Enphase Energy (ENPH) 10-Q 27 Apr 21 2021 Q1 Quarterly report Financial data

COVER PAGE

COVER PAGE - shares	3 Months Ended
	Mar. 31, 2021	Apr. 23, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2021
Document Transition Report	false
Entity File Number	001-35480
Entity Registrant Name	Enphase Energy, Inc.
Entity Incorporation, State	DE
Entity Tax Identification Number	20-4645388
Entity Address, Address Line One	47281 Bayside Parkway
Entity Address, City or Town	Fremont
Entity Address, State or Province	CA
Entity Address, Postal Zip Code	94538
City Area Code	877
Local Phone Number	774-7000
Title of 12(b) Security	Common Stock, $0.00001 par value per share
Trading Symbol	ENPH
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		135,698,889
Entity Central Index Key	0001463101
Amendment Flag	false
Document Fiscal Year Focus	2021
Current Fiscal Year End Date	--12-31
Document Fiscal Period Focus	Q1

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Current assets:
Cash and cash equivalents	$ 1,489,010	$ 679,379
Accounts receivable, net of allowances of $746 and $462 at March 31, 2021 and December 31, 2020, respectively	236,090	182,165
Inventory	34,876	41,764
Prepaid expenses and other assets	31,386	29,756
Total current assets	1,791,362	933,064
Property and equipment, net	53,648	42,985
Operating lease, right of use asset, net	16,688	17,683
Intangible assets, net	47,917	28,808
Goodwill	61,038	24,783
Other assets	91,315	59,875
Deferred tax assets, net	132,231	92,904
Total assets	2,194,199	1,200,102
Current liabilities:
Accounts payable	81,524	72,609
Accrued liabilities	115,172	76,542
Deferred revenues, current	49,118	47,665
Warranty obligations, current (includes $10,163 and $8,267 measured at fair value at March 31, 2021 and December 31, 2020, respectively)	14,303	11,260
Debt, current	84,356	325,967
Total current liabilities	344,473	534,043
Long-term liabilities:
Deferred revenues, noncurrent	142,985	125,473
Warranty obligations, noncurrent (includes $23,156 and $20,469 measured at fair value at March 31, 2021 and December 31, 2020, respectively)	40,250	34,653
Other liabilities	15,777	17,042
Debt, noncurrent	917,873	4,898
Total liabilities	1,461,358	716,109
Commitments and contingencies (Note 9)
Stockholders’ equity:
Common stock, $0.00001 par value, 200,000 shares and 200,000 shares authorized; and 135,691 shares and 128,962 shares issued and outstanding at March 31, 2021 and December 31, 2020, respectively	1	1
Additional paid-in capital	751,688	534,744
Accumulated deficit	(19,488)	(51,186)
Accumulated other comprehensive income	640	434
Total stockholders’ equity	732,841	483,993
Total liabilities and stockholders’ equity	$ 2,194,199	$ 1,200,102

CONDENSED CONSOLIDATED BALANC_2

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Accounts receivable, allowance	$ 746	$ 462
Product warranty, measured at fair value, current	10,163	8,267
Product warranty, measured at fair value, noncurrent	$ 23,156	$ 20,469
Common stock, par value (in usd per share)	$ 0.00001	$ 0.00001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	135,691,000	128,962,000
Common stock, shares outstanding (in shares)	135,691,000	128,962,000

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Statement [Abstract]
Net revenues	$ 301,754	$ 205,545
Cost of revenues	178,805	124,870
Gross profit	122,949	80,675
Operating expenses:
Research and development	21,818	11,876
Sales and marketing	19,622	11,772
General and administrative	20,123	12,315
Total operating expenses	61,563	35,963
Income from operations	61,386	44,712
Other income (expense), net
Interest income	73	1,091
Interest expense	(7,329)	(3,155)
Other (expense) income, net	573	(924)
Loss on partial repurchase of convertible notes	56,369	0
Change in fair value of derivatives	0	15,344
Total other income (expense), net	(63,052)	12,356
Income (loss) before income taxes	(1,666)	57,068
Income tax benefit	33,364	11,868
Net income	$ 31,698	$ 68,936
Net income per share:
Basic (in USD per share)	$ 0.24	$ 0.56
Diluted (in USD per share)	$ 0.22	$ 0.50
Shares used in per share calculation:
Basic (in shares)	131,303	123,531
Diluted (in shares)	146,442	138,104

CONDENSED CONSOLIDATED STATEM_2

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Statement of Comprehensive Income [Abstract]
Net income	$ 31,698	$ 68,936
Other comprehensive income (loss):
Foreign currency translation adjustments	206	(168)
Comprehensive income	$ 31,904	$ 68,768

CONDENSED CONSOLIDATED STATEM_3

CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock and paid-in capital	Accumulated deficit	Accumulated other comprehensive income (loss)
Balance, beginning of period at Dec. 31, 2019		$ 458,316	$ (185,181)	$ (923)
Issuance of common stock from exercise of equity awards and employee stock purchase plan		1,979
Payment of withholding taxes related to net share settlement of equity awards		(34,267)
Stock-based compensation expense		7,515
Net income	$ 68,936		68,936
Foreign currency translation adjustments	(168)			(168)
Balance, end of period at Mar. 31, 2020	316,207	433,543	(116,245)	(1,091)
Balance, beginning of period at Dec. 31, 2020	483,993	534,745	(51,186)	434
Issuance of common stock from exercise of equity awards and employee stock purchase plan		214
Payment of withholding taxes related to net share settlement of equity awards		(9,185)
Equity component of convertible notes issued, net of tax		207,962
Cost of convertible notes hedge related to the convertible notes issued, net of tax		(213,322)
Sale of warrants related to the convertible notes issued		220,800
Equity component of partial settlement of convertible notes		(966,483)
Cost of reacquired equity component on partial settlement of convertible notes		962,114
Stock-based compensation expense		14,844
Net income	31,698		31,698
Foreign currency translation adjustments	206			206
Balance, end of period at Mar. 31, 2021	$ 732,841	$ 751,689	$ (19,488)	$ 640

CONDENSED CONSOLIDATED STATEM_4

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities:
Net income	$ 31,698	$ 68,936
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	5,558	3,844
Provision for doubtful accounts	14	104
Non-cash interest expense	7,156	2,722
Loss on partial settlement of convertibles notes	56,369	0
Deemed repayment of convertible notes attributable to accreted debt discount	(15,579)	0
Change in fair value of debt security	(1,437)	0
Stock-based compensation	14,844	7,515
Change in fair value of derivatives	0	(15,344)
Deferred income taxes	(35,367)	(12,500)
Changes in operating assets and liabilities:
Accounts receivable	(53,719)	49,637
Inventory	6,888	(2,560)
Prepaid expenses and other assets	(5,040)	(5,009)
Accounts payable, accrued and other liabilities	36,376	(22,066)
Warranty obligations	8,640	403
Deferred revenues	19,440	(36,460)
Net cash provided by operating activities	75,841	39,222
Cash flows from investing activities:
Purchases of property and equipment	(9,940)	(3,353)
Investment in a private company	(25,000)	0
Business acquisitions, net of cash acquired	(55,239)	0
Net cash used in investing activities	(90,179)	(3,353)
Cash flows from financing activities:
Issuance of convertible notes, net of issuance costs	1,189,388	313,011
Purchase of convertible note hedges	(286,235)	(89,056)
Sale of warrants	220,800	71,552
Principal payments and financing fees on debt	(1,078)	(1,148)
Partial repurchase of convertible notes	(289,233)	0
Proceeds from exercise of equity awards and employee stock purchase plan	214	1,979
Payment of withholding taxes related to net share settlement of equity awards	(9,185)	(34,267)
Net cash provided by financing activities	824,671	262,071
Effect of exchange rate changes on cash and cash equivalents	(702)	(205)
Net increase in cash, cash equivalents and restricted cash	809,631	297,735
Cash, cash equivalents and restricted cash—Beginning of period	679,379	296,109
Cash. cash equivalents and restricted cash—End of period	1,489,010	593,844
Supplemental disclosures of non-cash investing and financing activities:
Total cash, cash equivalents, and restricted cash	1,489,010	593,844
Purchases of fixed assets included in accounts payable	7,301	585
Contingent consideration in connection with the acquisition	3,500	0
Convertible senior note issuance costs included in accounts payable and accrued expense	$ 991	$ 591

DESCRIPTION OF BUSINESS AND BAS

DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION	DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION Description of Business Enphase Energy, Inc. (the “Company”) is a global energy technology company. The Company delivers smart, easy-to-use solutions that manage solar generation, storage and communication on one platform. The Company revolutionized the solar industry with its microinverter technology and produces a fully integrated solar-plus-storage solution. Basis of Presentation and Consolidation The accompanying condensed consolidated financial statements are presented in accordance with accounting principles generally accepted in the United States (“U.S.”), or GAAP. The condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation. Unaudited Interim Financial Information These accompanying unaudited condensed consolidated financial statements have been prepared pursuant to the rules and regulations of the SEC for interim financial reporting. In the opinion of management, these unaudited condensed consolidated financial statements reflect all adjustments, consisting of normal recurring items, considered necessary to present fairly the Company’s financial condition, results of operations, comprehensive income, stockholders’ equity and cash flows for the interim periods indicated. The results of operations for the three months ended March 31, 2021 are not necessarily indicative of the operating results for the full year. Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of income and expenses during the reporting period. Significant estimates and assumptions reflected in the financial statements include revenue recognition, allowance for doubtful accounts, stock-based compensation, inventory valuation, accrued warranty obligations, fair value of investments, debt derivatives, convertible notes and contingent consideration, fair value of acquired intangible assets and goodwill, useful lives of acquired intangible assets and property and equipment, incremental borrowing rate for right-of-use assets and lease liability, legal contingencies, and tax valuation allowance. These estimates are based on information available as of the date of the financial statements; therefore, actual results could differ materially from management’s estimates using different assumptions or under different conditions. In light of the ongoing and quickly evolving COVID-19 pandemic, management has considered the impacts of the COVID-19 pandemic on the Company’s critical and significant accounting estimates and as of the date of issuance of these financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, judgments or revise the carrying value of its assets or liabilities. These estimates may change, as new events occur and additional information is obtained, and are recognized in the consolidated financial statements as soon as they become known. Actual results could differ from those estimates and any such differences may be material to the Company’s condensed financial statements. Summary of Significant Accounting Policies There have been no significant changes to the Company’s significant accounting policies in Note 2, “Summary of Significant Accounting Policies,” of the notes to consolidated financial statements included in Part II, Item 8 of the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2020.

