly through third parties, make an aggregate of $50.0 million in payments for goods and services to the Company, up to payments totaling $400.0 million in the aggregate. The exercise price of the third tranche of Walmart Warrant Shares will be an amount per share equal to ninety percent (90%) of the 30-day volume weighted average share price of the common stock as of the final vesting date of the second tranche of Walmart Warrant Shares, provided that, with limited exceptions, the exercise price for the third tranche will be no lower than $1.1893. The Walmart Warrant Shares are exercisable through July 20, 2027. The Walmart Warrant Shares provide for net share settlement that, if elected by the holders, will reduce the number of shares issued upon exercise to reflect net settlement of the exercise price. The Walmart Warrant Shares provide for certain adjustments that may be made to the exercise price and the number of shares of common stock issuable upon exercise due to customary anti-dilution provisions based on future events. These warrants are classified as equity instruments. Because the Walmart Warrant Shares contain performance criteria (i.e. aggregate purchase levels), which Walmart must achieve for the Walmart Warrant Shares to vest, as detailed above, the final measurement date for the Walmart Warrant Shares is the date on which the Walmart Warrant Shares vest. Prior to the final measurement, when achievement of the performance criteria has been deemed probable, the estimated fair value of Walmart Warrant Shares is being recorded as a reduction to revenue and an addition to additional paid-in capital based on the projected number of Walmart Warrant Shares expected to vest, the proportion of purchases by Walmart and its affiliates within the period relative to the aggregate purchase levels required for the Walmart Warrant Shares to vest and the then-current fair value of the related Walmart Warrant Shares. To the extent that projections change in the future as to the number of Walmart Warrant Shares that will vest, as well as changes in the fair value of the Walmart Warrant Shares, a cumulative catch-up adjustment will be recorded in the period in which the estimates change. At December 31, 2018 and 2017, 5,819,652 of the Walmart Warrant Shares had vested. The amount of provision for common stock warrants recorded as a reduction of revenue for the Walmart Warrant during the years ended December 31, 2018 and 2017 was $0.4 million and $12.4 million, respectively.

Revenue

Revenue	12 Months Ended
	Dec. 31, 2018
Revenue
Revenue	14. Revenue Disaggregation of revenue In the following table, revenue is disaggregated by major product line and timing of revenue recognition, net of the provision for common stock warrants, where applicable (in thousands): Major products/services lines Year ended December 31, 2018 2017 2016 Sales of fuel cell systems $ 75,146 $ 49,179 $ 30,008 Sale of hydrogen installations and other infrastructure 32,146 13,452 9,977 Services performed on fuel cell systems and related infrastructure 22,002 16,202 17,347 Power Purchase Agreements 22,869 12,869 13,687 Fuel delivered to customers 22,469 8,167 10,916 Other — 284 884 Net revenue $ 174,632 $ 100,153 $ 82,819 Contract balances The following table provides information about receivables, contract assets and contract liabilities from contracts with customers (in thousands): 2018 2017 Accounts receivable $ 37,347 $ 24,179 Contract assets 788 1,141 Contract liabilities 40,476 35,171 Contract assets relate to contracts for which revenue is recognized on a straight-line basis, however billings escalate over the life of a contract. The contract liabilities relate to the advance consideration received from customers for services that will be recognized over time (primarily fuel cell and related infrastructure services). These amounts are included within deferred revenue on the consolidated balance sheet. Significant changes in the contract assets and the contract liabilities balances during the period are as follows (in thousands): Contract assets Year ended December 31, 2018 Amortization of contract assets recognized at the beginning of the period $ (353) Revenue recognized and not billed as of the end of the period — Net change in contract assets $ (353) Contract liabilities Year ended December 31, 2018 Revenue recognized that was included in the contract liability balance as of the beginning of the period $ (10,783) Increases due to cash received, net of amounts recognized as revenue during the period 16,088 Net change in contract liabilities $ 5,305 Estimated future revenue The following table includes estimated revenue expected to be recognized in the future (sales of fuel cell systems and hydrogen installations are expected to be recognized as revenue within one year; sales of services and PPAs are expected to be recognized as revenue over five to seven years) related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period, excluding provision for common stock warrants as it is not readily estimable as it depends on the valuation of the common stock warrants when revenue is recognized (in thousands): Year ended December 31, 2018 2017 Sales of fuel cell systems $ 17,318 $ 26,298 Sale of hydrogen installations and other infrastructure 9,141 15,512 Services performed on fuel cell systems and related infrastructure 73,381 77,453 Power Purchase Agreements 111,533 130,042 Total estimated future revenue $ 211,373 $ 249,305 Contract costs Contract costs consists of capitalized commission fees and other expenses related to obtaining or fulfilling a contract. Capitalized contract costs at December 31, 2018 and 2017 were $0.2 million and zero, respectively. Expense related to the amortization of capitalized contract costs was not significant for the year ended December 31, 2018.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2018
Employee Benefit Plans
Employee Benefit Plans	15. Employee Benefit Plans 2011 Stock Option and Incentive Plan On May 12, 2011, the Company’s stockholders approved the 2011 Stock Option and Incentive Plan (the 2011 Plan). The 2011 Plan provided for the issuance of up to a maximum number of shares of common stock equal to the sum of (i) 1,000,000, plus (ii) the number of shares of common stock underlying any grants pursuant to the 2011 Plan or the Plug Power Inc. 1999 Stock Option and Incentive Plan that are forfeited, canceled, repurchased or are terminated (other than by exercise). The shares may be issued pursuant to stock options, stock appreciation rights, restricted stock awards and certain other equity-based awards granted to employees, directors and consultants of the Company. No grants may be made under the 2011 Plan after May 12, 2021. Through various amendments to the 2011 Plan approved by the Company’s stockholders, the number of shares of the Company’s common stock authorized for issuance under the 2011 Plan has been increased to 30.0 million. For the years ended December 31, 2018, 2017, and 2016, the Company recorded expense of approximately $7.4 million, $9.0 million, and $9.0 million, respectively, in connection with the 2011 Stock Option and Incentive Plan. At December 31, 2018, there were outstanding options to purchase approximately 22.0 million shares of Common Stock and 2.3 million shares available for future awards under the 2011 Plan, including adjustments for other types of share-based awards. Options for employees issued under this plan generally vest in equal annual installments over three years and expire ten years after issuance. Options granted to members of the Board generally vest one year after issuance. To date, options granted under the 2011 Plan have vesting provisions ranging from one to three years in duration and expire ten years after issuance. Compensation cost associated with employee stock options represented approximately $6.4 million, $8.6 million, and $9.0 million of the total share-based payment expense recorded for the years ended December 31, 2018, 2017, and 2016, respectively. The Company estimates the fair value of stock options using a Black-Scholes valuation model, and the resulting fair value is recorded as compensation cost on a straight-line basis over the option vesting period. Key inputs and assumptions used to estimate the fair value of stock options include the grant price of the award, the expected option term, volatility of the Company’s stock, an appropriate risk-free rate, and the Company’s dividend yield. Estimates of fair value are not intended to predict actual future events or the value ultimately realized by employees who receive equity awards, and subsequent events are not indicative of the reasonableness of the original estimates of fair value made by the Company. The assumptions made for purposes of estimating fair value under the Black-Scholes model for the 2,679,667, 5,485,863 and 3,702,500 options granted during the years ended December 31, 2018, 2017 and 2016, respectively, were as follows: 2018 2017 2016 Expected term of options (years) 6 6 6 Risk free interest rate 2.81% - 2.88% 1.89% - 2.16% 1.27% - 1.69% Volatility 98.31%-98.89% 99.24%-102.16% 103.87% - 104.88% There was no expected dividend yield for the employee stock options granted. The Company’s estimate of an expected option term was calculated in accordance with the simplified method for calculating the expected term assumption. The estimated stock price volatility was derived from the Company’s actual historic stock prices over the past six years, which represents the Company’s best estimate of expected volatility. A summary of stock option activity for the year December 31, 2018 is as follows (in thousands except share amounts): Weighted Weighted Average Average Remaining Aggregate Exercise Contractual Intrinsic Shares Price Terms Value Options outstanding at December 31, 2017 19,872,029 $ 2.66 7.7 $ — Granted 2,679,667 1.96 — — Exercised (276,668) 0.63 — — Forfeited (246,378) 2.91 — — Expired (71,500) 29.51 — — Options outstanding at December 31, 2018 21,957,150 $ 2.51 7.1 $ 1,432 Options exercisable at December 31, 2018 14,964,985 2.74 6.3 1,431 Options unvested at December 31, 2018 6,992,165 $ 2.01 8.9 $ 1,467 The weighted average grant date fair value of options granted during the years ended December 31, 2018, 2017 and 2016 was $1.55, $1.67, and $1.39, respectively. As of December 31, 2018, there was approximately $8.8 million of unrecognized compensation cost related to stock option awards to be recognized over the next three years with all of this expected to vest. The total fair value of stock options that vested during the years ended December 31, 2018 and 2017 was approximately $7.1 million and $9.6 million, respectively. Restricted stock awards generally vest in equal installments over a period of one to three years. Restricted stock awards are valued based on the closing price of the Company’s common stock on the date of grant, and compensation cost is recorded on a straight-line basis over the share vesting period. The Company recorded expense associated with its restricted stock awards of approximately $966 thousand, $335 thousand, and $88 thousand, for the years ended December 31, 2018, 2017 and 2016, respectively. Additionally, for the years ended December 31, 2018, 2017 and 2016, there was $3.9 million, $208 thousand, and $43 thousand, respectively, of unrecognized compensation cost related to restricted stock awards to be recognized over the next three years. A summary of restricted stock activity for the year ended December 31, 2018 is as follows (in thousands except share amounts): Aggregate Intrinsic Shares Value Unvested restricted stock at December 31, 2017 234,744 $ — Granted 2,347,347 — Vested (234,744) — Unvested restricted stock at December 31, 2018 2,347,347 $ 2,911 401(k) Savings & Retirement Plan The Company offers a 401(k) Savings & Retirement Plan to eligible employees meeting certain age and service requirements. This plan permits participants to contribute 100% of their salary, up to the maximum allowable by the Internal Revenue Service regulations. Participants are immediately vested in their voluntary contributions plus actual earnings or less actual losses thereon. Participants are vested in the Company’s matching contribution based on years of service completed. Participants are fully vested upon completion of three years of service. During 2018, the Company began funding its matching contribution in a combination of cash and common stock. Accordingly, the Company has issued 633,827 shares of common stock to the Plug Power Inc. 401(k) Savings & Retirement Plan during 2018. The Company’s expense for this plan was approximately $1.8 million, $1.6 million, and $1.4 million for years ended December 31, 2018, 2017 and 2016, respectively. Non-Employee Director Compensation Each non-employee director is paid an annual retainer for their services, in the form of either cash or stock compensation. The Company granted 107,389, 148,077, and 105,479 shares of stock to non-employee directors as compensation for the years ended December 31, 2018, 2017, and 2016, respectively. All common stock issued is fully vested at the time of issuance and is valued at fair value on the date of issuance. The Company’s share-based compensation expense for this plan was approximately $261 thousand, $276 thousand, and $267 thousand for the years ended December 31, 2018, 2017, and 2016 respectively.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Measurements
Fair Value Measurements	16. Fair Value Measurements The following table summarizes the financial instruments measured at fair value on a recurring basis in the consolidated balance sheets (in thousands): Quoted Prices Significant Significant in Active Other Other Markets for Observable Unobservable Identical Items Inputs Inputs Total (Level 1) (Level 2) (Level 3) Balance at December 31, 2018 Common stock warrant liability $ 105 $ — $ — $ 105 Balance at December 31, 2017 Common stock warrant liability $ 4,391 $ — $ — $ 4,391 Derivative Liabilities The Company’s common stock warrant liability represents the only asset or liability classified financial instrument measured at fair value on a recurring basis in the consolidated balance sheets. The fair value measurement is determined by using Level 3 inputs due to the lack of active and observable markets that can be used to price identical assets. Level 3 inputs are unobservable inputs and should be used to determine fair value only when observable inputs are not available. Unobservable inputs should be developed based on the best information available in the circumstances, which might include internally generated data and assumptions being used to price the asset or liability. Fair value of the common stock warrant liability is based on the Black-Scholes pricing model which is based, in part, upon unobservable inputs for which there is little or no market data, requiring the Company to develop its own assumptions. The Company used the following assumptions for its liability-classified common stock warrants: Year ended December 31, 2018 December 31, 2017 Risk-free interest rate 1.64% - 2.66% 1.01% - 2.01% Volatility 18.40% - 81.69% 43.60% - 108.77% Expected average term 0.01 - 1.53 0.14 - 5.23 There was no expected dividend yield for the warrants granted. If factors change and different assumptions are used, the warrant liability and the change in estimated fair value could be materially different. Generally, as the market price of our common stock increases, the fair value of the warrants increase, and conversely, as the market price of our common stock decreases, the fair value of the warrants decrease. Also, a significant increase in the volatility of the market price of the Company’s common stock, in isolation, would result in significantly higher fair value measurements; and a significant decrease in volatility would result in significantly lower fair value measurements. The following table shows the activity in the common stock warrant liability (in thousands): 2018 2017 Beginning of period $ 4,391 $ 11,387 Change in fair value of common stock warrants (4,286) 15,188 Issuance of common stock warrants — 4,905 Exercise of common stock warrants — (27,089) End of period $ 105 $ 4,391 Equity Instruments The fair value measurement of the Company’s equity-classified common stock warrants further described in Note 13, Warrant Transaction Agreements, is determined by using Level 3 inputs due to the lack of active and observable markets that can be used to price identical instruments. Fair value of the equity-classified common stock warrants is based on the Monte Carlo pricing model which is based, in part, upon unobservable inputs for which there is little or no market data, requiring the Company to develop its own assumptions. The Company used the following assumptions for its equity-classified common stock warrants for the years ended December 31, 2018 and 2017: Year ended December 31, 2018 December 31, 2017 Risk-free interest rate 2.60% - 3.02% 2.30% - 2.47% Volatility 75.00% - 85.00% 85.00% - 90.00% Expected average term 8.26 - 9.30 9.26 - 10.00 The Monte Carlo pricing models used in the determination of the fair value of the equity-classified warrants also incorporate assumptions involving future revenues associated with Amazon and Walmart, and related timing. The following table represents the fair value per warrant on the execution date of the transaction agreements and as of December 31, 2018 and 2017: Amazon Warrant Shares Walmart Warrant Shares Issuance date - first tranche $ 1.15 $ 1.88 As of vesting date - second tranche, first installment 2.16 — As of vesting date - second tranche, second installment 1.54 — As of December 2018 - second tranche 0.98 0.92 As of December 2017 - second tranche 2.13 1.92