REVENUE RECOGNITION

REVENUE RECOGNITION	3 Months Ended
	Mar. 31, 2021
Revenue from Contract with Customer [Abstract]
REVENUE RECOGNITION	REVENUE RECOGNITION Disaggregated Revenue The Company has one business activity, which is the design, manufacture and sale of solutions for the solar photovoltaic (“PV”) industry. Disaggregated revenue by primary geographical market and timing of revenue recognition for the Company’s single product line are as follows: Three Months Ended 2021 2020 (In thousands) Primary geographical markets: U.S. $ 247,782 $ 179,600 International 53,972 25,945 Total $ 301,754 $ 205,545 Timing of revenue recognition: Products delivered at a point in time $ 288,871 $ 194,679 Products and services delivered over time 12,883 10,866 Total $ 301,754 $ 205,545 Contract Balances Receivables, and contract assets and contract liabilities from contracts with customers are as follows: March 31, December 31, (In thousands) Receivables $ 236,090 $ 182,165 Short-term contract assets (Prepaid expenses and other assets) 19,338 17,879 Long-term contract assets (Other assets) 57,400 51,986 Short-term contract liabilities (Deferred revenues) 49,118 47,665 Long-term contract liabilities (Deferred revenues) 142,985 125,473 The Company receives payments from customers based upon contractual billing schedules. Accounts receivable are recorded when the right to consideration becomes unconditional. Contract assets include deferred product costs and commissions associated with the deferred revenue and will be amortized along with the associated revenue. The Company had no asset impairment charges related to contract assets in the three months ended March 31, 2021. Significant changes in the balances of contract assets (prepaid expenses and other assets) during the period are as follows (in thousands): Contract Assets Contract Assets, beginning of period $ 69,865 Amount recognized (4,955) Increase 11,828 Contract Assets, end of period $ 76,738 Contract liabilities are recorded as deferred revenue on the accompanying condensed consolidated balance sheets and include payments received in advance of performance obligations under the contract and are realized when the associated revenue is recognized under the contract. Significant changes in the balances of contract liabilities (deferred revenues) during the period are as follows (in thousands): Contract Liabilities Contract Liabilities, beginning of period $ 173,138 Revenue recognized (16,235) Increase due to billings 35,200 Contract Liabilities, end of period $ 192,103 Remaining Performance Obligations Estimated revenue expected to be recognized in future periods related to performance obligations that are unsatisfied or partially unsatisfied at the end of the reporting period are as follows: March 31, (In thousands) Fiscal year: 2021 (remaining nine months) $ 38,116 2022 44,067 2023 38,220 2024 32,994 2025 26,021 Thereafter 12,685 Total $ 192,103

OTHER FINANCIAL INFORMATION

OTHER FINANCIAL INFORMATION	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
OTHER FINANCIAL INFORMATION	OTHER FINANCIAL INFORMATION Inventory Inventory consist of the following: March 31, December 31, (In thousands) Raw materials $ 10,611 $ 10,140 Finished goods 24,265 31,624 Total inventory $ 34,876 $ 41,764 Accrued Liabilities Accrued liabilities consist of the following: March 31, December 31, (In thousands) Salaries, commissions, incentive compensation and benefits $ 17,466 $ 6,634 Customer rebates and sales incentives 55,397 36,622 Freight 11,691 10,300 Operating lease liabilities, current 4,772 4,542 Liability due to supply agreements 4,310 5,500 Contingent consideration 4,193 — Other 17,343 12,944 Total accrued liabilities $ 115,172 $ 76,542

BUSINESS COMBINATION

BUSINESS COMBINATION	3 Months Ended
	Mar. 31, 2021
Business Combinations [Abstract]
BUSINESS COMBINATION	BUSINESS COMBINATION Acquisition of Sofdesk Inc. (“Sofdesk”) On January 25, 2021, the Company completed the acquisition of 100% of the shares of Sofdesk, a privately-held company. Sofdesk provides design tools and services software for residential solar installers and roofing companies and will enhance the Company’s digital transformation efforts. As part of the purchase price, the Company (i) paid approximately $32.0 million in cash on January 25, 2021 and (ii) is liable for up to approximately $3.7 million of contingent consideration payable during the first quarter of 2022, of which the Company recorded a liability of approximately $3.5 million representing the fair value of the contingent consideration. The contingent consideration is subject to remeasurement at each reporting period until paid. The acquisition date fair value of the purchase price was approximately $35.5 million, which consisted of the following (in thousands): Cash consideration $ 31,988 Fair value of contingent consideration 3,500 Total $ 35,488 In addition to the purchase price discussed above, the Company will be obligated to pay up to approximately $3.7 million, during the first quarter of 2022, subject to continued employment of key employees of Sofdesk. As this payment is contingent upon the continuous service of the employees, it is being accounted for as a post-combination expense and will be recognized ratably over the one year period. The acquisition has been accounted for as a business combination under the acquisition method, and accordingly, the total purchase price is allocated to the tangible and identifiable intangible assets acquired and liabilities assumed based on their respective fair values on the acquisition date. The results of operations of Sofdesk have been included in the Company’s condensed consolidated statement of operations from the acquisition date, though Sofdesk’s results of operations was not material for the three months ended March 31, 2021. The following table summarizes the preliminary estimated fair values of the assets acquired and liabilities assumed at the acquisition date, which are subject to change within the measurement period as the fair value assessments are finalized (in thousands): Net tangible assets acquired $ 1,441 Intangible assets 9,200 Deferred tax asset 457 Goodwill 24,390 Net assets acquired $ 35,488 The excess of the consideration paid over the fair values assigned to the assets acquired and liabilities assumed represents the goodwill resulting from the acquisition. Goodwill is primarily attributable to expected synergies in the Company’s solar offerings and cross-selling opportunities. Intangible assets consist primarily of developed technology, customer relationship intangibles and trade name intangibles. Intangible assets attributable to developed technology include a combination of unpatented technology, trade secrets, computer software and research processes that represent the foundation for the existing and planned new products to facilitate the generation of new content. Customer relationship intangibles relate to Sofdesk’s software ability to sell current and future offerings, as well as products built around the current offering, to its existing customers. Trade name intangibles are attributable to marketing goods and services under the Solargraf TM and Roofgraf TM brands. The following table shows the fair value of the separately identifiable intangible assets at the time of acquisition and the period over which each intangible asset will be amortized: Preliminary Fair Value Useful Life (In thousands) (Years) Developed technology $ 6,900 5 Customer relationship 1,800 5 Trade Name 500 5 Total identifiable intangible assets $ 9,200 The Company incurred costs related to this acquisition of $1.7 million that were recorded in general and administrative expenses in the accompanying condensed consolidated statements of operations for the three months ended March 31, 2021. Acquisition of DIN’s Solar Design Services Business (“DIN”) On March 31, 2021, the Company completed its acquisition of DIN’s solar design services business. DIN's solar design services business provides outsourced proposal drawings and permit plan sets for residential solar installers in North America and will enhance the Company’s digital transformation effort. As part of the purchase price, the Company paid approximately $24.8 million in cash at closing on March 31, 2021. The acquisition has been accounted for as a business combination under the acquisition method; accordingly, the total purchase price is allocated to the tangible and identifiable intangible assets acquired and liabilities assumed based on their respective fair values on the acquisition date. In addition to the purchase price summarized above, the Company will be obligated to pay up to i) approximately $5.0 million in equal monthly installments over the course of one year following the acquisition date and ii) approximately $5.0 million payable on the one year anniversary following the acquisition date subject to achievement of certain revenue and operational targets. As both the additional payments require continuous employment of certain key employees of DIN and are subject to other conditions, these payments are being accounted for as post-combination expense and will be recognized ratably over the one year period. The following table summarizes the preliminary estimated fair values of the assets acquired and liabilities assumed at the acquisition date, which are subject to change within the measurement period as the fair value assessments are finalized (in thousands): Net tangible assets acquired $ 1,541 Intangible assets 11,700 Goodwill 11,544 Net assets acquired $ 24,785 The excess of the consideration paid over the fair values assigned to the assets acquired and liabilities assumed represents the goodwill resulting from the acquisition. Goodwill is primarily attributable to expected synergies in the Company’s solar offerings and cross-selling opportunities. Intangible assets consist primarily of customer relationship intangibles. Customer relationship intangibles relate to the ability of the acquired DIN solar design services business to sell current and future offering, as well as products built around the current offering, to its existing customers. The following table shows the fair value of the separately identifiable intangible assets at the time of acquisition and the period over which each intangible asset will be amortized: Preliminary Fair Value Useful Life (In thousands) (Years) Customer relationship $ 11,700 5 The Company incurred costs related to this acquisition of $1.4 million that were recorded in general and administrative expenses in the accompanying condensed consolidated statements of operations for the three months ended March 31, 2021.

GOODWILL AND INTANGIBLE ASSETS

GOODWILL AND INTANGIBLE ASSETS	3 Months Ended
	Mar. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
GOODWILL AND INTANGIBLE ASSETS	The Company’s goodwill and purchased intangible assets as of March 31, 2021 and December 31, 2020 are as follows: March 31, 2021 December 31, 2020 Gross Additions Accumulated Amortization Net Gross Additions Accumulated Amortization Net (In thousands) Goodwill $ 24,783 $ 36,255 $ — $ 61,038 $ 24,783 $ — $ — $ 24,783 Intangible assets: Other indefinite-lived intangibles 286 — — 286 286 — — 286 Intangible assets with finite lives: Developed technology 13,100 6,900 (6,075) 13,925 13,100 — (5,276) 7,824 Customer relationships 26,421 13,500 (6,653) 33,268 23,100 3,321 (5,723) 20,698 Trade names — 500 (62) 438 — — — — Total purchased intangible assets $ 39,807 $ 20,900 $ (12,790) $ 47,917 $ 36,486 $ 3,321 $ (10,999) $ 28,808 Amortization expense related to finite-lived intangible assets are as follows: Three Months Ended 2021 2020 (In thousands) Developed technology $ 799 $ 546 Customer relationships 930 701 Trade names 62 — Total amortization expense $ 1,791 $ 1,247 Amortization of developed technology, customer relationships and trade names is recorded to sales and marketing expense.

WARRANTY OBLIGATIONS

WARRANTY OBLIGATIONS	3 Months Ended
	Mar. 31, 2021
Product Warranties Disclosures [Abstract]
WARRANTY OBLIGATIONS	WARRANTY OBLIGATIONS The Company’s warranty activities were as follows: Three Months Ended 2021 2020 (In thousands) Warranty obligations, beginning of period $ 45,913 $ 37,098 Accruals for warranties issued during period 3,894 1,524 Changes in estimates 7,655 1,677 Settlements (2,930) (3,270) Increase due to accretion expense 943 774 Other (922) (302) Warranty obligations, end of period 54,553 37,501 Less: current portion (14,303) (9,678) Noncurrent $ 40,250 $ 27,823 Changes in Estimates In the three months ended March 31, 2021 and 2020, the Company recorded a $6.3 million and $1.5 million, respectively, increase to warranty expense based on continuing analysis of field performance data and diagnostic root-cause failure analysis primarily relating to its prior generation products. The Company also recorded additional warranty expense of $1.3 million and $0.2 million in the three months ended March 31, 2021 and 2020, respectively, related to unit costs for prior generation microinverter replacement mainly driven by tariffs.

FAIR VALUE MEASUREMENTS

FAIR VALUE MEASUREMENTS	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS The accounting guidance defines fair value as the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. When determining the fair value measurements for assets and liabilities recorded at fair value, the Company considers the principal or most advantageous market in which it would transact and considers assumptions that market participants would use when pricing the asset or liability, such as inherent risk, transfer restrictions, and risk of nonperformance. The fair value hierarchy requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. An asset’s or liability’s categorization within the fair value hierarchy is based upon the lowest level of input that is significant to the fair value measurement. Three levels of inputs may be used to measure fair value: • Level 1 - Valuations based on quoted prices in active markets for identical assets or liabilities that the Company is able to access. Since valuations are based on quoted prices that are readily and regularly available in an active market, valuation of such assets or liabilities do not entail a significant degree of judgment. • Level 2 - Valuations based on one or more quoted prices in markets that are not active or for which all significant inputs are observable, either directly or indirectly. • Level 3 - Valuations based on inputs that are unobservable and significant to the overall fair value measurement. March 31, 2021 December 31, 2020 (In thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Assets: Cash and cash equivalents: Money market funds $ 1,463,750 $ — $ — $ 654,699 $ — $ — Other assets Investment in debt security — — 26,437 — — — Total assets measured at fair value $ 1,463,750 $ — $ 26,437 $ 654,699 $ — $ — Liabilities: Accrued liabilities Contingent consideration $ — $ — $ 3,540 $ — $ — $ — Warranty obligations Current — — 10,163 — — 8,267 Non-current — — 23,156 — — 20,469 Total warranty obligations measured at fair value — — 33,319 — — 28,736 Total liabilities measured at fair value $ — $ — $ 36,859 $ — $ — $ 28,736 Level 1. The Company considers all highly liquid investments, such as certificates of deposit and money market instruments with maturities of three months or less at the time of acquisition to be cash equivalents. For all periods presented, its cash balances consist of amounts held in non-interest-bearing and interest-bearing deposits and money market accounts and are within Level 1 of the fair value hierarchy because they are valued using quoted market prices for identical instruments in active markets. Level 2. Notes due 2028 , Notes due 2026 , Notes due 2025 and Notes due 2024. The Company carries the Notes due 2028, Notes due 2026, Notes due 2025 and Notes due 2024 (as defined below) at face value less unamortized discount and issuance costs on its condensed consolidated balance sheets. The fair value of the Notes due 2028, Notes due 2026, Notes due 2025 and Notes due 2024 was $538.3 million, $596.1 million, $218.7 million and $9.1 million, respectively, as of March 31, 2021 based on the closing trading prices per $100 principal amount as of the last day of trading for the period. The Company considers the fair value of the Notes due 2028, Notes due 2026, Notes due 2025 and Notes due 2024 to be a Level 2 measurement as they are not actively traded. Level 3. Investment in debt security. In January 2021, the Company invested approximately $25.0 million in a privately-held company. The Company concluded the investment qualifies as an investment in a debt security as it accrues interest and principal plus accrued interest become payable back to the Company at certain dates unless it is converted to equity at a pre-determined price. As the investment includes a conversion option, the Company has elected to account for this investment under the fair value option and any change in fair value of the investment is recognized in “Other income (expense)”, net in the Company’s condensed consolidated statement of operations for that period. Further, the Company has concluded that the Company’s investment in debt security is considered to be a Level 3 measurement due to the use of significant unobservable inputs in the valuation model. These assumption include implied yield and change in estimated term of investment being held-to-maturity. Three Months Ended 2021 (In thousands) Initial investment (January 2021) $ 25,000 Fair value adjustments included in other income (expenses), net 1,437 Balance at end of period $ 26,437 Contingent consideration. The estimated fair value of the contingent consideration incurred in connection with the Company’s acquisition of Sofdesk is considered to be a Level 3 measurement due to the use of significant unobservable inputs. These unobservable inputs include probability assessment of expected future customer count over the period in which the obligation is expected to be settled. The value was determined using a discounted risk-neutral expected (probability-weighted) cash flow methodology. The resulting expected contingent consideration payment is discounted back to present value using our cost of debt. The fair value of contingent consideration arrangement is reassessed quarterly based on assumptions used in the Company’s latest projections and input provided by management. Any change in the fair value estimate, which could include accretion of interest expense due to passage of time as well as any changes in the inputs to the model, is recorded in the Company’s condensed consolidated statement of operations for that period. The following table reflects the activity for the Company’s contingent consideration liabilities measured at fair value using Level 3 inputs for the three months ended March 31, 2021 (in thousands): Three Months Ended 2021 (In thousands) Balance at acquisition (January 25, 2021) $ 3,500 Fair value adjustments included in other income (expense), net 40 Balance at end of period $ 3,540 Warranty obligations. Fair Value Option for Warranty Obligations Related to Microinverters and Other Products Sold Since January 1, 2014 The Company estimates the fair value of warranty obligations by calculating the warranty obligations in the same manner as for sales prior to January 1, 2014 and applying an expected present value technique to that result. The expected present value technique, an income approach, converts future amounts into a single current discounted amount. In addition to the key estimates of failure rates, claim rates and replacement costs, the Company used certain Level 3 inputs which are unobservable and significant to the overall fair value measurement. Such additional assumptions included a discount rate based on the Company’s credit-adjusted risk-free rate and compensation comprised of a profit element and risk premium required of a market participant to assume the obligation. The following table provides information regarding changes in nonfinancial liabilities related to the Company’s warranty obligations measured at fair value on a recurring basis using significant unobservable inputs (Level 3) for the periods indicated. Three Months Ended 2021 2020 (In thousands) Balance at beginning of period $ 28,736 $ 19,806 Accruals for warranties issued during period 3,894 1,524 Changes in estimates 2,583 615 Settlements (1,915) (1,993) Increase due to accretion expense 943 774 Other (922) (301) Balance at end of period $ 33,319 $ 20,425 Quantitative and Qualitative Information about Level 3 Fair Value Measurements As of March 31, 2021 and December 31, 2020, the significant unobservable inputs used in the fair value measurement of the Company’s liabilities designated as Level 3 are as follows: Percent Used (Weighted Average) Item Measured at Fair Value Valuation Technique Description of Significant Unobservable Input March 31, December 31, Warranty obligations for microinverters sold since January 1, 2014 Discounted cash flows Profit element and risk premium 15% 15% Credit-adjusted risk-free rate 13% 13% Sensitivity of Level 3 Inputs - Warranty Obligations Each of the significant unobservable inputs is independent of the other. The profit element and risk premium are estimated based on requirements of a third-party participant willing to assume the Company’s warranty obligations. The credit‑adjusted risk‑free rate (“discount rate”) is determined by reference to the Company’s own credit standing at the fair value measurement date. Increasing the profit element and risk premium input by 100 basis points would result in a $0.3 million increase to the liability. Decreasing the profit element and risk premium by 100 basis points would result in a $0.3 million reduction of the liability. Increasing the discount rate by 100 basis points would result in a $1.5 million reduction of the liability. Decreasing the discount rate by 100 basis points would result in a $1.7 million increase to the liability.