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Taxes
Income Taxes	17. Income Taxes The components of loss before income taxes and the income tax benefit for the years ended December 31, 2018, 2017 and 2016, by jurisdiction, are as follows (in thousands): 2018 2017 2016 U.S. Foreign Total U.S. Foreign Total U.S. Foreign Total Loss before income taxes $ (85,925) $ (1,407) $ (87,332) $ (125,871) $ (1,209) $ (127,080) $ (56,317) $ (1,562) $ (57,879) Income tax benefit 9,217 — 9,217 — — — — 392 392 Net loss attributable to the Company $ (76,708) $ (1,407) $ (78,115) $ (125,871) $ (1,209) $ (127,080) $ (56,317) $ (1,170) $ (57,487) The significant components of deferred income tax expense (benefit) for the years ended December 31, 2018, 2017 and 2016, by jurisdiction, are as follows (in thousands): 2018 2017 2016 U.S. Foreign Total U.S. Foreign Total U.S. Foreign Total Deferred tax (benefit) expense $ (10,182) $ 933 $ (9,249) $ 7,675 $ (531) $ 7,144 $ (6,420) $ (1,299) $ (7,719) Net operating loss carryforward generated (10,038) (665) (10,703) (19,117) (17) (19,134) (16,727) (2,827) (19,554) Rate change impact on net operating loss carryforwards — — — 23,609 — 23,609 — — — Valuation allowance increase (decrease) 11,003 (268) 10,735 (12,167) 548 (11,619) 23,147 4,126 27,273 Provision for income taxes $ (9,217) $ — $ (9,217) $ — $ — $ — $ — $ — $ — The Company’s effective income tax rate differed from the federal statutory rate as follows: 2018 2017 2016 U.S. Federal statutory tax rate (21.0) % (35.0) % (35.0) % Deferred state taxes (1.9) % (1.4) % (3.1) % Common stock warrant liability (1.0) % 4.2 % (2.6) % Provision to return and deferred tax asset adjustments — % 5.9 % (2.9) % Change in unrecognized tax benefits — % — % (0.7) % Change in U.S. Federal statutory tax rate — % 33.5 % — % Other, net 0.4 % 2.0 % (1.6) % Change in valuation allowance 12.9 % (9.2) % 45.2 % (10.6) % 0.0 % (0.7) % Deferred income taxes reflect the net tax effects of temporary differences between the carrying amounts of certain assets and liabilities for financial reporting and the amounts used for income tax purposes. Significant components of the Company’s deferred tax assets and liabilities as of December 31, 2018 and 2017 are as follows (in thousands): U.S. Foreign Total 2018 2017 2018 2017 2018 2017 Intangible assets $ — $ — $ 1,146 $ 1,698 $ 1,146 $ 1,698 Deferred revenue 9,304 8,083 — — 9,304 8,083 Interest expense 5,239 — — — 5,239 — Other reserves and accruals 592 850 — — 592 850 Tax credit carryforwards 1,865 1,378 1,200 1,304 3,065 2,682 Amortization of stock-based compensation 8,442 6,904 — — 8,442 6,904 Non-compensatory warrants 3,597 4,555 — — 3,597 4,555 Capitalized research & development expenditures 19,116 14,496 4,294 4,666 23,410 19,162 Net operating loss carryforwards 49,058 39,437 9,306 8,641 58,364 48,078 Total deferred tax asset 97,213 75,703 15,946 16,309 113,159 92,012 Valuation allowance (84,567) (73,564) (15,926) (16,194) (100,493) (89,758) Net deferred tax assets $ 12,646 $ 2,139 $ 20 $ 115 $ 12,666 $ 2,254 Intangible assets (37) (76) — — (37) (76) Convertible debt (9,217) — — — (9,217) — Other reserves and accruals — — (20) — (20) — Property, plant and equipment and right of use assets (3,392) (1,854) — (115) (3,392) (1,969) Section 382 recognized built in loss — (209) — — — (209) Deferred tax liability $ (12,646) $ (2,139) $ (20) $ (115) $ (12,666) $ (2,254) Net $ — $ — $ — $ — $ — $ — The Company has recorded a valuation allowance, as a result of uncertainties related to the realization of its net deferred tax asset, at December 31, 2018 and 2017 of approximately $100.4 million and $89.8 million, respectively. A reconciliation of the current year change in valuation allowance is as follows (in thousands): U.S. Foreign Total Increase in valuation allowance for current year increase in net operating losses $ 10,038 $ 464 $ 10,502 Increase (decrease) in valuation allowance for current year net increase (decrease) in deferred tax assets other than net operating losses 1,665 (281) 1,384 Decrease in valuation allowance as a result of foreign currency fluctuation — (451) (451) Decrease in valuation allowance due to change in tax rates (700) — (700) Net increase (decrease) in valuation allowance $ 11,003 $ (268) $ 10,735 The deferred tax assets have been offset by a full valuation allowance because it is more likely than not that the tax benefits of the net operating loss carryforwards and other deferred tax assets may not be realized due to cumulative losses. Under Internal Revenue Code (IRC) Section 382, the use of loss carryforwards may be limited if a change in ownership of a company occurs. If it is determined that due to transactions involving the Company’s shares owned by its 5 percent or greater shareholders a change of ownership has occurred under the provisions of IRC Section 382, the Company's federal and state net operating loss carryforwards could be subject to significant IRC Section 382 limitations. Based on studies of the changes in ownership of the Company, it has been determined that an IRC Section 382 ownership change occurred in 2013 that limited the amount of pre-change net operating losses that can be used in future years to $13.5 million. These net operating loss carryforwards will expire, if unused, at various dates from 2020 through 2033. Net operating losses of $195.4 million incurred after the most recent ownership change are not subject to IRC Section 382 and are available for use in future years. Accordingly, the Company's deferred tax assets include $208.8 million of U.S. net operating loss carryforwards. The net operating loss carryforwards available at December 31, 2018, include $42.2 million of net operating loss that was generated in 2018 that does not expire. The remainder, if unused, will expire at various dates from 2032 through 2037. The ownership change in 2013 also resulted in net unrealized built-in losses per IRS Notice 2003-65 which should result in recognized built in losses during the five year recognition period. These recognized built in losses will translate into unfavorable book to tax add backs in the Company’s 2018 U.S corporate income tax return of approximately $0.9 million that resulted in a gross deferred tax liability of zero at December 31, 2018. This gross deferred tax liability offsets existing gross deferred tax assets effectively reducing the valuation allowance. This has no impact on the Company’s current financial position, results of operations, or cash flows because of the full valuation allowance. Approximately $1.9 million of research credit carryforwards generated after the most recent IRC Section 382 ownership change are included in the Company's deferred tax assets. Due to limitations under IRC Section 382, research credit carryforwards existing prior to the most recent IRC Section 382 ownership change will not be used and are not reflected in the Company's gross deferred tax asset at December 31, 2018. The remaining credit carryforwards will expire during the periods 2033 through 2038. At December 31, 2018, the Company has unused Canadian net operating loss carryforwards of approximately $13.6 million. The net operating loss carryforwards if unused will expire at various dates from 2026 through 2034. At December 31, 2018, the Company has Scientific Research and Experimental Development (SR&ED) expenditures of $16.5 million available to offset future taxable income. These (SR&ED) expenditures have no expiry date. At December 31, 2017, the Company has Canadian ITC credit carryforwards of $1.2 million available to offset future income tax. These credit carryforwards if unused will expire at various dates from 2022 through 2028. At December 31, 2018, the Company has unused French net operating loss carryforwards of approximately $17.6 million. The net operating loss may carryforward indefinitely or until the Company changes its activity. As of December 31, 2018, the Company has no un-repatriated foreign earnings. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): 2018 2017 2016 Unrecognized tax benefits balance at beginning of year $ — $ — $ 437 Reductions for tax positions of prior years — — (469) Currency translation — — 32 Unrecognized tax benefits balance at end of year $ — $ — $ — The Company files income tax returns in the U.S. federal jurisdiction and various state and foreign jurisdictions. In the normal course of business, the Company is subject to examination by taxing authorities. Open tax years in the US range from 2014 to 2017. Open tax years in the foreign jurisdictions range from 2009 to 2018. However, upon examination in subsequent years, if net operating losses carryforwards and tax credit carryforwards are utilized, the US and foreign jurisdictions can reduce net operating loss carryforwards and tax credit carryforwards utilized in the year being examined if they do not agree with the carryforward amount. As of December 31, 2018, the Company was not under audit in the U.S. or non-U.S. taxing jurisdictions. The Tax Cuts and Jobs Act (the "Act") was enacted on December 22, 2017. The Act makes broad and complex changes to the U.S. tax code including a reduction of the U.S. federal corporate tax rate from 35 percent to 21 percent effective for the 2018 tax year. Accordingly, federal deferred tax assets were adjusted in 2017 by $42.5 million to reflect the reduction in tax rates. Also, in 2017 the valuation allowance was reduced by $42.5 million resulting in no change to the net deferred tax asset. The deferred tax asset adjustments reduced the tax benefit of the current year losses by 33.5% as shown in the effective tax rate schedule. The valuation allowance rate impact includes an offsetting 33.5% for the tax rate reduction resulting in no change to the provision for income taxes.

Commitment and Contingencies

Commitment and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies
Commitments and Contingencies	18. Commitments and Contingencies Operating Leases – as Lessor As of December 31, 2018, the Company has several noncancelable operating leases (as lessor), primarily associated with assets deployed at customer sites. These leases expire over the next one to seven years. Leases contain termination clauses with associated penalties, the amount of which cause the likelihood of cancelation to be remote. Future minimum lease payments under noncancelable operating leases (with initial or remaining lease terms in excess of one year) as of December 31, 2018 are (in thousands): 2019 $ 27,234 2020 25,503 2021 20,781 2022 12,044 2023 8,146 2024 and thereafter 7,946 Total future minimum lease payments $ 101,654 Operating Leases – as Lessee As of December 31 2018, the Company has several noncancelable operating leases (as lessee), primarily associated with sale/leaseback transactions that are partially secured by restricted cash (see also Note 1, Nature of Operations) as summarized below. These leases expire over the next one to seven years. Minimum rent payments under operating leases are recognized on a straight‑line basis over the term of the lease. Leases contain termination clauses with associated penalties, the amount of which cause the likelihood of cancelation to be remote. Future minimum lease payments under noncancelable operating leases (with initial or remaining lease terms in excess of one year) as of December 31, 2018 are (in thousands): 2019 $ 22,582 2020 21,255 2021 16,512 2022 11,060 2023 10,325 2024 and thereafter 13,352 Total future minimum lease payments 95,086 Less imputed lease interest (24,171) Total lease liabilities $ 70,915 The Company has received cash for future services to be performed associated with certain sale/leaseback transactions and recorded the balance as a finance obligation. The outstanding balance of this obligation at December 31, 2018 is $37.0 million, $5.7 million and $31.3 million of which is classified as short-term and long-term, respectively, on the accompanying consolidated balance sheet. The outstanding balance of this obligation at December 31, 2017 was $10.4 million, $2.5 million and $7.9 million of which is classified as short-term and long-term, respectively, on the accompanying consolidated balance sheet. The amount is amortized using the effective interest method. The fair value of this finance obligation approximates the carrying value as of December 31, 2018. Rental expense for all operating leases was $15.8 million and $13.4 million for the years ended December 31, 2018 and 2017, respectively. The gross profit on sale/leaseback transactions for all operating leases was $16.4 million for the year ended December 31, 2018. Right of use assets obtained in exchange for new operating lease liabilities were $42.2 million for the year ended December 31, 2018. At December 31, 2018 and 2017, security deposits associated with sale/leaseback transactions were $6.8 million $8.3 million, respectively, and are included in other assets on the consolidated balance sheet. Other information related to the operating leases is presented in the following table: Year ended December 31, 2018 Cash payments $ 14,926 Weighted average remaining lease term (years) Weighted average discount rate Finance Leases – As Lessee During the year ended December 31, 2017, the Company entered into sale/leaseback transactions, which were accounted for as finance leases and reported as part of finance obligations on the Company’s consolidated balance sheet. In June 2018, the timing and amount of the lease payments from certain previous sale/leaseback transactions were amended to extend the due date. The outstanding balance of finance obligations related to sale/leaseback transactions at December 31, 2018 was $81.9 million. The fair value of the finance obligation approximates the carrying value as of December 31, 2018. Future minimum lease payments under noncancelable finance leases related to sale/leaseback transactions (with initial or remaining lease terms in excess of one year) as of December 31, 2018 are (in thousands): 2019 $ 59,151 2020 6,722 2021 6,722 2022 4,975 2023 3,166 2024 and thereafter 16,154 Total future minimum lease payments 96,890 Less imputed lease interest (14,986) Total lease liabilities $ 81,904 Finance lease costs include amortization of the right of use assets (i.e. depreciation expense) and interest of lease liabilities (i.e. interest and other expense, net in the consolidated statement of operations). Finance lease costs for the year ended December 31, 2018 is (in thousands): Year ended December 31, 2018 Amortization of right of use asset $ 7,549 Interest on finance obligations 6,908 Total finance lease cost $ 14,457 Right of use assets obtained in exchange for new finance lease liabilities were $2.2 million for the year ended December 31, 2018. The Company has a finance lease associated with its property and equipment in Latham, New York. Liabilities relating to this agreement of $2.5 million has been recorded as a finance obligation, in the accompanying consolidated balance sheet as of December 31, 2018. The fair value of this finance obligation approximates the carrying value as of December 31, 2018. Upon the adoption of ASC 842, the Company reclassified property and equipment of $1.5 million, subject to a finance lease, to right of use assets. Other information related to the finance leases is presented in the following table: Year ended December 31, 2018 Cash payments $ 33,715 Weighted average remaining lease term (years) Weighted average discount rate Restricted Cash In connection with certain of the above noted sale/leaseback agreements, cash of $34.3 million is required to be restricted as security and will be released over the lease term. The Company also has certain letters of credit backed by security deposits totaling $36.8 million that are security for the above noted sale/leaseback agreements. The Company also has letters of credit in the aggregate amount of $0.5 million at December 31, 2018 associated with a finance obligation from the sale/leaseback of its building. Cash collateralizing this letter of credit is also considered restricted cash. Litigation Legal matters are defended and handled in the ordinary course of business. The Company has established accruals for matters for which management considers a loss to be probable and reasonably estimable. It is the opinion of management that facts known at the present time do not indicate that such litigation, after taking into account insurance coverage and the aforementioned accruals, will have a material adverse impact on our results of operations, financial position, or cash flows. Concentrations of credit risk Concentrations of credit risk with respect to receivables exist due to the limited number of select customers with whom the Company has initial commercial sales arrangements. To mitigate credit risk, the Company performs appropriate evaluation of a prospective customer’s financial condition. At December 31, 2018, three customers comprise approximately 52.3% of the total accounts receivable balance. At December 31, 2017, three customers comprised approximately 59.0% of the total accounts receivable balance. For the year ended December 31, 2018, 66.7% of total consolidated revenues were associated primarily with two customers. For the year ended December 31, 2017 and 2016, 71.8% and 34.1% of total consolidated revenues were associated primarily with two customers. For purposes of assigning a customer to a sale/leaseback transaction completed with a financial institution, the Company considers the end user of the assets to be the ultimate customer.