DEBT

DEBT	Mar. 01, 2021	Mar. 31, 2021
Debt Disclosure [Abstract]
DEBT	Convertible Senior Notes due 2026 On March 1, 2021, the Company issued $575.0 million aggregate principal amount of 0.0% convertible senior notes due 2026 (the “Notes due 2026”). In addition, on March 12, 2021, the Company issued an additional $57.5 million aggregate principal amount of the Notes due 2026 pursuant to the initial purchasers’ full exercise of the over-allotment option for additional Notes due 2026. The Notes due 2026 will not bear regular interest, and the principal amount of the Notes due 2026 will not accrete. The Notes due 2026 are general unsecured obligations and are governed by an indenture between the Company and U.S. Bank National Association, as trustee. The Notes due 2026 will mature on March 1, 2026, unless earlier repurchased by the Company or converted at the option of the holders. The Company received approximately $623.0 million in net proceeds, after deducting the initial purchasers’ discount, from the issuance of the Notes due 2026. The initial conversion rate for the Notes due 2026 is 3.2523 shares of common stock per $1,000 principal amount of the Notes due 2026 (which represents an initial conversion price of approximately $307.47 per share). The conversion rate for the Notes due 2026 will be subject to adjustment upon the occurrence of certain specified events but will not be adjusted for accrued and unpaid interest. In addition, if a make-whole fundamental change or a redemption with respect to the Notes due 2026 occurs prior to the maturity date, under certain circumstances as specified in the relevant indenture, the Company will increase the conversion rate for the Notes due 2026 by a number of additional shares of the Company’s common stock for a holder that elects to convert its notes in connection with such make-whole fundamental change or redemption. Upon conversion, the Company will settle conversions of Notes due 2026 through payment or delivery, as the case may be, of cash, shares of its common stock or a combination of cash and shares of its common stock, at the Company’s election. The Company may not redeem the Notes due 2026 prior to the September 6, 2023. The Company may redeem for cash all or any portion of the Notes due 2026, at the Company’s election, on or after September 6, 2023, if the last reported sale price of the Company’s common stock has been greater than or equal to 130% of the conversion price then in effect for the Notes due 2026 (i.e., $399.71, which is 130% of the current conversion price for the Notes due 2026) for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading day period ending on, and including, the trading day immediately preceding the date on which the Company provides notice of redemption. The redemption price will equal 100% of the principal amount of the Notes due 2026 to be redeemed, plus accrued and unpaid special interest, if any, to, but excluding, the relevant redemption date for the Notes due 2026. The redemption price will be increased as described in the relevant indentures by a number of additional shares of the Company in connection with such optional redemption by the Company. No sinking fund is provided for the Notes due 2026. The Notes due 2026 may be converted on any day prior to the close of business on the business day immediately preceding September 1, 2025, in multiples of $1,000 principal amount, at the option of the holder only under any of the following circumstances: (1) during any calendar quarter commencing after the calendar quarter ending on June 30, 2021 (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 of the Notes due 2026 (i.e., $399.71, which is 130% of the current conversion price for the Notes due 2026) 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 relevant indenture) 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 Notes due 2026 on each such trading day; (3) if the Company calls any or all of the Notes due 2026 for redemption, at any time prior to the close of business on the scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after June 30, 2021 until the close of business on the second scheduled trading day immediately preceding the maturity date of March 1, 2026, holders of the Notes due 2026 may convert their notes at any time, regardless of the foregoing circumstances. Upon the occurrence of a fundamental change (as defined in the relevant indenture), holders may require the Company to repurchase all or a portion of their Notes due 2026 for cash at a price equal to 100% of the principal amount of the notes to be repurchased plus any accrued and unpaid special interest, if any, to, but excluding, the fundamental change repurchase date. In accounting for the issuance of the Notes due 2026 on March 1, 2021, the Company separated the Notes due 2026 into liability and equity components. The carrying amount of the liability component of approximately $509.0 million was calculated by using a discount rate of 4.44%, which was the Company’s borrowing rate on the date of the issuance of the Notes due 2026 for a similar debt instrument without the conversion feature. The carrying amount of the equity component of approximately $123.5 million, representing the conversion option, was determined by deducting the fair value of the liability component from the par value of the Notes due 2026. The equity component of the Notes due 2026 is included in additional paid-in capital in the condensed consolidated balance sheet and is not remeasured as long as it continues to meet the conditions for equity classification. The difference between the principal amount of the Notes due 2026 and the liability component (the “debt discount”) is amortized to interest expense using the effective interest method over the term of the Notes due 2026. Debt issuance costs for the issuance of the Notes due 2026 were approximately $10.0 million, consisting of initial purchasers' discount and other issuance costs. In accounting for the transaction costs, the Company allocated the total amount incurred to the liability and equity components using the same proportions as the proceeds from the Notes due 2026. Transaction costs attributable to the liability component were approximately $8.1 million, were recorded as debt issuance cost (presented as contra debt in the condensed consolidated balance sheet) and are being amortized to interest expense over the term of the Notes due 2026. The transaction costs attributable to the equity component were approximately $2.0 million and were netted with the equity component in stockholders’ equity. As of March 31, 2021, the unamortized deferred issuance cost for the Notes due 2026 was $7.9 million, on the condensed consolidated balance sheet. The following table presents the total amount of interest cost recognized in the statement of operations relating to the Notes due 2026: Three Months Ended 2021 (In thousands) Amortization of debt discount $ 1,786 Amortization of debt issuance costs 135 Total interest cost recognized $ 1,921 The effective interest rate on the liability component Notes due 2026 was 4.44% for the three months ended March 31, 2021, which remains unchanged from the date of issuance. The remaining unamortized debt discount was $121.7 million as of March 31, 2021, and will be amortized over approximately 4.9 years from March 31, 2021. Notes due 2026 Hedge and Warrant Transactions In connection with the offering of the Notes due 2026 (including in connection with the issuance of additional Notes due 2026 upon the initial purchasers’ exercise of their over-allotment option), the Company entered into privately-negotiated convertible note hedge transactions (the “2026 Hedge Transactions”) pursuant to which the Company has the option to purchase a total of approximately 2.1 million shares of its common stock (subject to anti-dilution adjustments), which is the same number of shares initially issuable upon conversion of the Notes due 2026, at a price of $307.47 per share, which is the initial conversion price of the Notes due 2026. The total cost of the 2026 Hedge Transactions was approximately $124.6 million. The 2026 Hedge Transactions are expected generally to reduce potential dilution to the Company’s common stock upon any conversion of the Notes due 2026 and/or offset any cash payments the Company is required to make in excess of the principal amount of converted notes, as the case may be. Additionally, the Company separately entered into privately-negotiated warrant transactions, including in connection with the issuance of additional Notes due 2026 upon the initial purchasers’ exercise of their over-allotment option (the “2026 Warrants”), whereby the Company sold warrants to acquire approximately 2.1 million shares of the Company’s common stock (subject to anti-dilution adjustments) at an initial strike price of $397.91 per share. The Company received aggregate proceeds of approximately $97.4 million from the sale of the 2026 Warrants. If the market value per share of the Company’s common stock, as measured under the 2026 Warrants, exceeds the strike price of the Warrants, the 2026 Warrants will have a dilutive effect on the Company’s earnings per share, unless the Company elects, subject to certain conditions, to settle the 2026 Warrants in cash. Taken together, the purchase of the 2026 Hedge Transactions and the sale of the 2026 Warrants are intended to reduce potential dilution from the conversion of the Notes due 2026 and to effectively increase the overall conversion price from $307.47 to $397.91 per share. The 2026 Warrants are only exercisable on the applicable expiration dates in accordance with the 2026 Warrants. Subject to the other terms of the 2026 Warrants, the first expiration date applicable to the Warrants is June 1, 2026, and the final expiration date applicable to the 2026 Warrants is July 27, 2026. Given that the transactions meet certain accounting criteria, the Notes due 2026 hedge and the warrants transactions are recorded in stockholders’ equity, and they are not accounted for as derivatives and are not remeasured each reporting period.	DEBT The following table provides information regarding the Company’s debt. March 31, December 31, (In thousands) Convertible notes Notes due 2028 $ 575,000 $ — Less: unamortized discount and issuance costs (164,905) — Carrying amount of Notes due 2028 410,095 — Notes due 2026 632,500 — Less: unamortized discount and issuance costs (129,630) — Carrying amount of Notes due 2026 502,870 — Notes due 2025 102,260 320,000 Less: unamortized discount and issuance costs (19,657) (64,979) Carrying amount of Notes due 2025 82,603 255,021 Notes due 2024 1,068 88,140 Less: unamortized discount and issuance costs (217) (19,119) Carrying amount of Notes due 2024 851 69,021 Notes due 2023 5,000 5,000 Less: unamortized issuance costs (92) (102) Carrying amount of Notes due 2023 4,908 4,898 Sale of long-term financing receivable recorded as debt 902 1,925 Total carrying amount of debt 1,002,229 330,865 Less: current portion of convertible notes and long-term financing receivable recorded as debt (84,356) (325,967) Long-term debt $ 917,873 $ 4,898 Convertible Senior Notes due 2028 On March 1, 2021, the Company issued $575 million aggregate principal amount of 0.0% convertible senior notes due 2028 (the “Notes due 2028”). The Notes due 2028 will not bear regular interest, and the principal amount of the Notes due 2028 will not accrete. The Notes due 2028 are general unsecured obligations and are governed by an indenture between the Company and U.S. Bank National Association, as trustee. The Notes due 2028 will mature on March 1, 2028, unless earlier repurchased by the Company or converted at the option of the holders. The Company received approximately $566.4 million in net proceeds, after deducting the initial purchasers’ discount, from the issuance of the Notes due 2028. The initial conversion rate for the Notes due 2028 is 3.5104 shares of common stock per $1,000 principal amount of the Notes due 2028 (which represents an initial conversion price of approximately $284.87 per share). The conversion rate for the Notes due 2028 will be subject to adjustment upon the occurrence of certain specified events but will not be adjusted for any accrued and unpaid special interest, if any. In addition, if a make-whole fundamental change or a redemption with respect to the Notes due 2028 occurs prior to the maturity date, under certain circumstances as specified in the relevant indenture, the Company will increase the conversion rate for the Notes due 2028 by a number of additional shares of the Company’s common stock for a holder that elects to convert its notes in connection with such make-whole fundamental change or redemption. Upon conversion, the Company will settle conversions of the Notes due 2028 through payment or delivery, as the case may be, of cash, shares of its common stock or a combination of cash and shares of its common stock, at the Company’s election. The Company may not redeem the Notes due 2028 prior to September 6, 2024. The Company may redeem for cash all or any portion of the Notes due 2028, at the Company’s election, on or after September 6, 2024, if the last reported sale price of the Company’s common stock has been greater than or equal to 130% of the conversion price then in effect for the Notes due 2028 (i.e. $370.33, which is 130% of the current conversion price for the Notes due 2028) for at least 20 trading days (whether or not consecutive) 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. The redemption price will equal 100% of the principal amount of the Notes due 2028 to be redeemed, plus accrued and unpaid special interest, if any to, but excluding, the relevant redemption date. No sinking fund is provided for the Notes due 2028. The Notes due 2028 may be converted on any day prior to the close of business on the business day immediately preceding September 1, 2027, in multiples of $1,000 principal amount, at the option of the holder only under any of the following circumstances: (1) during any calendar quarter commencing after the calendar quarter ending on June 30, 2021 (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 for the Notes due 2028 (i.e, $370.33 which is 130% of the current conversion price for the Notes due 2028) 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 relevant indenture) 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 due 2028 on each such trading day; (3) if the Company calls any or all of the Notes due 2028 for redemption, at any time prior to the close of business on the scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after September 1, 2027 until the close of business on the second scheduled trading day immediately preceding the maturity date of March 1, 2028, holders of the Notes due 2028 may convert their notes at any time, regardless of the foregoing circumstances. Upon the occurrence of a fundamental change (as defined in the relevant indenture), holders may require the Company to repurchase all or a portion of their Notes due 2028 for cash at a price equal to 100% of the principal amount of the notes to be repurchased plus any accrued and unpaid special interest, if any, to, but excluding, the fundamental change repurchase date. In accounting for the issuance of the Notes due 2028 on March 1, 2021, the Company separated the Notes due 2028 into liability and equity components. The carrying amount of the liability component of approximately $415.0 million was calculated by using a discount rate of 4.77%, which was the Company’s borrowing rate on the date of the issuance of the Notes due 2028 for a similar debt instrument without the conversion feature. The carrying amount of the equity component of approximately $160.0 million, representing the conversion option, was determined by deducting the fair value of the liability component from the par value of the Notes due 2028. The equity component of the Notes due 2028 is included in additional paid-in capital in the condensed consolidated balance sheet and is not remeasured as long as it continues to meet the conditions for equity classification. The difference between the principal amount of the Notes due 2028 and the liability component (the “debt discount”) is amortized to interest expense using the effective interest method over the term of the Notes due 2028. Debt issuance costs for the issuance of the Notes due 2028 were approximately $9.1 million, consisting of initial purchasers' discount and other issuance costs. In accounting for the transaction costs, the Company allocated the total amount incurred to the liability and equity components using the same proportions as the proceeds from the Notes due 2028. Transaction costs attributable to the liability component were approximately $6.6 million, were recorded as debt issuance cost (presented as contra debt in the condensed consolidated balance sheet) and are being amortized to interest expense over the term of the Notes due 2028. The transaction costs attributable to the equity component were approximately $2.5 million and were netted with the equity component in stockholders’ equity. As of March 31, 2021, the unamortized deferred issuance cost for the Notes due 2028 was $6.5 million, on the condensed consolidated balance sheet. The following table presents the total amount of interest cost recognized in the statement of operations relating to the Notes due 2028: Three Months Ended 2021 (In thousands) Amortization of debt discount $ 1,611 Amortization of debt issuance costs 79 Total interest cost recognized $ 1,690 The effective interest rate on the liability component Notes due 2028 was 4.77% for the three months ended March 31, 2021, which remains unchanged from the date of issuance. The remaining unamortized debt discount was $158.4 million as of March 31, 2021, and will be amortized over approximately 6.9 years from March 31, 2021. Notes due 2028 Hedge and Warrant Transactions In connection with the offering of the Notes due 2028, the Company entered into privately-negotiated convertible note hedge transactions (“Notes due 2028 Hedge”) pursuant to which the Company has the option to purchase a total of approximately 2.0 million shares of its common stock (subject to anti-dilution adjustments), which is the same number of shares initially issuable upon conversion of the Notes due 2028, at a price of $284.87 per share, which is the initial conversion price of the Notes due 2028. The total cost of the convertible note hedge transactions was approximately $161.6 million. The convertible note hedge transactions are expected generally to reduce potential dilution to the Company’s common stock upon any conversion of the Notes due 2028 and/or offset any cash payments the Company is required to make in excess of the principal amount of converted notes, as the case may be. Additionally, the Company separately entered into privately-negotiated warrant transactions (the “2028 Warrants”) whereby the Company sold warrants to acquire approximately 2.0 million shares of the Company’s common stock (subject to anti-dilution adjustments) at an initial strike price of $397.91 per share. The Company received aggregate proceeds of approximately $123.4 million from the sale of the Warrants. If the market value per share of the Company’s common stock, as measured under the 2028 Warrants, exceeds the strike price of the 2028 Warrants, the 2028 Warrants will have a dilutive effect on the Company’s earnings per share, unless the Company elects, subject to certain conditions, to settle the 2028 Warrants in cash. Taken together, the purchase of the Notes due 2028 Hedge and the sale of the 2028 Warrants are intended to reduce potential dilution from the conversion of the Notes due 2028 and to effectively increase the overall conversion price from $284.87 to $397.91 per share. The 2028 Warrants are only exercisable on the applicable expiration dates in accordance with the Notes due 2028 Hedge. Subject to the other terms of the Warrants, the first expiration date applicable to the Notes due 2028 Hedge is June 1, 2028, and the final expiration date applicable to the Notes due 2028 Hedge is July 27, 2028. Given that the transactions meet certain accounting criteria, the Notes due 2028 Hedge and the 2028 Warrants transactions are recorded in stockholders’ equity, and they are not accounted for as derivatives and are not remeasured each reporting period. Convertible Senior Notes due 2025 On March 9, 2020, the Company issued $320.0 million aggregate principal amount of the Notes due 2025 (the “Notes due 2025”). The Notes due 2025 are general unsecured obligations and bear interest at an annual rate of 0.25% per year, payable semi-annually on March 1 and September 1 of each year, beginning September 1, 2020. The Notes due 2025 are governed by an indenture between the Company and U.S. Bank National Association, as trustee. The Notes due 2025 will mature on March 1, 2025, unless earlier repurchased by the Company or converted at the option of the holders. The Company may not redeem the notes prior to the maturity date, and no sinking fund is provided for the notes. The Notes due 2025 may be converted, under certain circumstances as described below, based on an initial conversion rate of 12.2637 shares of common stock per $1,000 principal amount (which represents an initial conversion price of $81.54 per share). The conversion rate for the Notes due 2025 will be subject to adjustment upon the occurrence of certain specified events but will not be adjusted for accrued and unpaid interest. In addition, upon the occurrence of a make-whole fundamental change (as defined in the relevant indenture), the Company will, in certain circumstances, increase the conversion rate by a number of additional shares for a holder that elects to convert its notes in connection with such make-whole fundamental change. The Company received approximately $313.0 million in net proceeds, after deducting the initial purchasers’ discount, from the issuance of the Notes due 2025. The Notes due 2025 may be converted prior to the close of business on the business day immediately preceding September 1, 2024, in multiples of $1,000 principal amount, at the option of the holder only under the following circumstances: (1) during any calendar quarter commencing after the calendar quarter ending on June 30, 2020 (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 relevant indenture) 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 on each such trading day; or (3) upon the occurrence of specified corporate events. On and after September 1, 2024 until the close of business on the second scheduled trading day immediately preceding the maturity date of March 1, 2025, holders may convert their notes at any time, regardless of the foregoing circumstances. Upon the occurrence of a fundamental change (as defined in the relevant indenture), holders may require the Company to repurchase all or a portion of their Notes due 2025 for cash at a price equal to 100% of the principal amount of the notes to be repurchased plus any accrued and unpaid interest to, but excluding, the fundamental change repurchase date. As of March 31, 2021, the sale price of the Company’s common stock was greater than or equal to $106.00 (130% of the notes conversion price) for at least 20 trading days (whether consecutive or not) during a period of 30 consecutive trading days preceding the quarter-ended March 31, 2021. As a result, as of March 31, 2021, the Notes due 2025 are convertible at the holders’ option through June 30, 2021. Accordingly, the Company classified the net carrying amount of the Notes due 2025 of $102.3 million as Debt, current on the condensed consolidated balance sheet as of March 31, 2021. From April 1, 2021 through April 27, 2021, the Company has received the request for conversion of approximately $0.1 million in principal amount of Notes due 2025, of which the Company has elected to settle the aggregate principal amount of the Notes due 2025 in a combination of cash and any excess in shares of the Company’s common stock in accordance with the applicable indenture. Such conversion will be settled in May 2021. For the period from March 9, 2020, the issuance date, through May 19, 2020, the number of authorized and unissued shares of the Company’s common stock that are not reserved for other purposes was less than the maximum number of underlying shares that would be required to settle the Notes due 2025 into equity. Accordingly, unless and until the Company had a number of authorized shares that were not issued or reserved for any other purpose that equaled or exceeded the maximum number of underlying shares (“share reservation condition”), the Company would have been required to pay to the converting holder in respect of each $1,000 principal amount of notes being converted solely in cash in an amount equal to the sum of the daily conversion values for each of the 20 consecutive trading days during the related observation period. However, following satisfaction of the share reservation condition, the Company could settle conversions of notes through payment or delivery, as the case may be, of cash, shares of the Company’s common stock or a combination of cash and shares of its common stock, at the Company’s election. As further discussed below, the Company satisfied the share reservation condition during May 2020. In accounting for the issuance of the Notes due 2025, on March 9, 2020, the conversion option of the Notes due 2025 was deemed an embedded derivative requiring bifurcation from the Notes due 2025 (“host contract”) and separate accounting as an embedded derivative liability, as a result of the Company not having the necessary number of authorized but unissued shares of its common stock available to settle the conversion option of the Notes due 2025 in shares. The proceeds from the Notes due 2025 were first allocated to the embedded derivative liability and the remaining proceeds were then allocated to the host contract. On March 9, 2020, the carrying amount of the embedded derivative liability of $68.7 million representing the conversion option was determined using the Binomial Lattice model and the remaining $251.3 million was allocated to the host contract. The difference between the principal amount of the Notes due 2025 and the fair value of the host contract (the “debt discount”) is amortized to interest expense using the effective interest method over the term of the Notes due 2025. On May 20, 2020, at the Company’s annual meeting of stockholders, the stockholders approved an amendment to the Amended and Restated Certificate of Incorporation to increase the number of authorized shares of the Company’s common stock, par value $0.00001 per share, from 150,000,000 shares to 200,000,000 shares (the “Amendment”). The Amendment became effective upon filing with the Secretary of State of Delaware on May 20, 2020. As a result, the Company satisfied the share reservation condition. The Company may now settle the Notes due 2025 and warrants issued in conjunction with the