Unaudited Quarterly Financial D

Unaudited Quarterly Financial Data	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure
Unaudited Quarterly Financial Data	19. Unaudited Quarterly Financial Data (in thousands, except per share data) Quarters ended March 31, June 30, September 30, December 31, 2018 2018 2018 2018 Net revenue (1): Sales of fuel cell systems and related infrastructure $ 10,613 $ 18,820 $ 36,668 $ 41,191 Services performed on fuel cell systems and related infrastructure (2) 5,483 5,691 5,156 5,672 Power Purchase Agreements 5,372 5,438 5,555 6,504 Fuel delivered to customers 4,950 5,280 5,786 6,453 Net revenue 26,418 35,229 53,165 59,820 Gross (loss) profit (3,984) (2,310) 4,409 4,507 Operating expenses 16,957 20,668 17,054 17,426 Operating loss (20,941) (22,978) (12,645) (12,919) Net loss attributable to common shareholders (19,848) (25,881) (15,578) (16,860) Loss per share: Basic and Diluted $ (0.09) $ (0.12) $ (0.07) $ (0.08) Quarters ended March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Net revenue (1): Sales of fuel cell systems and related infrastructure $ 2,197 $ 7,650 $ 38,060 $ 14,724 Services performed on fuel cell systems and related infrastructure (2) 4,372 3,908 2,217 5,706 Power Purchase Agreements 4,311 4,945 (1,663) 5,276 Fuel delivered to customers 3,491 3,440 (4,149) 5,384 Other 87 64 128 5 Net revenue 14,458 20,007 34,593 31,095 Gross loss (4,479) (3,543) (19,410) (657) Operating expenses (3) 15,143 24,529 16,971 17,060 Operating loss (19,622) (28,072) (36,381) (17,717) Net loss attributable to common shareholders (27,074) (42,645) (41,008) (19,451) Loss per share: Basic and Diluted $ (0.14) $ (0.19) $ (0.18) $ (0.09) (1) Revenue amounts reported in prior periods have been reclassified to be presented net of provision for common stock warrants, as referenced in Note 2, Summary of Significant Accounting Policies. (2) Presentation of certain service arrangements and the amortization of the associated finance obligations for 2018 and 2017 has been corrected resulting in changes to previously reported interim financial information. See Note 2, Summary of Significant Accounting Policies, for details. (3) Operating expenses in the second quarter of 2017 includes the impact of $7.1 million charge related to the fair value of Amazon Warrant Shares as discussed in Note 13, Warrant Transaction Agreements.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the financial statements of the Company and its wholly-owned subsidiaries. All significant intercompany balances and transactions have been eliminated in consolidation.
Leases	Leases The Company is a lessee in several noncancelable (1) operating leases, primarily related to sale/leaseback transactions with financial institutions for deployment of the Company’s products at certain customer sites, and (2) finance leases, also primarily related to sale/leaseback transactions with financial institutions for similar commercial purposes. The Company accounts for leases in accordance with ASC Topic 842, Leases (ASC Topic 842). See Recently Adopted Accounting Standards, as the Company has early adopted ASC Topic 842 in 2018. The Company determines if an arrangement is or contains a lease at contract inception. The Company recognizes a right of use (ROU) asset and a lease liability (i.e. finance obligation) at the lease commencement date. For operating leases, the lease liability is initially measured at the present value of the unpaid lease payments at the lease commencement date. For finance leases, the lease liability is initially measured in the same manner and date as for operating leases, and is subsequently measured at amortized cost using the effective interest method. Key estimates and judgments include how the Company determines (1) the discount rate it uses to discount the unpaid lease payments to present value, (2) lease term and (3) lease payments. · ASC Topic 842 requires a lessee to discount its unpaid lease payments using the interest rate implicit in the lease or, if that rate cannot be readily determined, its incremental borrowing rate. Generally, the Company cannot determine the interest rate implicit in the lease because it does not have access to the lessor’s estimated residual value or the amount of the lessor’s deferred initial direct costs. Therefore, the Company generally uses its incremental borrowing rate as the discount rate for the lease. The Company’s incremental borrowing rate for a lease is the rate of interest it would have to pay on a collateralized basis to borrow an amount equal to the lease payments under similar terms. · The lease term for all of the Company’s leases includes the noncancelable period of the lease, plus any additional periods covered by either a Company option to extend (or not to terminate) the lease that the Company is reasonably certain to exercise, or an option to extend (or not to terminate) the lease controlled by the lessor. · Lease payments included in the measurement of the lease liability comprise fixed payments, and the exercise price of a Company option to purchase the underlying asset if the Company is reasonably certain to exercise the option. The ROU asset is initially measured at cost, which comprises the initial amount of the lease liability adjusted for lease payments made at or before the lease commencement date, plus any initial direct costs incurred less any lease incentives received. For operating leases, the ROU asset is subsequently measured throughout the lease term at the carrying amount of the lease liability, plus initial direct costs, plus (minus) any prepaid (accrued) lease payments, less the unamortized balance of lease incentives received. Lease expense for lease payments is recognized on a straight-line basis over the lease term. For finance leases, the ROU asset is subsequently amortized using the straight-line method from the lease commencement date to the earlier of the end of its useful life or the end of the lease term unless the lease transfers ownership of the underlying asset to the Company or the Company is reasonably certain to exercise an option to purchase the underlying asset. In those cases, the ROU asset is amortized over the useful life of the underlying asset. Amortization of the ROU asset is recognized and presented separately from interest expense on the lease liability. The Company’s leases do not contain variable lease payments. ROU assets for operating and finance leases are periodically reviewed for impairment losses. The Company uses the long-lived assets impairment guidance in ASC Subtopic 360-10, Property, Plant, and Equipment – Overall , to determine whether an ROU asset is impaired, and if so, the amount of the impairment loss to recognize. No impairment losses have been recognized to date. The Company monitors for events or changes in circumstances that require a reassessment of one of its leases. When a reassessment results in the remeasurement of a lease liability, a corresponding adjustment is made to the carrying amount of the corresponding ROU asset. Operating and finance lease ROU assets are presented within leased property, net on the consolidated balance sheet. The current portion of operating and finance lease liabilities is included in finance obligations within current liabilities and the long-term portion is presented in finance obligations within noncurrent liabilities on the consolidated balance sheet. The Company has elected not to recognize ROU assets and lease liabilities for short-term leases that have a lease term of 12 months or less. The Company has elected to apply the short-term lease recognition and measurement exemption for other classes of leased assets. The Company recognizes the lease payments associated with its short-term leases as an expense on a straight-line basis over the lease term. Adoption of ASC Topic 842 - Transition Approach As discussed in Recent Accounting Pronouncements, effective January 1, 2018, the Company early adopted ASC Topic 842. Further, ASU 2018-11 was issued in July 2018, which allowed for a modified retrospective basis of accounting for the transition. The Company selected this transition method and, therefore, restatement of prior period consolidated financial statements or presentation of comparative disclosures is not necessary. While determining the impact from the transition, the Company elected the practical expedients allowed under ASC 842. The Company did not reassess (1) whether any expired or existing contracts are or contain leases, (2) the lease classification for any expired or existing leases, or (3) initial direct costs for any existing leases. No modifications to leases effective upon January 1, 2018 were noted where hindsight expedients were necessary to apply.
Revenue Recognition	Revenue Recognition The Company enters into contracts that may contain one or a combination of fuel cell systems and infrastructure, installation, maintenance, spare parts, fuel delivery and other support services. Contracts containing fuel cell systems and related infrastructure may be sold, or provided to customers under a PPA. The Company does not include a right of return on its products other than rights related to standard warranty provisions that permit repair or replacement of defective goods. The Company accrues for anticipated standard warranty costs at the same time that revenue is recognized for the related product, or when circumstances indicate that warranty costs will be incurred, as applicable. Only a limited number of fuel cell units are under standard warranty. Revenue is measured based on the transaction price specified in a contract with a customer, subject to the allocation of the transaction price to distinct performance obligations as discussed below. The Company recognizes revenue when it satisfies a performance obligation by transferring control over a product or service to a customer. The Company accounts for each distinct performance obligation of within its arrangements as a distinct unit of accounting if the items under the performance obligation have value to the customer on a standalone basis. The Company considers a performance obligation to be distinct and have a standalone value if the customer can benefit from the good or service either on its own or together with other resources readily available to the customer and the Company’s promise to transfer the goods or service to the customer is separately identifiable from other promises in the contract. The Company allocates revenue to each distinct performance obligation based on relative standalone selling prices. Payment terms for fuel cells, infrastructure and service are invoiced with terms ranging from 30 to 90 days. Service is prepaid upfront in a majority of the arrangements. The Company does not adjust the transaction price for a significant financing component when the performance obligation is expected to be fulfilled within a year. The Company presents the provision for common stock warrants within each revenue-related line item on the consolidated statements of operations. This presentation reflects a discount that those common stock warrants represent, and therefore revenue is net of these non-cash charges. The provision of common stock warrants is allocated to the relevant revenue-related line items based upon the expected mix of the revenue for each respective contract. Nature of goods and services The following is a description of principal activities from which the Company generates its revenue. (i) Revenue from sales of fuel cell systems and related infrastructure represents sales of our GenDrive units, GenSure stationary backup power units, as well as hydrogen fueling infrastructure. The Company considers comparable list prices, as well as historical average pricing approaches to determine stand-alone selling prices. Once relative standalone selling prices are determined, the Company proportionately allocates the transaction price to each performance obligation within the customer arrangement. The allocated transaction price related to fuel cell systems, spare parts and hydrogen infrastructure is recognized as revenue at a point in time which usually occurs at shipment (and occasionally upon delivery) for fuel cells and spare parts and upon customer acceptance for hydrogen infrastructure, depending on the terms of the arrangement, the point at which transfer of control passes to the customer and the performance obligation has been satisfied. (ii) Revenue from services performed on fuel cell systems and related infrastructure represents revenue earned on our service and maintenance contracts and sales of spare parts. The transaction price allocated to services as discussed above is generally recognized as revenue over time on a straight-line basis over the expected service period. In substantially all of its commercial transactions, the Company sells extended maintenance contracts that generally provide for a five to ten year service period from the date of product installation. Services include monitoring, technical support, maintenance and services that provide for 97-98% uptime of the fleet. These services are accounted for as a separate performance obligation, and accordingly, revenue generated from these transactions, subject to the proportional allocation of transaction price, is deferred and recognized in income over the term of the contract, generally on a straight-line basis. Additionally, the Company may enter into annual service and extended maintenance contracts that are billed monthly. Revenue generated from these transactions is recognized in income on a straight-line basis over the term of the contract. Costs are recognized as incurred over the term of the contract. Sales of spare parts are included within service revenue on the accompanying consolidated statements of operations. When costs are projected to exceed revenues over the life of the contract, an accrual for loss contracts is recorded. Costs are estimated based upon historical experience and consider the estimated impact of the Company’s cost reduction initiatives. The actual results may differ from these estimates. When costs are projected to exceed revenues over the life of an extended maintenance contract, an accrual for loss contracts is recorded. Costs are estimated based upon historical experience and consider the estimated impact of the Company’s cost reduction initiatives. The actual results may differ from these estimates. Upon expiration of the extended maintenance contracts, customers either choose to extend the contract or switch to purchasing spare parts and maintaining the fuel cell systems on their own. (iii) Revenue from PPAs primarily represents payments received from customers who make monthly payments to access the Company’s GenKey solution. When fuel cell systems and related infrastructure are provided to customers through a PPA, revenues associated with these agreements are treated as rental income and recognized on a straight-line basis over the life of the agreements. In conjunction with entering into a PPA with a customer, the Company may enter into sale/leaseback transactions with third-party financial institutions, whereby the fuel cells, a majority of the related infrastructure, and, in some cases service are sold to the third-party financial institution and leased back to the Company through either an operating or finance lease. Certain of the Company’s sale/leaseback transactions with third-party financial institutions are required to be accounted for as financing leases. As a result, no upfront revenue was recognized at the closing of these transactions and a finance obligation for each lease was established. The fuel cell systems and related infrastructure that are provided to customers through these PPAs are considered leased property on the accompanying consolidated balance sheet. Costs to service the leased property, depreciation of the leased property, and other related costs are considered cost of PPA revenue on the accompanying consolidated statements of operations. Interest cost associated with finance leases is presented within interest and other expense, net on the accompanying consolidated statement of operations. The Company also has sale/leaseback transactions with financial institutions, which were required to be accounted for as an operating lease. The Company has rental expense associated with these sale/leaseback agreements with financial institutions. Rental expense is recognized on a straight-line basis over the life of the agreements and is characterized as cost of PPA revenue on the accompanying consolidated statements of operations. The Company adopted ASC Topic 842, effective January 1, 2018. As part of the adoption, the Company elected the practical expedient to not separate lease and non-lease components (i.e. maintenance services) within its rental income related to all PPA-related assets. (iv) Revenue associated with fuel delivered to customers represents the sale of hydrogen to customers that has been purchased by the Company from a third party or generated on site. The Company purchases hydrogen fuel from suppliers in certain cases (and produces hydrogen onsite) and sells to its customers upon delivery. Revenue and cost of revenue related to this fuel is recorded as dispensed, and is included in the respective “Fuel delivered to customers” lines on the consolidated statements of operations. Contract costs The Company expects that incremental commission fees paid to employees as a result of obtaining sales contracts are recoverable and therefore the Company capitalizes them as contract costs. Capitalized commission fees are amortized on a straight-line basis over the period of time over which the transfer of goods or services to which the assets relate occur, typically ranging from 5 to 10 years. Amortization of the capitalized commission fees is included in selling, general, and administrative expenses. The Company recognizes the incremental costs of obtaining contracts as an expense when incurred if the amortization period of the assets that the Company otherwise would have recognized is one year or less. These costs are included in selling, general, and administrative expenses. Adoption of ASC Topic 606 - Transition Approach As discussed in Recent Accounting Pronouncements, on January 1, 2018, the Company adopted ASC Topic 606, Revenue from Contracts with Customers (ASC Topic 606), which offers two transition approaches: full retrospective and modified retrospective. The Company chose the modified retrospective approach as its transition method and will not experience a significant effect on the timing and amount of revenue recognized or the amount of revenue allocated to the identified performance obligations. There was an insignificant amount of historical contract acquisition costs that were expensed and were not capitalized upon adoption of ASC Topic 606. However, upon adoption, contract acquisition costs of $0.1 million were capitalized and are being amortized as described above.
Cash Equivalents	Cash Equivalents For purposes of the consolidated statements of cash flows, the Company considers all highly-liquid debt instruments with original maturities of three months or less to be cash equivalents. At December 31, 2018 and 2017, cash equivalents consist of money market accounts. The Company’s cash and cash equivalents are deposited with financial institutions located in the U.S. and may at times exceed insured limits.
Common Stock Warrant Accounting	Common Stock Warrant Accounting The Company accounts for common stock warrants as either derivative liabilities or as equity instruments depending on the specific terms of the respective warrant agreements.
Derivative Liabilities	Derivative Liabilities Registered common stock warrants that require the issuance of registered shares upon exercise and do not sufficiently preclude an implied right to cash settlement are accounted for as derivative liabilities. We currently classify these derivative warrant liabilities on the consolidated balance sheet as a long-term liability, which are revalued at each balance sheet date subsequent to the initial issuance, using the Black-Scholes pricing model. This pricing model, which is based, in part, upon unobservable inputs for which there is little or no market data, requires the Company to develop its own assumptions. Changes in the fair value of the warrants are reflected in the consolidated statements of operations as change in fair value of common stock warrant liability.
Accounts Receivable	Accounts Receivable Accounts receivable are stated at the amount billed or billable to customers and are ordinarily due between 30 and 60 days after the issuance of the invoice. Receivables are reserved or written off based on individual credit evaluation and specific circumstances of the customer. The allowance for doubtful accounts and related receivable are reduced when the amount is deemed uncollectible. As of December 31, 2018, and 2017, the allowance for doubtful accounts was $249 thousand.
Inventory	Inventory Inventories are valued at the lower of cost, determined on a first-in, first-out basis, and net realizable value. All inventory, including spare parts inventory held at service locations, is not relieved until the customer has received the product, at which time the risks and rewards of ownership have transferred.
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment are originally recorded at cost or, if acquired as part of business combination, at fair value. Maintenance and repairs are expensed as costs are incurred. Depreciation on plant and equipment, which includes depreciation on the Company’s primary manufacturing facility, which is accounted for as a financing obligation, is calculated on the straight-line method over the estimated useful lives of the assets. The Company records depreciation and amortization over the following estimated useful lives: Buildings 20 years Building improvements 5 ‑ 20 years Software, machinery and equipment 1 ‑ 15 years Gains and losses resulting from the sale of property and equipment are recorded in current operations.
Leased Property	Leased Property Leased property primarily consists of the cost of assets deployed related to finance leases. Depreciation expense is recorded on a straight-line basis over the shorter of the lease term or the estimated useful life of the asset, generally six to seven years, and is included in cost of revenue for PPAs in the accompanying consolidated statements of operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets Long-lived assets, such as property, plant, and equipment, leased property and purchased intangibles subject to amortization, are reviewed for potential impairment whenever events or changes in circumstances indicate that the carrying amount of an asset or asset group may not be recoverable. Recoverability of assets to be held and used is measured by a comparison of the carrying amount of an asset or asset group to estimated undiscounted future cash flows expected to be generated by the asset or asset group. If the carrying amount of an asset or asset group exceeds its estimated undiscounted future cash flows, an impairment charge is recognized by the amount by which the carrying amount of the asset or asset group exceeds the fair value of the asset or asset group. Fair value is determined through various valuation techniques, including discounted cash flow models, quoted market values and third party independent appraisals, as considered necessary. Assets to be disposed of and considered held for sale would be separately presented in the consolidated balance sheet and reported at the lower of the carrying amount or fair value less costs to sell, and are no longer depreciated. The assets and liabilities of a disposal group classified as held for sale would be presented separately in the appropriate asset and liability sections of the consolidated balance sheet.
Goodwill	Goodwill Goodwill is an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. Goodwill is reviewed for impairment at least annually. The Company has the option to perform a qualitative assessment to determine whether it is more-likely-than-not that the fair value of a reporting unit is less than its carrying amount prior to performing the two-step goodwill impairment test. If this is the case, the two-step goodwill impairment test is required. If it is more-likely-than-not that the fair value of a reporting unit is greater than its carrying amount, the two-step goodwill impairment test is not required. The Company performs an impairment review of goodwill on an annual basis at December 1, and when a triggering event is determined to have occurred between annual impairment tests. For the years ended December 31, 2018, 2017, and 2016, the Company performed a qualitative assessment of goodwill for its single reporting unit based on multiple factors including market capitalization, and determined that it is not more likely than not that the fair value of its reporting unit is less than the carrying amount.
Intangible Assets	Intangible Assets Intangible assets consist of acquired technology, customer relationships and trademarks, and are amortized using a straight-line method over their useful lives of 5 - 10 years. Additionally, the intangible assets are reviewed for impairment when certain triggering events occur.
Product Warranty Reserve	Product Warranty Reserve Aside from when included in the sale of an extended maintenance contract, the Company provides a one to two year standard product warranty to customers from date of installation of GenDrive units, and the GenSure sales generally include a two year standard product warranty. We currently estimate the costs of satisfying warranty claims based on an analysis of past experience and provide for future claims in the period the revenue is recognized. Factors that affect our warranty liability include the number of installed units, estimated material costs, estimated travel, and labor costs. The warranty reserve is included within the other current liabilities on the accompanying consolidated balance sheet.
Equity Instruments	Equity Instruments Common stock warrants that meet certain applicable requirements of ASC Subtopic 815-40, Derivatives and Hedging – Contracts in Entity’s Own Equity , and other related guidance, including the ability of the Company to settle the warrants without the issuance of registered shares or the absence of rights of the grantee to require cash settlement, are accounted for as equity instruments. The Company classifies these equity instruments within additional paid-in capital on the accompanying consolidated balance sheets. Common stock warrants accounted for as equity instruments represent the warrants issued to Amazon and Walmart as discussed in Note 13, Warrant Transaction Agreements. These warrants are remeasured at each financial reporting date prior to vesting, using the Monte Carlo pricing model. Once these warrants vest, they are no longer remeasured. This pricing model, which is based, in part, upon unobservable inputs for which there is little or no market data, requires the Company to develop its own assumptions. Changes in fair value resulting from remeasurement of common stock warrants issued in connection with the Amazon Transaction Agreement and the Walmart Transaction Agreement, as described in Note 13, Warrant Transaction Agreements and are recorded as cumulative catch up adjustments as a reduction of revenue.
Redeemable Preferred Stock	Redeemable Preferred Stock We account for redeemable preferred stock as temporary equity in accordance with applicable accounting guidance in FASB ASC Topic 480, Distinguishing Liabilities from Equity . Dividends on the redeemable preferred stock are accounted for as an increase in the net loss attributable to common shareholders.
Income Taxes	Income Taxes Income taxes are accounted for under the asset and liability method. Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax basis and operating loss and tax credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in the period that includes the enactment date. A valuation allowance is recorded to reduce the carrying amounts of deferred tax assets if it is more likely than not that such assets will not be realized. The Company accounts for uncertain tax positions in accordance with FASB ASC No. 740-10-25, Income Taxes-Overall-Recognition . The Company recognizes in its consolidated financial statements the impact of a tax position only if that position is more likely than not to be sustained on audit, based on the technical merits of the position.
Foreign Currency Translation	Foreign Currency Translation Foreign currency translation adjustments arising from conversion of the Company’s foreign subsidiary’s financial statements to U.S. dollars for reporting purposes are included in accumulated other comprehensive income in stockholders’ equity on the accompanying consolidated balance sheets. Transaction gains and losses resulting from the effect of exchange rate changes on transactions denominated in currencies other than the functional currency of the Company’s operations give rise to realized foreign currency transaction gains and losses, and are included in interest and other income and interest and other expense, respectively, in the accompanying consolidated statements of operations.
Research and Development.	Research and Development Costs related to research and development activities by the Company are expensed as incurred.
Stock-Based Compensation	Stock-Based Compensation The Company maintains employee stock-based compensation plans, which are described more fully in Note 15, Employee Benefit Plans. Stock-based compensation represents the cost related to stock-based awards granted to employees and directors. The Company measures stock-based compensation cost at grant date, based on the fair value of the award, and recognizes the cost as expense on a straight-line basis over the option’s requisite service period. The Company estimates the fair value of stock-based awards using a Black-Scholes valuation model. Stock-based compensation expense is recorded in cost of revenue associated with sales of fuel cell systems and related infrastructure, cost of revenue for services performed on fuel cell systems and related infrastructure, research and development expense and selling, general and administrative expenses in the accompanying consolidated statements of operations based on the employees’ respective function. The Company records deferred tax assets for awards that result in deductions on the Company’s income tax returns, based upon the amount of compensation cost recognized and the Company's statutory tax rate. Differences between the deferred tax assets recognized for financial reporting purposes and the actual tax deduction reported on the Company's income tax return are recorded in the income statement. No tax benefit or expense for stock-based compensation has been recorded during the years ended December 31, 2018, 2017 and 2016 since the Company remains in a net operating loss (NOL) position.
Convertible Senior Notes	Convertible Senior Notes The Company accounts for the issued Convertible Senior Notes with separate liability and equity components. The carrying amount of the liability component was initially determined by estimating the fair value of a similar debt instrument that does not have an associated convertible feature. The carrying amount of the equity component representing the conversion option was determined by deducting the estimated fair value of the liability component from the par value of the Convertible Senior Notes as a whole as of the date of issuance. This difference represents a debt discount that is amortized to interest expense, with a corresponding increase to the carrying amount of the liability component, over the term of the Convertible Senior Notes using the effective interest rate method. The equity component is not remeasured as long as it continues to meet the conditions for equity classification. The Company has allocated issuance costs incurred to the liability and equity components. Issuance costs attributable to the liability component are being amortized to expense over the respective term of the Convertible Senior Notes, and issuance costs attributable to the equity components were netted with the respective equity component in additional paid-in capital.
Use of Estimates	Use of Estimates The consolidated financial statements of the Company have been prepared in conformity with U.S. generally accepted accounting principles, which require management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Reclassifications and correction of Immaterial Errors	Reclassifications and Correction of Immaterial Errors Reclassifications are made, whenever necessary, to prior period financial statements to conform to the current period presentation. The provision for common stock warrants presented historically as one line item on the consolidated statements of operations has been allocated to each of the relevant revenue line items. This reclassification did not have an impact on gross profit (loss) or net loss within the consolidated statements of operations or major categories within the consolidated statements of cash flows in the periods presented. In the third quarter of 2018, it was determined that the presentation in our consolidated statements of operations of certain service arrangements and the amortization of the associated finance obligations had not been appropriately accounted for resulting in an overstatement of our revenue and cost of revenue. This previous presentation resulted in a gross up of these line items and had no impact on our gross profit (loss) or net loss. The Company corrected the 2017 and 2016 annual financial statements to be consistent with the current period presentation and will correct comparable financial information in future filings. The amount reclassified from revenue on service performed on fuel cell systems and related infrastructure to cost of revenue on PPAs for years ended December 31, 2017 and 2016 was $3.8 million and $3.6 million, respectively. The amount reclassified from cost of revenue on service performed on fuel cell systems and related infrastructure to cost of revenue on PPAs for the years ended December 31, 2017 and 2016 was $3.1 million and $3.1 million, respectively. The Company does not consider the impact of the prior period correction to be material to the prior period consolidated financial statements.
Subsequent Events	Subsequent Events The Company evaluates subsequent events at the date of the balance sheet as well as conditions that arise after the balance sheet date but before the consolidated financial statements are issued. The effects of conditions that existed at the balance sheet date are recognized in the consolidated financial statements. Events and conditions arising after the balance sheet date but before the consolidated financial statements are issued are evaluated to determine if disclosure is required to keep the consolidated financial statements from being misleading. To the extent such events and conditions exist, if any, disclosures are made regarding the nature of events and the estimated financial effects for those events and conditions.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In September 2018, the Company early adopted ASC Topic 842, as amended, effective January 1, 2018 and elected the available practical expedients. This adoption had a material impact on the Company’s consolidated statements of operations in that it allowed the Company to recognize gross profit on sale/leaseback transactions of $16.4 million. The previous accounting standard only allowed revenue on sale/leaseback transactions to be recognized up to the amount of cost of goods sold and gross profit was deferred. Under ASC Topic 842, revenue can be recognized in full. Another impact from the adoption of ASC Topic 842 was the recognition of right of use assets and finance obligations for operating leases on the consolidated balance sheet, as well as expanded disclosures. The table below summarizes the impact of this initial adoption to the consolidated balance sheet as of January 1, 2018 (in thousands): Recognition of right of use asset $ 34,416 Decrease in accrued expenses 385 Recognition of finance obligation (34,161) Decrease in prepaid expenses and other assets (3,229) Decrease in leased property, net of accumulated depreciation (563) Increase in accumulated deficit 3,487 In addition, the consolidated statements of operations for the year ended December 31, 2018 was impacted by a decrease of depreciation expenses of $0.3 million. See Note 18, Commitments and Contingencies for gross profit recognized on sale/leaseback transactions accounted for under ASC Topic 842. In June 2014, an accounting update, ASC Topic 606, Revenue from Contracts with Customers , was issued that replaces the existing revenue recognition framework regarding contracts with customers. The Company adopted this accounting update as of January 1, 2018. The standard outlines a five-step model whereby revenue is recognized as performance obligations within a contract are satisfied. The standard also requires new, expanded disclosures regarding revenue recognition. The Company did not experience a significant effect on the timing and amount of revenue recognized or the amount of revenue allocated to the identified performance obligations. There is an insignificant amount of historical contract acquisition costs that were expensed under prior guidance and were not capitalized upon adoption of ASC Topic 606. However, in subsequent periods, contract acquisition costs are capitalized in accordance with ASC Topic 606. In October 2016, an accounting update was issued to simplify how an entity should recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. Consequently, the amendments in this update eliminate the exception for an intra-entity transfer of an asset other than inventory. Two common examples of assets included in the scope of this update are intellectual property and property, plant, and equipment. The Company adopted this update on January 1, 2018 and it did not have any effect on the consolidated financial statements because our net tax position is zero. In November 2016, an accounting update was issued to reduce the existing diversity in the classification and presentation of changes in restricted cash on the statements of cash flows. This accounting update was adopted retrospectively by the Company on January 1, 2018. The adoption of this update impacts the cash flows from financing activities due to the change in the presentation of restricted cash within the consolidated statements of cash flows. Net cash flows from financing activities and change in cash and cash equivalents, which now includes restricted cash, decreased by $11.4 million for the year ended December 31, 2017 and increased by $6.8 million for the year ended December 31, 2016. Recently Issued and Not Yet Adopted Accounting Pronouncements In November 2018, an accounting update was issued to clarify the interaction between ASC Topic 808, Collaborative Arrangements, and Topic 606. Adoption of this update is effective for all reporting periods beginning after December 15, 2019. The Company is evaluating the impact this update will have on the consolidated financial statements. In August 2018, an accounting update was issued to help entities evaluate the accounting for fees paid by a customer in cloud computing arrangement (hosting arrangement) by providing guidance for determining when the arrangement includes a software license. The amendments in this update are effective for public companies beginning after December 15, 2019. Early adoption of the amendments in this update are permitted. The Company is evaluating the adoption method and impact this update will have on the consolidated financial statements. In June 2018, an accounting update was issued to simplify the accounting for nonemployee share-based payment transactions resulting from expanding the scope of ASC Topic 718, Compensation-Stock Compensation , to include share-based payment transactions for acquiring goods and services from nonemployees. An entity should apply the requirements of ASC Topic 718 to nonemployee awards except for specific guidance on inputs to an option pricing model and the attribution of cost (that is, the period of time over which share-based payment awards vest and the pattern of cost recognition over that period). The amendments specify that ASC Topic 718 applies to all share-based payment transactions in which a grantor acquires goods or services to be used or consumed in a grantor’s own operations by issuing share-based payment awards. The amendments also clarify that ASC Topic 718 does not apply to share-based payments used to effectively provide (1) financing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract accounted for under ASC Topic 606. The amendments in this accounting update are effective for public business entities for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. Early adoption is permitted, but no earlier than an entity’s adoption date of ASC Topic 606. The Company does not believe this update will have a material effect on the consolidated financial statements. In January 2017, an accounting update was issued to simplify how an entity is required to test goodwill for impairment by eliminating Step 2 from the goodwill impairment test. Step 2 measures a goodwill impairment loss by comparing the implied fair value of a reporting unit’s goodwill with the carrying amount of that goodwill. This accounting update is effective for years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company is evaluating the impact this update will have on the consolidated financial statements. In August 2016, an accounting update was issued to reduce the existing diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. This accounting update is effective for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. Early adoption is permitted, including adoption in an interim period. The Company is evaluating the impact this update will have on the consolidated financial statements.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share
Schedule of Property Plant and Equipment Useful Lives	Buildings 20 years Building improvements 5 ‑ 20 years Software, machinery and equipment 1 ‑ 15 years
Summary of impact due to adoption of new accounting pronouncements	Recognition of right of use asset $ 34,416 Decrease in accrued expenses 385 Recognition of finance obligation (34,161) Decrease in prepaid expenses and other assets (3,229) Decrease in leased property, net of accumulated depreciation (563) Increase in accumulated deficit 3,487