Notes due 2025 through payment or delivery, as the case may be, of cash, shares of its common stock or a combination of cash and shares of its common stock, at the Company’s election. Accordingly, on May 20, 2020, the embedded derivative liability was remeasured at a fair value of $116.3 million and was then reclassified to additional paid-in-capital in the condensed consolidated balance sheet in the second quarter of 2020 and is no longer remeasured as long as it continues to meet the conditions for equity classification. The Company recorded the change in the fair value of the embedded derivative in other expense, net in the condensed consolidated statement of operations during the three months ended March 31, 2021. The Company separated the Notes due 2025 into liability and equity components, this resulted in a tax basis difference associated with the liability component that represents a temporary difference. The Company recognized the deferred taxes of $0.2 million for the tax effect of that temporary difference as an adjustment to the equity component included in additional paid-in capital in the condensed consolidated balance sheet. Debt issuance costs for the issuance of the Notes due 2025 were approximately $7.6 million, consisting of initial purchasers' discount and other issuance costs. In accounting for the transaction costs, the Company allocated the total amount incurred to the Notes due 2025 host contract. Transaction costs were recorded as debt issuance cost (presented as contra debt in the condensed consolidated balance sheet) and are being amortized to interest expense over the term of the Notes due 2025. Concurrently with the offering of the Notes due 2026 and Notes due 2028, the Company entered into separately- and privately-negotiated transactions to repurchase approximately $217.7 million aggregate principal amount of the Notes due 2025. The Company paid $217.7 million in cash and issued approximately 1.67 million shares of its common stock to the holders of the repurchased notes with an aggregate fair value of $302.7 million, representing the conversion value in excess of the principal amount of the Notes due 2025, which were fully offset by shares received from the Company’s settlement of the associated note hedging arrangements discussed below. The total amount of $217.7 million paid to partially settle the repurchases of the Notes due 2025 was allocated between the liability and equity components of the amount extinguished by determining the fair value of the liability component immediately prior to the notes repurchase and allocating that portion of the conversion price to the liability component in the amount of $184.5 million. The residual of the conversion price of $4.3 million of the repurchased Notes due 2025, net of inducement loss of $37.5 million for additional shares issued, was allocated to the equity component of the repurchased Notes due 2025 as an increase of additional paid-in capital. The fair value of the notes settlement for such repurchases was calculated using a discount rate of 4.35%, representing an estimate of the Company's borrowing rate at the date of repurchase with a remaining expected life of approximately 4.1 years. As part of the settlement of the repurchase of the Notes due 2025, the Company wrote-off the $38.5 million unamortized debt discount and $4.1 million debt issuance cost apportioned to the principal amount of Notes due 2025 repurchased. The Company recorded a loss on partial settlement of the repurchased Notes due 2025 of $9.4 million in Other income (expense), net, representing the difference between the consideration attributed to the liability component and the sum of the net carrying amount of the liability component and unamortized debt issuance costs. Further, the Company also recorded loss on inducement of $37.5 million in Other income (expense), net, representing the difference between the fair value of the shares that would have been issued under the original conversion terms with respect to the repurchased Notes due 2025. Following the repurchase transactions summarized above, as of March 31, 2021, $102.3 million aggregate principal amount of the Notes due 2025 remained outstanding. The following table presents the total amount of interest cost recognized relating to the Notes due 2025 (in thousands): Three Months Ended 2021 2020 (In thousands) Contractual interest expense $ 150 $ 49 Amortization of debt discount 2,389 743 Amortization of debt issuance costs 294 87 Total interest cost recognized $ 2,833 $ 879 The derived effective interest rate on the Notes due 2025 host contract was determined to be 5.18%, which remain unchanged from the date of issuance. The remaining unamortized debt discount was $17.7 million and $58.6 million as of March 31, 2021 and December 31, 2020, respectively, and will be amortized over approximately 3.9 years from March 31, 2021. Notes due 2025 Hedge and Warrant Transactions In connection with the offering of the Notes due 2025, the Company entered into privately-negotiated convertible note hedge transactions (the “2025 Hedge Transactions”) pursuant to which the Company has the option to purchase a total of approximately 3.9 million shares of its common stock (subject to anti-dilution adjustments), which is the same number of shares initially issuable upon conversion of the notes, at a price of $81.54 per share, which is the initial conversion price of the Notes due 2025. The total cost of the convertible note hedge transactions was approximately $89.1 million. The convertible note hedge transactions are expected generally to reduce potential dilution to the Company’s common stock upon any conversion of the Notes due 2025 and/or offset any cash payments the Company is required to make in excess of the principal amount of converted notes, as the case may be. Additionally, the Company separately entered into privately-negotiated warrant transactions in connection with the offering of the Notes due 2025 (the “2025 Warrants”) whereby the Company sold warrants to acquire approximately 3.9 million shares of the Company’s common stock (subject to anti-dilution adjustments) at an initial strike price of $106.94 per share. The Company received aggregate proceeds of approximately $71.6 million from the sale of the 2025 Warrants. If the market value per share of the Company’s common stock, as measured under the 2025 Warrants, exceeds the strike price of the 2025 Warrants, the 2025 Warrants will have a dilutive effect on the Company’s earnings per share, unless the Company elects, subject to certain conditions, to settle the 2025 Warrants in cash. Taken together, the purchase of the convertible note hedges in connection with the 2025 Hedge Transactions and the sale of the 2025 Warrants are intended to reduce potential dilution from the conversion of the Notes due 2025 and to effectively increase the overall conversion price from $81.54 to $106.94 per share. The 2025 Warrants are only exercisable on the applicable expiration dates in accordance with the agreements relating to each of the 2025 Warrants. Subject to the other terms of the 2025 Warrants, the first expiration date applicable to the 2025 Warrants is June 1, 2025, and the final expiration date applicable to the 2025 Warrants is September 23, 2025. During the first quarter of 2021, in connection with the repurchase of $217.7 million aggregate principal amount of the Notes due 2025 summarized above, the Company entered into partial unwind agreements with respect to certain of the “2025 Hedge Transactions and the 2025 Warrants. In connection with these unwind transactions, the Company received shares of the Company’s common stock as a termination payment for the portion of the 2025 Hedge Transactions that were unwound, and the Company issued shares of its common stock as a termination payment for the portion of the 2025 Warrants that were unwound. As a result of the unwind agreements for the 2025 Hedge Transactions and the 2025 Warrants, the Company received 1.9 million of the Company’s common stock from the 2025 Hedge Transactions settlement and issued 1.8 million of the Company’s common stock from the 2025 Warrants that were unwound. Following the unwind transactions summarized above, as of March 31, 2021, options to purchase approximately 1.3 million shares of common stock remained outstanding under the 2025 Hedge Transactions, and 2025 Warrants exercisable to purchase approximately 1.3 million shares remained outstanding. For the period from March 9, 2020, the issuance date of the 2025 Hedge Transactions and 2025 Warrants, through May 19, 2020, the number of authorized and unissued shares of the Company’s common stock that are not reserved for other purposes was less than the maximum number of underlying shares that will be required to settle the Notes due 2025 through the delivery of shares of the Company’s common stock. Accordingly, the 2025 Hedge Transactions and 2025 Warrants could only be settled on net cash settlement basis. As a result, the 2025 Hedge Transactions and 2025 Warrants were classified as a Convertible notes hedge asset and Warrants liability, respectively, in the condensed consolidated balance sheet and the change in fair value of derivatives was included in other expense, net in the condensed consolidated statement of operations. On May 20, 2020, at the Company’s annual meeting of stockholders, the stockholders approved the Amendment and satisfied the share reservation condition (as discussed above), and as a result, the Convertible notes hedge asset and Warrants liabilities were remeasured at a fair value of $117.1 million and $96.4 million, respectively, and were then reclassified to additional paid-in-capital in the condensed consolidated balance sheet in the second quarter of 2020 and is no longer remeasured as long as they continue to meet the conditions for equity classification. The change in the fair value of the Convertible notes hedge asset and Warrants liability were recorded in other expense, net in the condensed consolidated statements of operations during the three months ended March 31, 2021. Convertible Senior Notes due 2024 On June 5, 2019, the Company issued $132.0 million aggregate principal amount of 1.0% convertible senior notes due 2024 (the “Notes due 2024”). The Notes due 2024 are general unsecured obligations and bear interest at an annual rate of 1.0% per year, payable semi-annually on June 1 and December 1 of each year, beginning December 1, 2019. The Notes due 2024 are governed by an indenture between the Company and U.S. Bank National Association, as trustee. The Notes due 2024 will mature on June 1, 2024, unless earlier repurchased by the Company or converted at the option of the holders. The Company may not redeem the notes prior to the maturity date, and no sinking fund is provided for the notes. The Notes due 2024 may be converted, under certain circumstances as described below, based on an initial conversion rate of 48.7781 shares of common stock per $1,000 principal amount (which represents an initial conversion price of $20.50 per share). The conversion rate for the Notes due 2024 will be subject to adjustment upon the occurrence of certain specified events but will not be adjusted for accrued and unpaid interest. In addition, upon the occurrence of a make-whole fundamental change (as defined in the relevant indenture), the Company will, in certain circumstances, increase the conversion rate by a number of additional shares for a holder that elects to convert its notes in connection with such make-whole fundamental change. The Company received approximately $128.0 million in net proceeds, after deducting the initial purchasers’ discount, from the issuance of the Notes due 2024. The Notes due 2024 may be converted on any day prior to the close of business on the business day immediately preceding December 1, 2023, in multiples of $1,000 principal amount, at the option of the holder only under any of the following circumstances: (1) during any calendar quarter commencing after the calendar quarter ending on September 30, 2019 (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 $26.65 (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 relevant indenture) 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 on each such trading day; or (3) upon the occurrence of specified corporate events. On and after December 1, 2023 until the close of business on the second scheduled trading day immediately preceding the maturity date of June 1, 2024, holders may convert their notes at any time, regardless of the foregoing circumstances. Upon the occurrence of a fundamental change (as defined in the relevant indenture), holders may require the Company to repurchase all or a portion of their Notes due 2024 for cash at a price equal to 100% of the principal amount of the notes to be repurchased plus any accrued and unpaid interest to, but excluding, the fundamental change repurchase date. As of March 31, 2021, the sale price of the Company’s common stock was greater than or equal to $26.65 (130% of the notes conversion price) for at least 20 trading days (whether consecutive or not) during a period of 30 consecutive trading days preceding the quarter-ended March 31, 2021. As a result, as of March 31, 2021, the Notes due 2024 are convertible at the holders’ option through June 30, 2021. Accordingly, the Company classified the net carrying amount of the Notes due 2024 of $0.9 million as Debt, current on the condensed consolidated balance sheet as of March 31, 2021. From April 1, 20

COMMITMENTS AND CONTINGENCIES

COMMITMENTS AND CONTINGENCIES	3 Months Ended
	Mar. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES Operating Leases The Company leases office facilities under noncancelable operating leases that expire on various dates through 2031, some of which may include options to extend the leases for up to 12 years. The components of lease expense are presented as follows: Three Months Ended 2021 2020 (In thousands) Operating lease costs $ 1,631 $ 1,222 The components of lease liabilities are presented as follows: March 31, December 31, (In thousands) Operating lease liabilities, current (Accrued liabilities) $ 4,772 $ 4,542 Operating lease liabilities, noncurrent (Other liabilities) 13,899 15,209 Total operating lease liabilities $ 18,671 $ 19,751 Supplemental lease information: Weighted average remaining lease term 6.3 years 6.4 years Weighted average discount rate 7.6% 7.7% Supplemental cash flow and other information related to operating leases, are as follows: Three Months Ended 2021 2020 (In thousands) Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 1,361 $ 1,014 Non-cash investing activities: Lease liabilities arising from obtaining right-of-use assets $ — $ 2,941 Undiscounted cash flows of operating lease liabilities as of March 31, 2021 are as follows: Lease Amounts (In thousands) Year: 2021 (remaining nine months) $ 4,457 2022 4,660 2023 4,038 2024 3,053 2025 2,262 2026 and thereafter 3,957 Total lease payments 22,427 Less: imputed lease interest (3,756) Total lease liabilities $ 18,671 Purchase Obligations The Company has contractual obligations related to component inventory that its contract manufacturers procure on its behalf in accordance with its production forecast as well as other inventory related purchase commitments. As of March 31, 2021, these purchase obligations totaled approximately $177.2 million. Litigation The Company is subject to various legal proceedings relating to claims arising out of its operations that have not been fully resolved. The outcome of litigation is inherently uncertain. If one or more legal matters were resolved against the Company in a reporting period for amounts above management’s expectations, the Company’s business, results of operations, financial position and cash flows for that reporting period could be materially adversely affected. As of April 27, 2021, the Company is not currently a party to any matters that the management expects will have an adverse material effect on the Company’s condensed consolidated financial position, results of operations or cash flows. Contingencies On March 26, 2020, the Office of the United States Trade Representative (the “USTR”) announced certain exclusion requests related to tariffs on Chinese imported microinverter products that fit the dimensions and weight limits within a Section 301 Tariff exclusion under U.S. note 20(ss)(40) to subchapter III of chapter 99 of the Harmonized Tariff Schedule of the United States (the “Tariff Exclusion”). The Tariff Exclusion applies to covered products under the China Section 301 Tariff Actions (“Section 301 Tariffs”) taken by the USTR exported from China to the United States from September 24, 2018 until August 7, 2020. Accordingly, the Company sought refunds totaling approximately $38.9 million plus approximately $0.6 million accrued interest on tariffs previously paid from September 24, 2018 to March 31, 2020 for certain microinverters that qualify for the Tariff Exclusion. The refund request was subject to review and approval by the U.S. Customs and Border Protection; therefore, the Company assessed the probable loss recovery in the year ended December 31, 2020 was equal to the approved refund requests available to the Company. As of December 31, 2020, the Company had received $24.8 million of tariff refunds and accrued for the remaining $14.7 million tariff refunds that were approved, however, not yet received on or before December 31, 2020. As of March 31, 2021, the Company received the remaining $14.7 million tariff refunds. For the year ended December 31, 2020, the Company recorded $38.9 million as a reduction to cost of revenues in the Company’s condensed consolidated statement of operations as the approved refunds relate to paid tariffs previously recorded to cost of revenues, therefore, the Company recorded the corresponding approved tariff refunds as credits to cost of revenues in the fourth quarter of 2020. For the year ended December 31, 2020, the Company recorded the $0.6 million accrued interest as interest income in the condensed consolidated statement of operations. The tariff refund receivable of zero and $14.7 million was recorded as a reduction of accounts payable to Flex Ltd. and affiliates (“Flex”), the Company’s manufacturing partner and the importer of record who will first receive the tariff refunds, on the Company’s condensed consolidated balance sheet as of March 31, 2021 and December 31, 2020, respectively. The Tariff Exclusion expired on August 7, 2020 and those microinverter products now are subject to tariffs. The Company continues to pay Section 301 Tariffs on its storage and communication products and other accessories imported from China which are not subject to the Tariff Exclusion.

STOCK-BASED COMPENSATION

STOCK-BASED COMPENSATION	3 Months Ended
	Mar. 31, 2021
Share-based Payment Arrangement [Abstract]
STOCK-BASED COMPENSATION	STOCK-BASED COMPENSATION Stock-based Compensation Expense Stock-based compensation expense for all stock-based awards expected to vest is measured at fair value on the date of grant and recognized ratably over the requisite service period. The following table summarizes the components of total stock-based compensation expense included in the condensed consolidated statements of operations for the periods presented. Three Months Ended 2021 2020 (In thousands) Cost of revenues $ 982 $ 606 Research and development 5,749 1,919 Sales and marketing 3,537 1,942 General and administrative 4,576 3,048 Total $ 14,844 $ 7,515 The following table summarizes the various types of stock-based compensation expense for the periods presented. Three Months Ended 2021 2020 (In thousands) Stock options, RSUs, and PSUs $ 13,840 $ 7,077 Employee stock purchase plan 1,004 438 Total $ 14,844 $ 7,515 As of March 31, 2021, there was approximately $107.4 million of total unrecognized stock-based compensation expense related to unvested equity awards, which are expected to be recognized over a weighted-average period of 3.0 years. Equity Awards Activity Stock Options The following is a summary of stock option activity. Number of Weighted- Weighted- Aggregate (1) (In thousands) (Years) (In thousands) Outstanding at December 31, 2020 2,532 $ 1.96 Granted — — Exercised (55) 3.91 $ 9,882 Canceled (1) 0.83 Outstanding at March 31, 2021 2,476 $ 1.92 3.5 $ 396,797 Vested and expected to vest at March 31, 2021 2,476 $ 1.92 3.5 $ 396,797 Exercisable at March 31, 2021 2,248 $ 1.92 3.5 $ 360,300 (1) The intrinsic value of options exercised is based upon the value of the Company’s stock at exercise. The intrinsic value of options outstanding, vested and expected to vest, and exercisable as of March 31, 2021 is based on the closing price of the last trading day during the period ended March 31, 2021. The Company’s stock fair value used in this computation was $162.16 per share. The following table summarizes information about stock options outstanding at March 31, 2021. Options Outstanding Options Exercisable Range of Exercise Prices Number of Weighted- Weighted- Number of Weighted- (In thousands) (Years) (In thousands) $0.70 —– $1.11 533 4.0 $ 0.85 496 $ 0.85 $1.29 —– $1.29 1,000 3.5 1.29 875 1.29 $1.31 —– $1.31 670 3.0 1.31 649 1.31 $1.39 —– $14.58 262 3.7 5.52 219 6.12 $64.17 —– $64.17 11 6.1 64.17 9 64.17 Total 2,476 3.5 $ 1.92 2,248 $ 1.92 Restricted Stock Units The following is a summary of RSU activity. Number of Weighted- Weighted- Aggregate (1) (In thousands) (Years) (In thousands) Outstanding at December 31, 2020 3,588 $ 27.61 Granted 168 183.78 Vested (847) 17.49 $ 163,030 Canceled (12) 88.18 Outstanding at March 31, 2021 2,897 $ 39.37 1.17 $ 469,766 Expected to vest at March 31, 2021 2,897 $ 39.37 1.17 $ 469,718 (1) The intrinsic value of RSUs vested is based upon the value of the Company’s stock when vested. The intrinsic value of RSUs outstanding and expected to vest as of March 31, 2021 is based on the closing price of the last trading day during the period ended March 31, 2021. The Company’s stock fair value used in this computation was $162.16 per share. Performance Stock Units The following is a summary of PSU activity. Number of Weighted- Weighted- Aggregate (1) (In thousands) (Years) (In thousands) Outstanding at December 31, 2020 494 $ 51.10 Granted 262 66.89 Vested (494) 59.19 $ 91,803 Canceled (247) 42.00 Outstanding at March 31, 2021 15 $ 193.74 0.4 $ 2,432 (1) The intrinsic value of PSUs vested is based upon the value of the Company’s stock when vested. The intrinsic value of PSUs outstanding and expected to vest as of March 31, 2021 is based on the closing price of the last trading day during the period ended March 31, 2021. The Company’s stock fair value used in this computation was $162.16 per share.