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share.
Schedule of components of the calculations of basic and diluted earnings per share:	The following table provides the components of the calculations of basic and diluted earnings per share (in thousands, except share amounts): Year ended December 31, 2018 2017 2016 Numerator: Net loss attributable to common shareholders $ (78,167) $ (130,178) $ (57,591) Denominator: Weighted average number of common shares outstanding 218,882,337 216,343,985 180,619,860
Schedule of potential dilutive common shares	At December 31, 2018 2017 2016 Stock options outstanding (1) 21,957,150 19,872,029 14,760,054 Restricted stock outstanding (2) 2,347,347 234,744 13,333 Common stock warrants (3) 115,824,142 115,824,242 14,501,600 Preferred stock (4) 17,933,591 2,782,075 17,490,078 Convertible Senior Notes (5) 43,630,020 - - Number of dilutive potential common shares 201,692,250 138,713,090 46,765,065 (1) During the years ended December 31, 2018, 2017, and 2016, the Company granted 2,679,667, 5,485,863, and 3,702,500 stock options, respectively. (2) During the years ended December 31, 2018, 2017, and 2016, the Company granted 2,347,347, 234,744, and zero shares of restricted stock, respectively. (3) In February 2013, the Company issued 23,637,500 warrants as part of an underwritten public offering with an exercise price of $0.15 per warrant. Of these warrants issued in February 2013, zero, 100 and 100 were unexercised as of December 31, 2018, 2017 and 2016. In January 2014, the Company issued 4,000,000 warrants as part of an underwritten public offering with an exercise price of $4.00 per warrant. In December 2016, as a result of additional public offerings, and pursuant to the effect of the anti-dilution provisions of these warrants, the exercise price of the $4.00 warrants was reduced to $0.65. Of these warrants issued in January 2014, all 4,000,000 warrants were exercised during the year ended December 31, 2017, as described in Note 11, Stockholders’ Equity. In December 2016, the Company issued 10,501,500 warrants as part of two concurrent underwritten public offerings with an exercise price of $1.50 per warrant. Of these warrants issued in December 2016, all 10,501,500 warrants were exercised during the year ended December 31, 2017, respectively, as described in Note 11, Stockholders’ Equity. In April 2017, the Company issued 5,250,750 warrants with an exercise price of $2.69 per warrant, as described in Note 11, Stockholders’ Equity. Of these warrants issued in April 2017, none have been exercised as of December 31, 2018. In April 2017, the Company issued warrants to acquire up to 55,286,696 of the Company’s common stock as part of a transaction agreement, subject to certain vesting events, as described in Note 13, Warrant Transaction Agreements. Of these warrants issued, none have been exercised as of December 31, 2018. In July 2017, the Company issued warrants to acquire up to 55,286,696 of the Company’s common stock as part of a transaction agreement, subject to certain vesting events, as described in Note 13, Warrant Transaction Agreements. Of these warrants issued, none have been exercised as of December 31, 2018. (4) The preferred stock amount represents the dilutive potential common shares of the Series C, D and E redeemable convertible preferred stock, based on the conversion price of the preferred stock as of December 31, 2018, 2017 and 2016, respectively. Of the 10,431 Series C redeemable preferred stock issued on May 16, 2013, 7,811 had been converted to common stock through December 31, 2017, with the remainder still outstanding. Of the 18,500 Series D redeemable convertible preferred stock issued on December 22, 2016, 3,700 shares have been redeemed and the remaining 14,800 have been converted to common stock during the year ended December 31, 2017. On November 1, 2018, the Company issued 35,000 shares of Series E redeemable convertible preferred stock. As of December 31, 2018, all Series E redeemable convertible preferred stock are outstanding. (5) In March 2018, the Company issued $100.0 million in aggregate principal amount of 5.5% Convertible Senior Notes. See Note 10, Convertible Senior Notes.

Inventory (Tables)

Inventory (Tables)	12 Months Ended
	Dec. 31, 2018
Inventory
Schedule of Inventory	Inventory as of December 31, 2018 and 2017 consists of the following (in thousands): December 31, 2018 December 31, 2017 Raw materials and supplies - production locations $ 32,941 $ 37,369 Raw materials and supplies - customer locations 6,755 5,482 Work-in-process 5,589 3,492 Finished goods 2,625 2,433 Inventory $ 47,910 $ 48,776

Property, Plant and Equipment (

Property, Plant and Equipment (Tables)	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment
Schedule of Property plant and equipment	Property, plant and equipment at December 31, 2018 and 2017 consists of the following (in thousands): December 31, December 31, 2018 2017 Land $ — $ 90 Buildings — 15,332 Building improvements 214 5,230 Software, machinery and equipment 27,058 21,350 Property, plant, and equipment 27,272 42,002 Less: accumulated depreciation (14,403) (31,588) Property, plant, and equipment, net $ 12,869 $ 10,414

Leased Property (Tables)

Leased Property (Tables)	12 Months Ended
	Dec. 31, 2018
Leased Property
Schedule of Leased Property	Leased property at December 31, 2018 and 2017 consists of the following (in thousands): December 31, December 31, 2018 2017 Right of use assets - operating $ 76,747 $ — Right of use assets - finance 39,905 — Capitalized costs of lessor assets 41,040 98,877 Less: accumulated depreciation (10,941) (11,812) Leased property, net $ 146,751 $ 87,065

Intangible Assets (Tables)

Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2018
Intangible Assets
Schedule of Intangible assets	The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of December 31, 2018 are as follows (in thousands): Weighted Average Gross Carrying Accumulated Amortization Period Amount Amortization Total Acquired technology 9 years $ 5,926 $ (2,176) $ 3,750 Customer relationships 10 years 260 (123) 137 Trademark 5 years 60 (57) 3 $ 6,246 $ (2,356) $ 3,890 The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of December 31, 2017 are as follows (in thousands): Weighted Average Gross Carrying Accumulated Amortization Period Amount Amortization Total Acquired technology 9 years $ 5,200 $ (1,593) $ 3,607 Customer relationships 10 years 260 (97) 163 Trademark 5 years 60 (45) 15 $ 5,520 $ (1,735) $ 3,785
Schedule of future amortization of intangible assets	Estimated amortization expense for subsequent years is as follows (in thousands): 2019 $ 534 2020 498 2021 498 2022 498 2023 498 2024 and thereafter 1,364 Total $ 3,890

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2018
Accrued Expenses
Schedule of Accrued Expenses	Accrued expenses at December 31, 2018 and 2017 consist of (in thousands): 2018 2017 Accrued payroll and compensation related costs $ 2,150 $ 1,473 Accrued accounts payable 1,790 5,821 Accrued sales and other taxes 1,478 1,278 Accrued interest 1,605 — Accrued litigation — 1,076 Accrued other 841 947 Total $ 7,864 $ 10,595

Convertible Senior Notes (Table

Convertible Senior Notes (Tables)	12 Months Ended
	Dec. 31, 2018
Convertible Senior Notes
Schedule of net proceeds from the Convertible Senior Notes	Amount (in thousands) Principal amount $ 100,000 Less initial purchasers' discount (3,250) Less cost of related capped call and common stock forward (43,500) Less other issuance costs (894) Net proceeds $ 52,356
Schedule of Convertible Senior Notes	The Convertible Senior Notes consist of the following at December 31, 2018 (in thousands): Principal amounts: Principal $ 100,000 Unamortized debt discount (1) (34,706) Unamortized debt issuance costs (1) (2,047) Net carrying amount $ 63,247 Carrying amount of the equity component (2) $ 37,702 1) Included in the consolidated balance sheet within Convertible senior notes, net and amortized over the remaining life of the Convertible senior notes using the effective interest rate method. 2) Included in the consolidated balance sheet within additional paid-in capital, net of $1.7 million in equity issuance costs and associated income tax benefit of $9.2 million.

Revenue (Tables)

Revenue (Tables)	12 Months Ended
	Dec. 31, 2018
Revenue
Schedule of disaggregation of revenue	In the following table, revenue is disaggregated by major product line and timing of revenue recognition, net of the provision for common stock warrants, where applicable (in thousands): Major products/services lines Year ended December 31, 2018 2017 2016 Sales of fuel cell systems $ 75,146 $ 49,179 $ 30,008 Sale of hydrogen installations and other infrastructure 32,146 13,452 9,977 Services performed on fuel cell systems and related infrastructure 22,002 16,202 17,347 Power Purchase Agreements 22,869 12,869 13,687 Fuel delivered to customers 22,469 8,167 10,916 Other — 284 884 Net revenue $ 174,632 $ 100,153 $ 82,819
Schedule of receivables, contract assets and contract liabilities from contracts with customers	The following table provides information about receivables, contract assets and contract liabilities from contracts with customers (in thousands): 2018 2017 Accounts receivable $ 37,347 $ 24,179 Contract assets 788 1,141 Contract liabilities 40,476 35,171
Schedule of changes in contract assets and the contract liabilities	Significant changes in the contract assets and the contract liabilities balances during the period are as follows (in thousands): Contract assets Year ended December 31, 2018 Amortization of contract assets recognized at the beginning of the period $ (353) Revenue recognized and not billed as of the end of the period — Net change in contract assets $ (353) Contract liabilities Year ended December 31, 2018 Revenue recognized that was included in the contract liability balance as of the beginning of the period $ (10,783) Increases due to cash received, net of amounts recognized as revenue during the period 16,088 Net change in contract liabilities $ 5,305
Schedule of Estimated future revenue	The following table includes estimated revenue expected to be recognized in the future (sales of fuel cell systems and hydrogen installations are expected to be recognized as revenue within one year; sales of services and PPAs are expected to be recognized as revenue over five to seven years) related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period, excluding provision for common stock warrants as it is not readily estimable as it depends on the valuation of the common stock warrants when revenue is recognized (in thousands): Year ended December 31, 2018 2017 Sales of fuel cell systems $ 17,318 $ 26,298 Sale of hydrogen installations and other infrastructure 9,141 15,512 Services performed on fuel cell systems and related infrastructure 73,381 77,453 Power Purchase Agreements 111,533 130,042 Total estimated future revenue $ 211,373 $ 249,305

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2018
Employee Benefit Plans
Assumptions made for the purpose of estimating fair value	2018 2017 2016 Expected term of options (years) 6 6 6 Risk free interest rate 2.81% - 2.88% 1.89% - 2.16% 1.27% - 1.69% Volatility 98.31%-98.89% 99.24%-102.16% 103.87% - 104.88%
Schedule of Share-based Compensation, Stock Options, Activity	A summary of stock option activity for the year December 31, 2018 is as follows (in thousands except share amounts): Weighted Weighted Average Average Remaining Aggregate Exercise Contractual Intrinsic Shares Price Terms Value Options outstanding at December 31, 2017 19,872,029 $ 2.66 7.7 $ — Granted 2,679,667 1.96 — — Exercised (276,668) 0.63 — — Forfeited (246,378) 2.91 — — Expired (71,500) 29.51 — — Options outstanding at December 31, 2018 21,957,150 $ 2.51 7.1 $ 1,432 Options exercisable at December 31, 2018 14,964,985 2.74 6.3 1,431 Options unvested at December 31, 2018 6,992,165 $ 2.01 8.9 $ 1,467
Nonvested Restricted Stock Shares Activity	A summary of restricted stock activity for the year ended December 31, 2018 is as follows (in thousands except share amounts): Aggregate Intrinsic Shares Value Unvested restricted stock at December 31, 2017 234,744 $ — Granted 2,347,347 — Vested (234,744) — Unvested restricted stock at December 31, 2018 2,347,347 $ 2,911

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Measurements
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis	The following table summarizes the financial instruments measured at fair value on a recurring basis in the consolidated balance sheets (in thousands): Quoted Prices Significant Significant in Active Other Other Markets for Observable Unobservable Identical Items Inputs Inputs Total (Level 1) (Level 2) (Level 3) Balance at December 31, 2018 Common stock warrant liability $ 105 $ — $ — $ 105 Balance at December 31, 2017 Common stock warrant liability $ 4,391 $ — $ — $ 4,391
Assumptions used to calculate common stock warrants	Year ended December 31, 2018 December 31, 2017 Risk-free interest rate 1.64% - 2.66% 1.01% - 2.01% Volatility 18.40% - 81.69% 43.60% - 108.77% Expected average term 0.01 - 1.53 0.14 - 5.23
Schedule of activity in the common stock warrant liability	The following table shows the activity in the common stock warrant liability (in thousands): 2018 2017 Beginning of period $ 4,391 $ 11,387 Change in fair value of common stock warrants (4,286) 15,188 Issuance of common stock warrants — 4,905 Exercise of common stock warrants — (27,089) End of period $ 105 $ 4,391
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation	Year ended December 31, 2018 December 31, 2017 Risk-free interest rate 2.60% - 3.02% 2.30% - 2.47% Volatility 75.00% - 85.00% 85.00% - 90.00% Expected average term 8.26 - 9.30 9.26 - 10.00
Schedule of fair value of warrants	Amazon Warrant Shares Walmart Warrant Shares Issuance date - first tranche $ 1.15 $ 1.88 As of vesting date - second tranche, first installment 2.16 — As of vesting date - second tranche, second installment 1.54 — As of December 2018 - second tranche 0.98 0.92 As of December 2017 - second tranche 2.13 1.92

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Taxes
Schedule Components of loss before income taxes and the provision for income taxes	The components of loss before income taxes and the income tax benefit for the years ended December 31, 2018, 2017 and 2016, by jurisdiction, are as follows (in thousands): 2018 2017 2016 U.S. Foreign Total U.S. Foreign Total U.S. Foreign Total Loss before income taxes $ (85,925) $ (1,407) $ (87,332) $ (125,871) $ (1,209) $ (127,080) $ (56,317) $ (1,562) $ (57,879) Income tax benefit 9,217 — 9,217 — — — — 392 392 Net loss attributable to the Company $ (76,708) $ (1,407) $ (78,115) $ (125,871) $ (1,209) $ (127,080) $ (56,317) $ (1,170) $ (57,487)
Schedule of Significant Components of Deferred Income Tax Expense (Benefit)	2018 2017 2016 U.S. Foreign Total U.S. Foreign Total U.S. Foreign Total Loss before income taxes $ (85,925) $ (1,407) $ (87,332) $ (125,871) $ (1,209) $ (127,080) $ (56,317) $ (1,562) $ (57,879) Income tax benefit 9,217 — 9,217 — — — — 392 392 Net loss attributable to the Company $ (76,708) $ (1,407) $ (78,115) $ (125,871) $ (1,209) $ (127,080) $ (56,317) $ (1,170) $ (57,487) The significant components of deferred income tax expense (benefit) for the years ended December 31, 2018, 2017 and 2016, by jurisdiction, are as follows (in thousands): 2018 2017 2016 U.S. Foreign Total U.S. Foreign Total U.S. Foreign Total Deferred tax (benefit) expense $ (10,182) $ 933 $ (9,249) $ 7,675 $ (531) $ 7,144 $ (6,420) $ (1,299) $ (7,719) Net operating loss carryforward generated (10,038) (665) (10,703) (19,117) (17) (19,134) (16,727) (2,827) (19,554) Rate change impact on net operating loss carryforwards — — — 23,609 — 23,609 — — — Valuation allowance increase (decrease) 11,003 (268) 10,735 (12,167) 548 (11,619) 23,147 4,126 27,273 Provision for income taxes $ (9,217) $ — $ (9,217) $ — $ — $ — $ — $ — $ —
Schedule of Effective Income Tax Rate Reconciliation	The Company’s effective income tax rate differed from the federal statutory rate as follows: 2018 2017 2016 U.S. Federal statutory tax rate (21.0) % (35.0) % (35.0) % Deferred state taxes (1.9) % (1.4) % (3.1) % Common stock warrant liability (1.0) % 4.2 % (2.6) % Provision to return and deferred tax asset adjustments — % 5.9 % (2.9) % Change in unrecognized tax benefits — % — % (0.7) % Change in U.S. Federal statutory tax rate — % 33.5 % — % Other, net 0.4 % 2.0 % (1.6) % Change in valuation allowance 12.9 % (9.2) % 45.2 % (10.6) % 0.0 % (0.7) %
Schedule of Deferred Tax Assets and Liabilities	Significant components of the Company’s deferred tax assets and liabilities as of December 31, 2018 and 2017 are as follows (in thousands): U.S. Foreign Total 2018 2017 2018 2017 2018 2017 Intangible assets $ — $ — $ 1,146 $ 1,698 $ 1,146 $ 1,698 Deferred revenue 9,304 8,083 — — 9,304 8,083 Interest expense 5,239 — — — 5,239 — Other reserves and accruals 592 850 — — 592 850 Tax credit carryforwards 1,865 1,378 1,200 1,304 3,065 2,682 Amortization of stock-based compensation 8,442 6,904 — — 8,442 6,904 Non-compensatory warrants 3,597 4,555 — — 3,597 4,555 Capitalized research & development expenditures 19,116 14,496 4,294 4,666 23,410 19,162 Net operating loss carryforwards 49,058 39,437 9,306 8,641 58,364 48,078 Total deferred tax asset 97,213 75,703 15,946 16,309 113,159 92,012 Valuation allowance (84,567) (73,564) (15,926) (16,194) (100,493) (89,758) Net deferred tax assets $ 12,646 $ 2,139 $ 20 $ 115 $ 12,666 $ 2,254 Intangible assets (37) (76) — — (37) (76) Convertible debt (9,217) — — — (9,217) — Other reserves and accruals — — (20) — (20) — Property, plant and equipment and right of use assets (3,392) (1,854) — (115) (3,392) (1,969) Section 382 recognized built in loss — (209) — — — (209) Deferred tax liability $ (12,646) $ (2,139) $ (20) $ (115) $ (12,666) $ (2,254) Net $ — $ — $ — $ — $ — $ —
Schedule of Valuation Allowance	A reconciliation of the current year change in valuation allowance is as follows (in thousands): U.S. Foreign Total Increase in valuation allowance for current year increase in net operating losses $ 10,038 $ 464 $ 10,502 Increase (decrease) in valuation allowance for current year net increase (decrease) in deferred tax assets other than net operating losses 1,665 (281) 1,384 Decrease in valuation allowance as a result of foreign currency fluctuation — (451) (451) Decrease in valuation allowance due to change in tax rates (700) — (700) Net increase (decrease) in valuation allowance $ 11,003 $ (268) $ 10,735
Schedule of Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): 2018 2017 2016 Unrecognized tax benefits balance at beginning of year $ — $ — $ 437 Reductions for tax positions of prior years — — (469) Currency translation — — 32 Unrecognized tax benefits balance at end of year $ — $ — $ —

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies
Schedule of future minimum lease payments under noncancellable operating leases - as lessor	Future minimum lease payments under noncancelable operating leases (with initial or remaining lease terms in excess of one year) as of December 31, 2018 are (in thousands): 2019 $ 27,234 2020 25,503 2021 20,781 2022 12,044 2023 8,146 2024 and thereafter 7,946 Total future minimum lease payments $ 101,654
Schedule of future minimum lease payments under noncancellable operating leases - as lessee	Future minimum lease payments under noncancelable operating leases (with initial or remaining lease terms in excess of one year) as of December 31, 2018 are (in thousands): 2019 $ 22,582 2020 21,255 2021 16,512 2022 11,060 2023 10,325 2024 and thereafter 13,352 Total future minimum lease payments 95,086 Less imputed lease interest (24,171) Total lease liabilities $ 70,915
Schedule of operating leases other information	Year ended December 31, 2018 Cash payments $ 14,926 Weighted average remaining lease term (years) Weighted average discount rate
Schedule of future minimum lease payments under noncancellable finance leases	Future minimum lease payments under noncancelable finance leases related to sale/leaseback transactions (with initial or remaining lease terms in excess of one year) as of December 31, 2018 are (in thousands): 2019 $ 59,151 2020 6,722 2021 6,722 2022 4,975 2023 3,166 2024 and thereafter 16,154 Total future minimum lease payments 96,890 Less imputed lease interest (14,986) Total lease liabilities $ 81,904
Schedule of finance lease cost	Finance lease costs for the year ended December 31, 2018 is (in thousands): Year ended December 31, 2018 Amortization of right of use asset $ 7,549 Interest on finance obligations 6,908 Total finance lease cost $ 14,457
Schedule of finance leases other information	Year ended December 31, 2018 Cash payments $ 33,715 Weighted average remaining lease term (years) Weighted average discount rate

Unaudited Quarterly Financial_2

Unaudited Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure
Schedule of Quarterly Financial Information	Quarters ended March 31, June 30, September 30, December 31, 2018 2018 2018 2018 Net revenue (1): Sales of fuel cell systems and related infrastructure $ 10,613 $ 18,820 $ 36,668 $ 41,191 Services performed on fuel cell systems and related infrastructure (2) 5,483 5,691 5,156 5,672 Power Purchase Agreements 5,372 5,438 5,555 6,504 Fuel delivered to customers 4,950 5,280 5,786 6,453 Net revenue 26,418 35,229 53,165 59,820 Gross (loss) profit (3,984) (2,310) 4,409 4,507 Operating expenses 16,957 20,668 17,054 17,426 Operating loss (20,941) (22,978) (12,645) (12,919) Net loss attributable to common shareholders (19,848) (25,881) (15,578) (16,860) Loss per share: Basic and Diluted $ (0.09) $ (0.12) $ (0.07) $ (0.08) Quarters ended March 31, June 30, September 30, December 31, 2017 2017 2017 2017 Net revenue (1): Sales of fuel cell systems and related infrastructure $ 2,197 $ 7,650 $ 38,060 $ 14,724 Services performed on fuel cell systems and related infrastructure (2) 4,372 3,908 2,217 5,706 Power Purchase Agreements 4,311 4,945 (1,663) 5,276 Fuel delivered to customers 3,491 3,440 (4,149) 5,384 Other 87 64 128 5 Net revenue 14,458 20,007 34,593 31,095 Gross loss (4,479) (3,543) (19,410) (657) Operating expenses (3) 15,143 24,529 16,971 17,060 Operating loss (19,622) (28,072) (36,381) (17,717) Net loss attributable to common shareholders (27,074) (42,645) (41,008) (19,451) Loss per share: Basic and Diluted $ (0.14) $ (0.19) $ (0.18) $ (0.09) (1) Revenue amounts reported in prior periods have been reclassified to be presented net of provision for common stock warrants, as referenced in Note 2, Summary of Significant Accounting Policies. (2) Presentation of certain service arrangements and the amortization of the associated finance obligations for 2018 and 2017 has been corrected resulting in changes to previously reported interim financial information. See Note 2, Summary of Significant Accounting Policies, for details. (3) Operating expenses in the second quarter of 2017 includes the impact of $7.1 million charge related to the fair value of Amazon Warrant Shares as discussed in Note 13, Warrant Transaction Agreements.

Nature of Operations - Liquidit

Nature of Operations - Liquidity (Details) - USD ($) $ / shares in Units, $ in Thousands	Mar. 08, 2018	Nov. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jun. 30, 2019	Jul. 31, 2017
Liquidity
Net loss attributable to common shareholders				$ 16,860	$ 15,578	$ 25,881	$ 19,848	$ 19,451	$ 41,008	$ 42,645	$ 27,074	$ 78,167	$ 130,178	$ 57,591
Accumulated deficit				1,260,290				1,178,636				1,260,290	1,178,636
Net cash used in operating activities												57,617	60,182	29,636
Net loss attributable to the Company												78,115	127,080	57,487
Non-cash charges												2,000
Net outflows from fluctuations in working capital and other assets and liabilities												4,600
Cash and cash equivalents				38,602				24,828				38,602	24,828
Net working capital				9,200				$ 3,900				9,200	3,900
Net cash used in investing activities												(19,572)	(44,363)	(58,075)
Net cash provided by financing activities												119,344	71,616	76,672
Proceeds from Issuance of Debt												95,856
Proceeds from borrowing of long-term debt													20,147	47,400
Increase in finance obligations												44,900
Repayments of Long-term Debt												16,190	$ 12,292	25,000
Purchase of Capped Call and Common Stock Forward												43,500
Remaining lease payments				82,200								82,200
Security on lease				80,400								80,400
Finance obligations and long-term debt classified as current liabilities				91,100								91,100
Convertible Senior Notes
Liquidity
Proceeds from Issuance of Debt	$ 52,356
Principal amount	100,000		$ 100,000	$ 100,000			$ 100,000					$ 100,000
Interest rate (as a percent)			5.50%				5.50%
Purchase of Capped Call and Common Stock Forward	$ 43,500		$ 43,500
Series E Redeemable Convertible Preferred Stock
Liquidity
Shares issued (in shares)		35,000
Redeemable convertible Preferred Stock, par value (in dollars per share)		$ 0.01		$ 0.01				$ 0.01				$ 0.01	$ 0.01
Aggregate proceeds from issuance of preferred stock		$ 30,900
NY Green Bank
Liquidity
Principal amount				$ 45,000								$ 45,000		$ 25,000		$ 45,000
Wells Fargo
Liquidity
Remaining lease payments				67,500								67,500
Letter of credit				$ 20,100								$ 20,100
Forecast
Liquidity
Amount due in second quarter															$ 50,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Summary of Significant Accounting Policies
Upfront revenue recognized	$ 0
Capitalized contract costs	200	$ 0		$ 100
Net cash flows from financing activities	119,344	71,616	$ 76,672
Impairment Loss	$ 0
ASU 2016-18
Summary of Significant Accounting Policies
Net cash flows from financing activities		11,400
Increase in cash and cash equivalents			6,800
Reclassified From Revenue
Summary of Significant Accounting Policies
Prior period amount reclassed		3,800	3,600
Reclassified From Cost of Revenue
Summary of Significant Accounting Policies
Prior period amount reclassed		$ 3,100	$ 3,100
Minimum
Summary of Significant Accounting Policies
Payment terms for fuel cells and its services	30 days
Extension period	5 years
Uptime of the fleet (as a percent)	97.00%
Amortization period of capitalized commission fees	5 years
Maximum
Summary of Significant Accounting Policies
Payment terms for fuel cells and its services	90 days
Extension period	10 years
Uptime of the fleet (as a percent)	98.00%
Amortization period of capitalized commission fees	10 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Receivable, Inventory, And Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue Recognition from Customers [Abstract]
Contract with Customer, Liability	$ 40,476
Accounts Receivable
Minimum number of days after an invoice is issued when accounts receivable is considered due	30 days
Maximum number of days after an invoice is issued when accounts receivable is considered due	60 days
Allowance for doubtful accounts receivable	$ 249	$ 249
Stock-Based Compensation
Tax benefit (expense) for stock-based compensation	$ 0	$ 0	$ 0
Minimum
Sale/leaseback transactions
Lease term	6 years
Intangible Assets
Intangible asset useful lives	5 years
Maximum
Sale/leaseback transactions
Lease term	7 years
Intangible Assets
Intangible asset useful lives	10 years
Buildings
Property, Plant and Equipment
Estimated useful life	20 years
Building improvements | Minimum
Property, Plant and Equipment
Estimated useful life	5 years
Building improvements | Maximum
Property, Plant and Equipment
Estimated useful life	20 years
Software, machinery and equipment | Minimum
Property, Plant and Equipment
Estimated useful life	1 year
Software, machinery and equipment | Maximum
Property, Plant and Equipment
Estimated useful life	15 years

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands	Jan. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary of Significant Accounting Policies
Sale/leaseback revenue and gross profit	$ 16,400
Decrease of depreciation expenses	300	$ 11,014	$ 9,190	$ 4,650
Decrease in accrued expenses		7,864	10,595
Recognition of finance obligation		(70,915)
Decrease in prepaid expenses and other assets		(4,654)	2,157	$ (2,386)
Decrease in leased property, net of accumulated depreciation		(146,751)	(87,065)
Increase in accumulated deficit		(1,260,290)	$ (1,178,636)
ASU 2016-02 | Restatement adjustment
Summary of Significant Accounting Policies
Decrease in leased property, net of accumulated depreciation		$ (1,500)
ASU 2016-02 | Early adoption | Restatement adjustment
Summary of Significant Accounting Policies
Recognition of right of use asset	34,416
Decrease in accrued expenses	385
Recognition of finance obligation	(34,161)
Decrease in prepaid expenses and other assets	(3,229)
Decrease in leased property, net of accumulated depreciation	(563)
Increase in accumulated deficit	$ 3,487

Earnings Per Share - Basic and

Earnings Per Share - Basic and Diluted Components (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss attributable to common shareholders	$ (16,860)	$ (15,578)	$ (25,881)	$ (19,848)	$ (19,451)	$ (41,008)	$ (42,645)	$ (27,074)	$ (78,167)	$ (130,178)	$ (57,591)
Denominator:
Weighted average number of common shares outstanding									218,882,337	216,343,985	180,619,860

Earnings Per Share - Dilutive P

Earnings Per Share - Dilutive Potential Common Shares (Details) - USD ($) $ / shares in Units, $ in Thousands	Nov. 01, 2018	Apr. 12, 2017	Dec. 22, 2016	May 16, 2013	Dec. 31, 2017	Apr. 30, 2017	Dec. 31, 2016	Feb. 28, 2013	Mar. 31, 2014	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2018	Mar. 08, 2018	Feb. 28, 2018	Jul. 31, 2017	Jul. 20, 2017	Apr. 04, 2017
Earnings Per Share
Number of dilutive potential common shares										201,692,250	138,713,090	46,765,065
Exercise price of warrants (in dollars per share)																	$ 2.1231
Stock options
Earnings Per Share
Stock options granted										2,679,667	5,485,863	3,702,500
Warrants issued in February, 2013
Earnings Per Share
Number of warrants issued (in shares)								23,637,500
Warrants outstanding (in shares)					100		100				100	100			100
Number of warrants exercised (in shares)											100	100
Number of warrants unexercised (in shares)										0
Exercise price of warrants (in dollars per share)					$ 0.15		$ 0.15	$ 0.15			$ 0.15	$ 0.15
Warrants issued in January, 2014
Earnings Per Share
Number of warrants issued (in shares)					4,000,000				4,000,000
Number of warrants exercised (in shares)						4,000,000
Exercise price of warrants (in dollars per share)						$ 0.65	$ 0.65		$ 4			0.65
Warrants issued in December 2016
Earnings Per Share
Number of warrants issued (in shares)			10,501,500			5,250,750	10,501,500
Shares of common stock that can be purchased from warrants issued (in shares)		5,250,750
Number of warrants exercised (in shares)		10,501,500									10,501,500
Exercise price of warrants (in dollars per share)			$ 1.50				$ 1.50					$ 1.50
Warrants issued in April 2017
Earnings Per Share
Warrant shares vested (in shares)										0
Number of warrants unexercised (in shares)										0
Exercise price of warrants (in dollars per share)						$ 2.69
Warrants issued with the Amazon.com, Inc transaction agreement
Earnings Per Share
Shares of common stock that can be purchased from warrants issued (in shares)																		55,286,696
Warrant shares vested (in shares)					13,094,217					20,368,782	13,094,217
Tranche one of warrants issued with the Amazon.com, Inc transaction agreement
Earnings Per Share
Warrant shares vested (in shares)																		5,819,652
Warrants issued with the Walmart transaction agreement
Earnings Per Share
Shares of common stock that can be purchased from warrants issued (in shares)																55,286,696
Tranche one of warrants issued with the Walmart Stores Inc transaction agreement
Earnings Per Share
Warrant shares vested (in shares)																	5,819,652
Series C redeemable convertible preferred stock
Earnings Per Share
Stock options granted				10,431
Shares issued upon conversion of redeemable stock (in shares)					7,811
Series D Redeemable Convertible Preferred Stock
Earnings Per Share
Number of preferred shares that had been converted to common stock											14,800
Shares issued (in shares)			18,500
Number of shares redeemed											3,700
Series E redeemable convertible preferred stock
Earnings Per Share
Shares issued upon conversion of redeemable stock (in shares)	35,000
Convertible Senior Notes
Earnings Per Share
Number of dilutive potential common shares										43,630,020
Principal amount										$ 100,000			$ 100,000	$ 100,000
Interest rate (as a percent)													5.50%
Stock options
Earnings Per Share
Number of dilutive potential common shares										21,957,150	19,872,029	14,760,054
Restricted stock
Earnings Per Share
Number of dilutive potential common shares										2,347,347	234,744	13,333
Stock options granted										2,347,347	234,744	0
Warrants
Earnings Per Share
Number of dilutive potential common shares										115,824,142	115,824,242	14,501,600
Preferred stock
Earnings Per Share
Number of dilutive potential common shares										17,933,591	2,782,075	17,490,078

Inventory (Details)

Inventory (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Inventory
Raw materials and supplies - production locations	$ 32,941	$ 37,369
Raw materials and supplies - customer locations	6,755	5,482
Work-in-process	5,589	3,492
Finished goods	2,625	2,433
Total Inventory	$ 47,910	$ 48,776

Property, Plant and Equipment_2

Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, plant and equipment
Property, plant, and equipment, gross	$ 27,272	$ 42,002
Less: accumulated depreciation	(14,403)	(31,588)
Property, plant, and equipment, net	12,869	10,414
Depreciation expense	2,600	1,900	$ 1,800
Land
Property, plant and equipment
Property, plant, and equipment, gross		90
Buildings
Property, plant and equipment
Property, plant, and equipment, gross		15,332
Building improvements
Property, plant and equipment
Property, plant, and equipment, gross	214	5,230
Software, machinery and equipment
Property, plant and equipment
Property, plant, and equipment, gross	$ 27,058	$ 21,350

Leased Property (Details)

Leased Property (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Leased Property
Right of use assets - operating	$ 76,747
Right of use asset - finance	39,905
Capitalized costs of lessor assets	41,040	$ 98,877
Less: accumulated depreciation	(10,941)	(11,812)
Leased property, net	146,751	87,065
Depreciation	$ 8,400	$ 7,300	$ 2,900

Intangible Assets - Gross Carry

Intangible Assets - Gross Carrying Amount (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Gross carrying amount and accumulated amortization of acquired identifiable intangible assets
Gross Carrying Amount	$ 6,246	$ 5,520
Accumulated amortization	(2,356)	(1,735)
Total	3,890	$ 3,785
Milestone payments	$ 2,900
Acquired technology
Gross carrying amount and accumulated amortization of acquired identifiable intangible assets
Weighted Average Amortization Period	9 years	9 years
Gross Carrying Amount	$ 5,926	$ 5,200
Accumulated amortization	(2,176)	(1,593)
Total	$ 3,750	$ 3,607
Customer relationships
Gross carrying amount and accumulated amortization of acquired identifiable intangible assets
Weighted Average Amortization Period	10 years	10 years
Gross Carrying Amount	$ 260	$ 260
Accumulated amortization	(123)	(97)
Total	$ 137	$ 163
Trademark
Gross carrying amount and accumulated amortization of acquired identifiable intangible assets
Weighted Average Amortization Period	5 years	5 years
Gross Carrying Amount	$ 60	$ 60
Accumulated amortization	(57)	(45)
Total	$ 3	$ 15

Intangible Assets - Estimated A

Intangible Assets - Estimated Amortization Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Intangible Assets
Amortization of Intangible Assets	$ 693	$ 593	$ 590
Estimated amortization expense
2019	534
2020	498
2021	498
2022	498
2023	498
2024 and thereafter	1,364
Total	$ 3,890	$ 3,785

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Expenses
Accrued payroll and compensation related costs	$ 2,150	$ 1,473
Accrued accounts payable	1,790	5,821
Accrued sales and other taxes	1,478	1,278
Accrued interest	1,605
Accrued litigation		1,076
Accrued other	841	947
Total	$ 7,864	$ 10,595

Long-Term Debt (Details)

Long-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Jul. 31, 2017	Dec. 31, 2016
Long-Term Debt
Percent of securities in foreign subsidiaries guaranteed to secure debt	65.00%
Maximum
Long-Term Debt
Additional fees	$ 1.8
Secured term loan facility
Long-Term Debt
Effective interest rate (as a percent)	11.90%
Term of loan agreement	3 years
2019	$ 17.2
Secured term loan facility | LIBOR
Long-Term Debt
Interest rate spread (as a percent)	9.50%
NY Green Bank
Long-Term Debt
Loan Amount	$ 45	$ 45	$ 25
Long-term borrowings	$ 17.2
Increase in term loan facility		$ 20

Convertible Senior Notes - Net

Convertible Senior Notes - Net proceeds (Details) - USD ($) $ in Thousands	Mar. 08, 2018	Mar. 31, 2018	Dec. 31, 2018
Convertible Senior Notes
Less cost of related capped call and common stock forward			$ (43,500)
Net proceeds			95,856
Convertible Senior Notes
Convertible Senior Notes
Interest rate (as a percent)		5.50%
Convertible Senior Notes
Principal amount	$ 100,000	$ 100,000	100,000
Initial purchasers' discount	(3,250)
Less cost of related capped call and common stock forward	(43,500)	$ (43,500)
Less other issuance costs	(894)		$ (1,700)
Net proceeds	$ 52,356

Convertible Senior Notes - Conv

Convertible Senior Notes - Conversion (Details)	Mar. 08, 2018USD ($)	Dec. 31, 2018USD ($)item$ / shares
Convertible Senior Notes
Ownership interest percentage		50.00%
Closing price of the company's stock | $ / shares		$ 1.24
Convertible Senior Notes
Convertible Senior Notes
Conversion ratio, principal amount		$ 1,000
Conversion rates for the notes (in shares)		436.3002
Conversion price, per share | $ / shares		$ 2.29
Trading days | item		20
Consecutive Trading days | item		30
Conversion price (as a percent)		130.00%
Number of business days		5 days
Number of consecutive trading days		5 days
Principal amount (as a percent)		98.00%
Principal amount of the repurchased Convertible Senior Notes(as a percent)		100.00%
Percentage of principal amount to be redeemed		100.00%
Carrying amount of the liability component		$ 58,200,000
Effective interest rate (as a percent)		16.00%
Transaction costs for issuance		$ 4,100,000
Initial purchasers' discount	$ 3,250,000
Other issuance costs	$ 894,000	1,700,000
Transaction costs attributable to the liability component		2,400,000
Transaction costs attributable to the equity component		$ 1,700,000
Remaining life of the Convertible Senior Notes		51 months

Convertible Senior Notes (Detai

Convertible Senior Notes (Details) - USD ($) $ in Thousands	Mar. 08, 2018	Dec. 31, 2018	Mar. 31, 2018
Convertible Senior Notes
Net carrying amount		$ 63,247
Convertible Senior Notes
Convertible Senior Notes
Principal amount	$ 100,000	100,000	$ 100,000
Unamortized debt discount		(34,706)
Unamortized debt issuance costs		(2,047)
Net carrying amount		63,247
Carrying amount of the equity component		37,702
Other issuance costs	$ 894	1,700
Income tax benefit on equity component		$ 9,200

Convertible Senior Notes - Capp

Convertible Senior Notes - Capped Call and Common Stock Forward (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)$ / sharesshares
Capped Call and Common Stock Forward
Share Price	$ 1.24
Capped Call
Capped Call and Common Stock Forward
Capped call options amount | $	$ 16
Cap price	$ 3.82
Premium (as a percent)	100.00%
Share Price	$ 1.91
Common Stock Forward
Capped Call and Common Stock Forward
Common stock shares issued | shares	14,397,906
Net cost incurred | $	$ 27.5

Stockholders' Equity - Common S

Stockholders' Equity - Common Stock and Warrants (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2018	Apr. 12, 2017	Apr. 03, 2017	Dec. 22, 2016	Dec. 31, 2017	Apr. 30, 2017	Dec. 31, 2016	Feb. 28, 2013	Mar. 31, 2014	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Feb. 28, 2018	Jul. 20, 2017
Stockholders' equity
Preferred stock, Shares authorized	5,000,000									5,000,000			5,000,000
Preferred stock, par value	$ 0.01									$ 0.01			$ 0.01
Common Stock Shares, Outstanding	219,157,998				228,486,366					219,157,998	228,486,366		219,157,998
Common Stock, par value (in dollars per share)	$ 0.01				$ 0.01					$ 0.01	$ 0.01		$ 0.01
Exercise price of warrants (in dollars per share)															$ 2.1231
Proceeds from exercise of warrants, net of transaction costs											$ 17,636	$ 111
Value of shares issued										$ 7,016	$ 22,992	$ 8,928
Series A Junior Participating Cumulative Preferred Stock
Stockholders' equity
Preferred stock, par value	$ 0.01				$ 0.01					$ 0.01	$ 0.01		$ 0.01
Common Stock Shares, Outstanding	0				0					0	0		0
At Market Issuance Sales Agreement
Stockholders' equity
Common Stock, par value (in dollars per share)			$ 0.01
Authorized amount			$ 75,000
Common stock shares issued										3,804,654
Value of shares issued										$ 7,000			$ 30,200
Warrants issued in February, 2013
Stockholders' equity
Class of Warrant or Right Issued								23,637,500
Exercise price of warrants (in dollars per share)					$ 0.15		$ 0.15	$ 0.15			$ 0.15	$ 0.15
Warrants outstanding (in shares)					100		100				100	100		100
Number of warrants exercised (in shares)											100	100
Warrants issued in December 2016
Stockholders' equity
Class of Warrant or Right Issued				10,501,500		5,250,750	10,501,500
Exercise price of warrants (in dollars per share)				$ 1.50			$ 1.50					$ 1.50
Number of warrants exercised (in shares)		10,501,500									10,501,500
Proceeds from exercise of warrants, net of transaction costs		$ 15,100
Shares of common stock that can be purchased from warrants issued		5,250,750
Per share price of shares of common stock		$ 2.69
Warrants issued in January, 2014
Stockholders' equity
Class of Warrant or Right Issued					4,000,000				4,000,000
Exercise price of warrants (in dollars per share)						$ 0.65	$ 0.65		$ 4			$ 0.65
Number of warrants exercised (in shares)						4,000,000
Proceeds from exercise of warrants, net of transaction costs						$ 2,600
Increase in additional paid-in capital						$ 27,100
Warrant Issued With Amazon And Walmart Stores Inc Transaction Agreement In 2017
Stockholders' equity
Number of warrants exercised (in shares)	26,188,434										18,913,869
Warrant Issued With Amazon And Walmart Stores Inc Transaction Agreement In 2017 | Maximum
Stockholders' equity
Class of Warrant or Right Issued											110,573,392

Redeemable Preferred Stock (Det

Redeemable Preferred Stock (Details)	Dec. 22, 2016shares	Nov. 30, 2018USD ($)shares	Dec. 31, 2018USD ($)installment$ / sharesshares	Dec. 31, 2017$ / sharesshares
Series C Redeemable Convertible Preferred Stock
Redeemable preferred stock
Redeemable stock issued, stated value (in dollars per share) | $ / shares			$ 0.01	$ 0.01
Preferred Stock, Shares Outstanding | shares			2,620	2,620
Dividend rate			8.00%
Original issue price | $			$ 248.794
Conversion price per share (in dollars per share) | $ / shares			$ 0.2343	$ 0.2343
Series D Redeemable Convertible Preferred Stock
Redeemable preferred stock
Shares issued (in shares) | shares	18,500
Series E Redeemable Convertible Preferred Stock
Redeemable preferred stock
Shares issued (in shares) | shares		35,000
Redeemable stock issued, par value (in dollars per share) | $		$ 0.01
Net proceeds from public offering | $		$ 30,900,000
Number of monthly installments | installment			13
Redemption value for each installment | $			$ 2,700,000
Discount on market price			15.00%
Conversion price per share | $ / shares			$ 2.31
Liquidation preference percentage			125.00%
Redeemable stock issued, initial stated value (in dollars per share) | $ / shares			$ 1,000

Warrant Transaction Agreements

Warrant Transaction Agreements - Amazon.com, Inc. Transaction Agreement (Details) $ / shares in Units, $ in Thousands	Apr. 04, 2017USD ($)installment$ / sharesshares	Dec. 31, 2017USD ($)shares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)shares	Dec. 31, 2016USD ($)	Jul. 20, 2017$ / shares
Class of Warrant or Right [Line Items]
Selling, general and administrative			$ 38,198	$ 45,010	$ 34,288
Exercise price of warrants (in dollars per share) | $ / shares						$ 2.1231
Warrants issued with the Amazon.com, Inc transaction agreement
Class of Warrant or Right [Line Items]
Shares of common stock that can be purchased from warrants issued (in shares) | shares	55,286,696
Cash payments to be received under agreement	$ 600,000
Warrant shares vested (in shares) | shares		13,094,217	20,368,782	13,094,217
Aggregate cash receipts for all installments	$ 200,000
Exercise price calculation	The exercise price of the third tranche of Amazon Warrant Shares will be an amount per share equal to ninety percent (90%) of the 30-day volume weighted average share price of the common stock as of the final vesting date of the second tranche of Amazon Warrant Shares
Provision for common stock warrants			$ 9,800	$ 17,300
Tranche one of warrants issued with the Amazon.com, Inc transaction agreement
Class of Warrant or Right [Line Items]
Warrant shares vested (in shares) | shares	5,819,652
Selling, general and administrative		$ 6,700
Tranche two of warrants issued with the Amazon.com, Inc. Transaction Agreement
Class of Warrant or Right [Line Items]
Warrant shares vested (in shares) | shares	29,098,260
Number of installments | installment	4
Number of shares per installment | shares	7,274,565
Cash receipt per installment	$ 50,000
Aggregate cash receipts for all installments	$ 200,000
Exercise price of warrants (in dollars per share) | $ / shares	$ 1.1893
Tranche three of warrants issued with the Amazon.com, Inc. Transaction Agreement
Class of Warrant or Right [Line Items]
Warrant shares vested (in shares) | shares	20,368,784
Number of installments | installment	8
Number of shares per installment | shares	2,546,098
Cash receipt per installment	$ 50,000
Aggregate cash receipts for all installments	$ 400,000

Warrant Transaction Agreement_2

Warrant Transaction Agreements - Walmart Stores, Inc. Transaction Agreement (Details) $ / shares in Units, $ in Thousands	Jul. 20, 2017USD ($)installment$ / sharesshares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Warrant Transaction Agreements
Selling, general and administrative		$ 38,198	$ 45,010	$ 34,288
Exercise price of warrants (in dollars per share) | $ / shares	$ 2.1231
Warrants issued with the Walmart Stores, Inc transaction agreement
Warrant Transaction Agreements
Shares of common stock that can be purchased from warrants issued (in shares) | shares	55,286,696
Cash payments to be received under agreement	$ 600,000
Warrant shares vested (in shares) | shares		5,819,652
Provision for common stock warrants		$ 400	$ 12,400
Exercise price calculation	The exercise price of the third tranche of Walmart Warrant Shares will be an amount per share equal to ninety percent (90%) of the 30-day volume weighted average share price of the common stock as of the final vesting date of the second tranche of Walmart Warrant Shares
Tranche one of warrants issued with the Walmart Stores Inc transaction agreement
Warrant Transaction Agreements
Warrant shares vested (in shares) | shares	5,819,652
Provision for common stock warrants	$ 10,900
Tranche two of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Warrant Transaction Agreements
Warrant shares vested (in shares) | shares	29,098,260
Number of installments | installment	4
Number of shares per installment | shares	7,274,565
Cash receipt per installment	$ 50,000
Aggregate cash receipts for all installments	$ 200,000
Tranche three of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Warrant Transaction Agreements
Warrant shares vested (in shares) | shares	20,368,784
Number of installments | installment	8
Number of shares per installment | shares	2,546,098
Cash receipt per installment	$ 50,000
Aggregate cash receipts for all installments	$ 400,000
Exercise price of warrants (in dollars per share) | $ / shares	$ 1.1893

Revenue - Disaggregation of Rev

Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue
Net revenue	$ 59,820	$ 53,165	$ 35,229	$ 26,418	$ 31,095	$ 34,593	$ 20,007	$ 14,458
Net revenue									$ 174,632	$ 100,153	$ 82,819
Sales of fuel cell systems
Revenue
Net revenue									75,146
Sale of hydrogen installations and other infrastructure
Revenue
Net revenue									32,146
Services performed on fuel cell systems and related infrastructure
Revenue
Net revenue	5,672	5,156	5,691	5,483	5,706	2,217	3,908	4,372	22,002	16,202	17,347
Power Purchase Agreements
Revenue
Net revenue	6,504	5,555	5,438	5,372	5,276	(1,663)	4,945	4,311	22,869	12,869	13,687
Fuel delivered to customers
Revenue
Net revenue	$ 6,453	$ 5,786	$ 5,280	$ 4,950	5,384	(4,149)	3,440	3,491	$ 22,469	8,167	10,916
Other
Revenue
Net revenue					$ 5	$ 128	$ 64	$ 87		284	$ 884
Calculated under Revenue Guidance in Effect before Topic 606
Revenue
Net revenue										100,153
Calculated under Revenue Guidance in Effect before Topic 606 | Sales of fuel cell systems
Revenue
Net revenue										49,179
Calculated under Revenue Guidance in Effect before Topic 606 | Sale of hydrogen installations and other infrastructure
Revenue
Net revenue										13,452
Calculated under Revenue Guidance in Effect before Topic 606 | Services performed on fuel cell systems and related infrastructure
Revenue
Net revenue										16,202
Calculated under Revenue Guidance in Effect before Topic 606 | Power Purchase Agreements
Revenue
Net revenue										12,869
Calculated under Revenue Guidance in Effect before Topic 606 | Fuel delivered to customers
Revenue
Net revenue										8,167
Calculated under Revenue Guidance in Effect before Topic 606 | Other
Revenue
Net revenue										$ 284

Revenue - Contract balances (De

Revenue - Contract balances (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Revenue
Accounts receivable	$ 37,347	$ 24,179
Contract assets	788
Contract liabilities	$ 40,476
Calculated under Revenue Guidance in Effect before Topic 606
Revenue
Accounts receivable		24,179
Contract assets		1,141
Contract liabilities		$ 35,171

Revenue - Changes in contract a

Revenue - Changes in contract assets and contract liabilities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Contract assets
Transferred to receivables from contract assets recognized at the beginning of the period	$ (353)
Net change in contract assets	(353)
Contract liabilities
Revenue recognized that was included in the contract liability balance as of the beginning of the period	(10,783)
Increases due to cash received, net of amounts recognized as revenue during the period	16,088
Net change in contract liabilities	$ 5,305

Revenue - Estimated future reve

Revenue - Estimated future revenue (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Revenue
Total estimated future revenue	$ 211,373
Sales of fuel cell systems
Revenue
Total estimated future revenue	17,318
Sale of hydrogen installations and other infrastructure
Revenue
Total estimated future revenue	9,141
Services performed on fuel cell systems and related infrastructure
Revenue
Total estimated future revenue	73,381
Power Purchase Agreements
Revenue
Total estimated future revenue	$ 111,533
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01 | Maximum | Sales of fuel cell systems
Revenue
Duration of estimated revenue expected to be recognized in future (in years)	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01 | Minimum | Sales of fuel cell systems
Revenue
Duration of estimated revenue expected to be recognized in future (in years)	7 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01 | Minimum | Power Purchase Agreements
Revenue
Duration of estimated revenue expected to be recognized in future (in years)	5 years
Calculated under Revenue Guidance in Effect before Topic 606
Revenue
Total estimated future revenue		$ 249,305
Calculated under Revenue Guidance in Effect before Topic 606 | Sales of fuel cell systems
Revenue
Total estimated future revenue		26,298
Calculated under Revenue Guidance in Effect before Topic 606 | Sale of hydrogen installations and other infrastructure
Revenue
Total estimated future revenue		15,512
Calculated under Revenue Guidance in Effect before Topic 606 | Services performed on fuel cell systems and related infrastructure
Revenue
Total estimated future revenue		77,453
Calculated under Revenue Guidance in Effect before Topic 606 | Power Purchase Agreements
Revenue
Total estimated future revenue		$ 130,042

Revenue - Others (Details)

Revenue - Others (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue
Capitalized contract costs	$ 0.2	$ 0.1	$ 0

Employee Benefit Plans - Assump

Employee Benefit Plans - Assumptions For Estimating Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 23, 2014	May 12, 2011
Employee Benefit Plans
Compensation cost	$ 7.4	$ 9	$ 9
Assumptions for estimating fair value
Dividend Yield	0.00%
Minimum
Assumptions for estimating fair value
Risk free rate (as a percent):	2.81%	1.89%	1.27%
Volatility (as a percent):	98.31%	99.24%	103.87%
Maximum
Assumptions for estimating fair value
Risk free rate (as a percent):	2.88%	2.16%	1.69%
Volatility (as a percent):	98.89%	102.16%	104.88%
Stock options
Employee Benefit Plans
Number of options outstanding (in shares)	21,957,150	19,872,029
Compensation cost	$ 6.4	$ 8.6	$ 9
Stock options granted	2,679,667	5,485,863	3,702,500
Assumptions for estimating fair value
Expected term of options (years)	6 years	6 years	6 years
the 2011 Plan | Stock options
Employee Benefit Plans
Maximum number of common stock shares available for issuance				30,000,000	1,000,000
Aggregate number of options granted (in shares)	22,000,000
Number of options available for issuance (in shares)	2,300,000
Expiration period	10 years
the 2011 Plan | Stock options | Minimum
Employee Benefit Plans
Vesting period	1 year
the 2011 Plan | Stock options | Maximum
Employee Benefit Plans
Vesting period	3 years
the 2011 Plan | Employees | Stock options
Employee Benefit Plans
Vesting period	3 years
Expiration period	10 years
the 2011 Plan | Board of Directors | Stock options
Employee Benefit Plans
Vesting period	1 year

Employee Benefit Plans - Stock

Employee Benefit Plans - Stock Activity, Weighted Average Exercise Price (Details) - Stock options - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares
Options outstanding, beginning balance (in shares)	19,872,029
Granted (in shares)	2,679,667	5,485,863	3,702,500
Exercised (in shares)	(276,668)
Forfeited (in shares)	(246,378)
Expired (in shares)	(71,500)
Options outstanding, end balance (in shares)	21,957,150	19,872,029
Options exercisable (in shares)	14,964,985
Options unvested (in shares)	6,992,165
Weighted Average Exercise Price
Options outstanding, beginning balance, weighted-average exercise price	$ 2.66
Granted, weighted-average exercise price	1.96
Exercised, weighted-average exercise price	0.63
Forfeited, weighted-average exercise price	2.91
Expired, weighted-average exercise price	29.51
Options outstanding, end balance, weighted-average exercise price	2.51	$ 2.66
Options exercisable, weighted-average exercise price	2.74
Options unvested, weighted-average exercise price	$ 2.01
Stock option activity additional disclosures
Options outstanding, weighted-average remaining contractual term	7 years 1 month 6 days	7 years 8 months 12 days
Options exercisable, weighted-average remaining contractual term	6 years 3 months 18 days
Options unvested, weighted-average remaining contractual term	8 years 10 months 24 days
Options outstanding, aggregate intrinsic value	$ 1,432
Options exercisable, aggregate intrinsic value	1,431
Options unvested, aggregate intrinsic value	$ 1,467
Weighted-average grant date fair value of options granted (per share)	$ 1.55	$ 1.67	$ 1.39
Unrecognized compensation cost	$ 8,800
Period for recognition	3 years
Fair value of stock options that vested during the period	$ 7,100	$ 9,600

Employee Benefit Plans - Restri

Employee Benefit Plans - Restricted Stock Activity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Benefit Plans
Compensation cost	$ 7,400	$ 9,000	$ 9,000
Restricted stock
Employee Benefit Plans
Compensation cost	966	335	88
Unrecognized compensation cost	$ 3,900	$ 208	$ 43
Period for recognition	3 years
Shares
Unvested restricted stock, beginning balance (in shares)	234,744
Granted (in shares)	2,347,347
Vested (in shares)	(234,744)
Unvested restricted stock, end balance (in shares)	2,347,347	234,744
Aggregate Intrinsic Value
Unvested restricted stock aggregate intrinsic value	$ 2,911
Restricted stock | Minimum
Employee Benefit Plans
Vesting period	1 year
Restricted stock | Maximum
Employee Benefit Plans
Vesting period	3 years

Employee Benefit Plans - 401(K)

Employee Benefit Plans - 401(K) Saving And Retirement Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Benefit Plan Compensation
Compensation cost	$ 7,400	$ 9,000	$ 9,000
Non Employee Director
Employee Benefit Plan Compensation
Granted (in shares)	107,389	148,077	105,479
Compensation cost	$ 261	$ 276	$ 267
Savings And Retirement Plan 401 K
401(K) Savings & Retirement Plan
Percent of salary employee is permitted to contribute	100.00%
Vesting period	3 years
Common stock, shares issued	633,827
Total expense (including issuance of shares)	$ 1,800	$ 1,600	$ 1,400

Fair Value Measurements - Recur

Fair Value Measurements - Recurring Basis (Details) - Recurring basis - Warrants - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value
Financial liabilities	$ 105	$ 4,391
Level 3
Fair Value
Financial liabilities	$ 105	$ 4,391

Fair Value Measurements - Valua

Fair Value Measurements - Valuation Technique (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017USD ($)
Derivative Liabilities | Expected dividend yield
Valuation technique for assets and liabilities measured and recorded at fair value
Derivative Liability, Measurement Input	0
Minimum | Equity Instruments | Risk free interest rate
Valuation technique for assets and liabilities measured and recorded at fair value
Equity Instrument, Measurement Input	2.60	2.30
Minimum | Equity Instruments | Volatility
Valuation technique for assets and liabilities measured and recorded at fair value
Equity Instrument, Measurement Input	75	85
Minimum | Equity Instruments | Expected average term
Valuation technique for assets and liabilities measured and recorded at fair value
Equity Instrument Expected average term	8 years 3 months 4 days	9 years 3 months 4 days
Minimum | Derivative Liabilities | Risk free interest rate
Valuation technique for assets and liabilities measured and recorded at fair value
Derivative Liability, Measurement Input	1.64	1.01
Minimum | Derivative Liabilities | Volatility
Valuation technique for assets and liabilities measured and recorded at fair value
Derivative Liability, Measurement Input	18.40	43.60
Minimum | Derivative Liabilities | Expected average term
Valuation technique for assets and liabilities measured and recorded at fair value
Derivative Liability, Expected average term	4 days	1 month 21 days
Maximum | Equity Instruments | Risk free interest rate
Valuation technique for assets and liabilities measured and recorded at fair value
Equity Instrument, Measurement Input	3.02	2.47
Maximum | Equity Instruments | Volatility
Valuation technique for assets and liabilities measured and recorded at fair value
Equity Instrument, Measurement Input	85	90
Maximum | Equity Instruments | Expected average term
Valuation technique for assets and liabilities measured and recorded at fair value
Equity Instrument Expected average term	9 years 3 months 18 days	10 years
Maximum | Derivative Liabilities | Risk free interest rate
Valuation technique for assets and liabilities measured and recorded at fair value
Derivative Liability, Measurement Input	2.66	2.01
Maximum | Derivative Liabilities | Volatility
Valuation technique for assets and liabilities measured and recorded at fair value
Derivative Liability, Measurement Input	81.69	108.77
Maximum | Derivative Liabilities | Expected average term
Valuation technique for assets and liabilities measured and recorded at fair value
Derivative Liability, Expected average term	1 year 6 months 11 days	5 years 2 months 23 days

Fair Value Measurements - Level

Fair Value Measurements - Level 3 Instruments Reconciliation (Details) - Warrants - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Reconciliations of the beginning and ending balances for liabilities measured at fair value on a recurring basis using significant unobservable inputs (i.e. Level 3)
Balance at the beginning of the period	$ 4,391	$ 11,387
Change in fair value of common stock warrants	(4,286)	15,188
Issuance of common stock warrants		4,905
Exercise of common stock warrants		(27,089)
Balance at the end of the period	$ 105	$ 4,391

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value of Warrants (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Tranche one of warrants issued with the Amazon.com, Inc transaction agreement
Fair Value
Fair value per warrant	$ 1.15
Tranche two of warrants issued with the Amazon.com, Inc. Transaction Agreement
Fair Value
As of vesting date - second tranche	1.54	$ 2.16
As of period end - second tranche	0.98	2.13
Tranche one of warrants issued with the Walmart Stores Inc transaction agreement
Fair Value
Fair value per warrant	1.88
Tranche two of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Fair Value
As of period end - second tranche	$ 0.92	$ 1.92

Income Taxes - Components Of In

Income Taxes - Components Of Income Tax (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of loss before income taxes and the provision for income taxes
Loss before income taxes	$ (87,332)	$ (127,080)	$ (57,879)
Income tax benefit	9,217		392
Net loss attributable to the Company	(78,115)	(127,080)	(57,487)
significant components deferred income tax (benefit) expense
Deferred tax (benefit) expense	(9,249)	7,144	(7,719)
Net operating loss carryforward generated	(10,703)	(19,134)	(19,554)
Rate change impact on net operating loss carryforwards		23,609
Valuation allowance increase (decrease)	10,735	(11,619)	27,273
U.S.
Components of loss before income taxes and the provision for income taxes
Loss before income taxes	(85,925)	(125,871)	(56,317)
Income tax benefit	9,217
Net loss attributable to the Company	(76,708)	(125,871)	(56,317)
significant components deferred income tax (benefit) expense
Deferred tax (benefit) expense	(10,182)	7,675	(6,420)
Net operating loss carryforward generated	(10,038)	(19,117)	(16,727)
Rate change impact on net operating loss carryforwards		23,609
Valuation allowance increase (decrease)	11,003	(12,167)	23,147
Foreign
Components of loss before income taxes and the provision for income taxes
Loss before income taxes	(1,407)	(1,209)	(1,562)
Income tax benefit			392
Net loss attributable to the Company	(1,407)	(1,209)	(1,170)
significant components deferred income tax (benefit) expense
Deferred tax (benefit) expense	933	(531)	(1,299)
Net operating loss carryforward generated	(665)	(17)	(2,827)
Valuation allowance increase (decrease)	$ (268)	$ 548	$ 4,126

Income Taxes - Effective Income

Income Taxes - Effective Income Tax Rate Reconciliation (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective income tax rate reconciliation
U.S. Federal statutory tax rate	(21.00%)	(35.00%)	(35.00%)
Deferred state taxes	(1.90%)	(1.40%)	(3.10%)
Common stock warrant liability	(1.00%)	4.20%	(2.60%)
Provision to return and deferred tax asset adjustments		5.90%	(2.90%)
Change in unrecognized tax benefits			(0.70%)
Change in U.S. Federal statutory tax rate		33.50%
Other, net	0.40%	2.00%	(1.60%)
Change in valuation allowance	12.90%	(9.20%)	45.20%
Total effective income tax rate	(10.60%)	0.00%	(0.70%)

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets And Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Significant components of the Company's deferred tax assets and liabilities
Intangible assets	$ 1,146	$ 1,698
Deferred revenue	9,304	8,083
Interest expense	5,239
Other reserves and accruals	592	850
Tax credit carryforwards	3,065	2,682
Amortization of stock-based compensation	8,442	6,904
Non-Compensatory Warrants	3,597	4,555
Capitalized research & development expenditures	23,410	19,162
Net operating loss carryforwards	58,364	48,078
Total deferred tax asset	113,159	92,012
Valuation allowance	(100,493)	(89,758)
Net deferred tax assets	12,666	2,254
Intangible assets	(37)	(76)
Convertible debt	(9,217)
Property, plant and equipment and right of use assets	(3,392)	(1,969)
Section 382 recognized built in loss	0	(209)
Deferred tax liability	(12,666)	(2,254)
U.S.
Significant components of the Company's deferred tax assets and liabilities
Deferred revenue	9,304	8,083
Interest expense	5,239
Other reserves and accruals	592	850
Tax credit carryforwards	1,865	1,378
Amortization of stock-based compensation	8,442	6,904
Non-Compensatory Warrants	3,597	4,555
Capitalized research & development expenditures	19,116	14,496
Net operating loss carryforwards	49,058	39,437
Total deferred tax asset	97,213	75,703
Valuation allowance	(84,567)	(73,564)
Net deferred tax assets	12,646	2,139
Intangible assets	(37)	(76)
Convertible debt	(9,217)
Property, plant and equipment and right of use assets	(3,392)	(1,854)
Section 382 recognized built in loss		(209)
Deferred tax liability	(12,646)	(2,139)
Foreign
Significant components of the Company's deferred tax assets and liabilities
Intangible assets	1,146	1,698
Tax credit carryforwards	1,200	1,304
Capitalized research & development expenditures	4,294	4,666
Net operating loss carryforwards	9,306	8,641
Total deferred tax asset	15,946	16,309
Valuation allowance	(15,926)	(16,194)
Net deferred tax assets	20	115
Property, plant and equipment and right of use assets		(115)
Deferred tax liability	$ (20)	$ (115)

Income Taxes - Change In Valuat

Income Taxes - Change In Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in valuation allowance
Increase in valuation allowance for current year increase in net operating losses	$ 10,502
Increase (decrease) in valuation allowance for current year net increase (decrease) in deferred tax assets other than net operating losses	1,384
Decrease in valuation allowance as a result of foreign currency fluctuation	(451)
Decrease in valuation allowance due to change in tax rates	(700)
Net decrease (increase) in valuation allowance	10,735	$ (11,619)	$ 27,273
U.S.
Change in valuation allowance
Increase in valuation allowance for current year increase in net operating losses	10,038
Increase (decrease) in valuation allowance for current year net increase (decrease) in deferred tax assets other than net operating losses	1,665
Decrease in valuation allowance due to change in tax rates	(700)
Net decrease (increase) in valuation allowance	11,003	(12,167)	23,147
Foreign
Change in valuation allowance
Increase in valuation allowance for current year increase in net operating losses	464
Increase (decrease) in valuation allowance for current year net increase (decrease) in deferred tax assets other than net operating losses	(281)
Decrease in valuation allowance as a result of foreign currency fluctuation	(451)
Net decrease (increase) in valuation allowance	$ (268)	$ 548	$ 4,126

Income Taxes - Unrecognized Tax

Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2017
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Unrecognized tax benefits balance at beginning of year		$ 437
Reductions for tax positions of prior years		(469)
Increases for currency translations		$ 32
Pre-change net operating losses that can be used in future years	$ 13,500
Net operating losses not subject to IRC section 382	195,000
Deferred tax assets, U.S. net operating loss carryforwards	208,000
Unused net operating loss carryforwards	$ 42,000
Unrecognized loss recognition period	5 years
Unrecognized loss from change in ownership that should be recognized during the recognition period	$ 900
Gross deferred tax liability	0		$ 209
Amount of net operating loss carryforwards that will expire due to IRC Section 382 limitations	1,900
Net operating loss carryforwards	58,364		48,078
Un-repatriated foreign earnings	0
Canada
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Research and experimental development expenditure carryforwards	16,500
Canadian ITC credit carryforwards			1,200
U.S.
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Gross deferred tax liability			209
Net operating loss carryforwards	49,058		39,437
Foreign
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Net operating loss carryforwards	9,306		$ 8,641
Foreign | French
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Unused net operating loss carryforwards	17,600
Foreign | Canada
Reconciliation of the beginning and ending amount of unrecognized tax benefits
Net operating loss carryforwards	$ 13,600

Income Taxes - Tax cuts and job

Income Taxes - Tax cuts and jobs act (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes
U.S. Federal statutory tax rate	21.00%	35.00%	35.00%
Write down of deferred tax assets	$ 42.5
Reduction in valuation allowance	$ 42.5
Reduction in tax benefit due to write down of deferred tax assets	33.50%
Valuation allowance rate offsetting tax rate reduction	33.50%

Commitments and Contingencies -

Commitments and Contingencies - Operating Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Future minimum lease payments under noncancellable operating leases - As Lessor
2019	$ 27,234
2020	25,503
2021	20,781
2022	12,044
2023	8,146
2024 and thereafter	7,946
Total future minimum lease payments	101,654
Future minimum lease payments under noncancellable operating leases - As Lessee
2019	22,582
2020	21,255
2021	16,512
2022	11,060
2023	10,325
2024 and thereafter	13,352
Total future minimum lease payments	95,086
Less imputed lease interest	(24,171)
Operating lease obligation	70,915
Rental expense for all operating lease	15,800	$ 13,400
Gross profit on sale leaseback transactions	16,400
Right of use assets obtained in exchange for new operating lease liabilities	42,200
Prepaid rent and security deposit	6,800	8,300
Other information of operating leases
Cash payments	$ 14,926
Weighted average remaining lease term (in years)	4 years 4 months 10 days
Weighted average discount rate (as a percent)	12.10%
Maximum
Operating Leases
Lease Term - as Lessor	7 years
Lease Term - as Lessee	7 years
Minimum
Operating Leases
Lease Term - as Lessor	1 year
Lease Term - as Lessee	1 year
Sale Leaseback Agreements
Future minimum lease payments under noncancellable operating leases - As Lessee
Operating lease obligation	$ 37,000	10,400
Finance obligations	96,890
Short term operating lease obligation	5,700	2,500
Long term operating lease obligation	$ 31,300	$ 7,900

Commitments and Contingencies_2

Commitments and Contingencies - Finance Leases (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Finance lease costs
Right of use assets obtained in exchange for new finance lease liabilities	$ 2,200
Restricted Cash
Leased property, net	146,751	$ 87,065
Sale Leaseback Agreements
Total Payments of future minimum lease payments
2019	59,151
2020	6,722
2021	6,722
2022	4,975
2023	3,166
2024 and thereafter	16,154
Total future minimum lease payments	96,890
Less imputed lease interest	(14,986)
Total lease liabilities	81,904
Finance lease costs
Amortization of right of use asset	7,549
Interest on finance obligations	6,908
Total finance lease cost	14,457
Other information
Cash payments	$ 33,715
Weighted average remaining lease term (years)	3 years 2 months 1 day
Weighted average discount rate	11.80%
Restricted Cash
Restricted cash	$ 34,300
Letter of credit	36,800
Finance obligation
Restricted Cash
Letter of credit	500
Finance obligation | Property and equipment
Total Payments of future minimum lease payments
Total future minimum lease payments	2,500
ASU 2016-02 | Restatement adjustment
Restricted Cash
Leased property, net	$ 1,500

Commitments and Contingencies_3

Commitments and Contingencies - Concentrations of Credit Risk (Details) - item	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounts receivable | Credit risk | Customers
Customer Concentration
Concentration Risk Number of Customers	3	3
Concentration risk (as a percent)	52.30%	59.00%
Revenues | Customer concentration
Customer Concentration
Concentration Risk Number of Customers	2	2	2
Concentration risk (as a percent)	66.70%	71.80%	34.10%

Unaudited Quarterly Financial_3

Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net revenue:
Net revenue	$ 59,820	$ 53,165	$ 35,229	$ 26,418	$ 31,095	$ 34,593	$ 20,007	$ 14,458
Gross (loss) profit	4,507	4,409	(2,310)	(3,984)	(657)	(19,410)	(3,543)	(4,479)	$ 2,622	$ (28,089)	$ 3,946
Operating expenses	17,426	17,054	20,668	16,957	17,060	16,971	24,529	15,143	72,105	73,703	55,465
Operating loss	(12,919)	(12,645)	(22,978)	(20,941)	(17,717)	(36,381)	(28,072)	(19,622)	(69,483)	(101,792)	(51,519)
Net loss attributable to common shareholders	$ (16,860)	$ (15,578)	$ (25,881)	$ (19,848)	$ (19,451)	$ (41,008)	$ (42,645)	$ (27,074)	$ (78,167)	$ (130,178)	$ (57,591)
Loss per share:
Basic and diluted (in dollars per share)	$ (0.08)	$ (0.07)	$ (0.12)	$ (0.09)	$ (0.09)	$ (0.18)	$ (0.19)	$ (0.14)	$ (0.36)	$ (0.60)	$ (0.32)
Tranche one of warrants issued with the Amazon.com, Inc transaction agreement
Net revenue:
Operating expenses							$ 7,100
Sales of fuel cell systems and related infrastructure
Net revenue:
Net revenue	$ 41,191	$ 36,668	$ 18,820	$ 10,613	$ 14,724	$ 38,060	7,650	$ 2,197	$ 107,292	$ 62,631	$ 39,985
Services performed on fuel cell systems and related infrastructure
Net revenue:
Net revenue	5,672	5,156	5,691	5,483	5,706	2,217	3,908	4,372	22,002	16,202	17,347
Power Purchase Agreements
Net revenue:
Net revenue	6,504	5,555	5,438	5,372	5,276	(1,663)	4,945	4,311	22,869	12,869	13,687
Fuel delivered to customers
Net revenue:
Net revenue	$ 6,453	$ 5,786	$ 5,280	$ 4,950	5,384	(4,149)	3,440	3,491	$ 22,469	8,167	10,916
Other
Net revenue:
Net revenue					$ 5	$ 128	$ 64	$ 87		$ 284	$ 884