INCOME TAXES

INCOME TAXES	3 Months Ended
	Mar. 31, 2021
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES For the three months ended March 31, 2021 the Company’s income tax benefit of $33.4 million on a net loss before income taxes of $1.7 million was calculated using the annualized effective tax rate method and was primarily due to tax deduction from employee stock compensation as a discrete event, partially offset by projected tax expense in the U.S. and foreign jurisdictions that are profitable. For the three months ended March 31, 2020, the Company’s income tax benefit of $11.9 million, on a net income before income taxes of $57.1 million was calculated using the annualized effective tax rate method and was primarily due to tax deduction from employee stock compensation as a discrete event, partially offset by projected tax expense in the U.S. and foreign jurisdictions that are profitable. For the three months ended March 31, 2021 and March 31, 2020, in accordance with FASB guidance for interim reporting of income tax, the Company has computed its benefit for income taxes based on a projected annual effective tax rate while excluding loss jurisdictions which cannot be benefited.

NET INCOME PER SHARE

NET INCOME PER SHARE	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
NET INCOME PER SHARE	NET INCOME PER SHARE Basic net income per share is computed by dividing net income by the weighted average number of shares of common stock outstanding during the period. Diluted net income per share is computed in a similar manner, but it also includes the effect of potential common shares outstanding during the period, when dilutive. Potential common shares include Stock Options, RSUs, PSUs, shares to be purchased under the Company’s ESPP, the Notes due 2023, Notes due 2024, 2024 Warrants, Notes due 2025, 2025 Warrants, Notes due 2026, 2026 Warrants, the Notes due 2028 and the 2028 Warrants. See Note 8. “Debt” of the notes to the condensed consolidated financial statements included in Part I, Item 1 of this Form 10-Q for additional information. The dilutive effect of potentially dilutive common shares is reflected in diluted earnings per share by application of the treasury stock method for stock options, RSUs, PSUs, the Notes due 2024, the 2024 Warrants, the Notes due 2025, the 2025 Warrants, the Notes due 2026, the 2026 Warrants, the Notes due 2028 and the 2028 Warrants. To the extent these potential common shares are antidilutive, they are excluded from the calculation of diluted net income per share. The following table presents the computation of basic and diluted net income per share for the periods presented. Three Months Ended 2021 2020 (In thousands, except per share data) Numerator: Net income $ 31,698 $ 68,936 Notes due 2023 interest and financing costs, net 44 44 Adjusted net income $ 31,742 $ 68,980 Denominator: Shares used in basic per share amounts: Weighted average common shares outstanding 131,303 123,531 Shares used in diluted per share amounts: Weighted average common shares outstanding 131,303 123,531 Effect of dilutive securities: Employee stock-based awards 5,735 8,609 Notes due 2023 900 900 Notes due 2024 2,984 2,936 2024 Warrants 2,506 2,128 Notes due 2025 1,713 — 2025 Warrants 1,301 — Weighted average common shares outstanding for diluted calculation 146,442 138,104 Basic and diluted net income per share Net income per share, basic $ 0.24 $ 0.56 Net income per share, diluted $ 0.22 $ 0.50 The following outstanding shares of common stock equivalents were excluded from the calculation of the diluted net income per share attributable to common stockholders because their effect would have been antidilutive. Three Months Ended 2021 2020 (In thousands) Employee stock-based awards 54 — Notes due 2026 682 — 2026 Warrants 1,070 — Notes due 2028 569 — 2028 Warrants 1,070 — Total 3,445 — Diluted earnings per share for the three months ended March 31, 2021 includes the dilutive effect of stock options, RSUs, PSUs, shares to be purchased under the ESPP, the Notes due 2023, the Notes due 2024, the 2024 Warrants, the Notes due 2025 and the 2025 Warrants. Certain common stock issuable under stock options, RSUs, PSUs, the Notes due 2026, the 2026 Warrants, the Notes due 2028 and the 2028 Warrants have been omitted from the diluted net income per share calculation because including such shares would have been antidilutive. Diluted earnings per share for the three months ended March 31, 2020 includes the dilutive effect of stock options, RSUs, PSUs, shares to be purchased under the ESPP, the Notes due 2023, the Notes due 2024 and the 2024 Warrants. For the three months ended March 31, 2020, no shares were antidilutive and excluded from diluted earnings per share.

RELATED PARTY

RELATED PARTY	3 Months Ended
	Mar. 31, 2021
Related Party Transactions [Abstract]
RELATED PARTY	RELATED PARTYIn 2018, a member of the Company’s board of directors and one of its principal stockholders, Thurman John Rodgers, purchased $5.0 million aggregate principal amount of the Notes due 2023 in a concurrent private placement. As of both March 31, 2021 and December 31, 2020, $5.0 million aggregate principal amount of the Notes due 2023 were outstanding. For additional information related to this purchase, see Note 8, “Debt,” of the notes to the condensed consolidated financial statements included in Part I, Item 1 of this Form 10-Q.

DESCRIPTION OF BUSINESS AND B_2

DESCRIPTION OF BUSINESS AND BASIS OF PRESENTATION (Policies)	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Consolidation	Basis of Presentation and Consolidation The accompanying condensed consolidated financial statements are presented in accordance with accounting principles generally accepted in the United States (“U.S.”), or GAAP. The condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany balances and transactions have been eliminated in consolidation.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of income and expenses during the reporting period. Significant estimates and assumptions reflected in the financial statements include revenue recognition, allowance for doubtful accounts, stock-based compensation, inventory valuation, accrued warranty obligations, fair value of investments, debt derivatives, convertible notes and contingent consideration, fair value of acquired intangible assets and goodwill, useful lives of acquired intangible assets and property and equipment, incremental borrowing rate for right-of-use assets and lease liability, legal contingencies, and tax valuation allowance. These estimates are based on information available as of the date of the financial statements; therefore, actual results could differ materially from management’s estimates using different assumptions or under different conditions. In light of the ongoing and quickly evolving COVID-19 pandemic, management has considered the impacts of the COVID-19 pandemic on the Company’s critical and significant accounting estimates and as of the date of issuance of these financial statements, the Company is not aware of any specific event or circumstance that would require the Company to update its estimates, judgments or revise the carrying value of its assets or liabilities. These estimates may change, as new events occur and additional information is obtained, and are recognized in the consolidated financial statements as soon as they become known. Actual results could differ from those estimates and any such differences may be material to the Company’s condensed financial statements.
Recently Issued Accounting Pronouncements Not Yet Effective and Recently Adopted Accounting Pronouncements	Recently Issued Accounting Pronouncements Not Yet Effective In August 2020, the FASB issued Account Standard Update (“ASU”) 2020-06, “Debt - Debt with Conversion and Other Options (subtopic 470-20) and Derivatives and Hedging - Contracts in Entity’s Own Equity (subtopic 815-40),” which reduces the number of accounting models in ASC 470-20 that require separate accounting for embedded conversion features. As a result, a convertible debt instrument will be accounted for as a single liability measured at its amortized cost as long as no other features require bifurcation and recognition as derivatives. By removing those separation models, the effective interest rate of convertible debt instruments will be closer to the coupon interest rate. Further, the diluted net income per share calculation for convertible instruments will require the Company to use the if-converted method. The treasury stock method should no longer be used to calculate diluted net income per share for convertible instruments. The amendment will be effective for the Company with annual period beginning January 1, 2022. The Company is evaluating the accounting, transition and disclosure requirements of the standard
Fair Value Measurement	The accounting guidance defines fair value as the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. When determining the fair value measurements for assets and liabilities recorded at fair value, the Company considers the principal or most advantageous market in which it would transact and considers assumptions that market participants would use when pricing the asset or liability, such as inherent risk, transfer restrictions, and risk of nonperformance. The fair value hierarchy requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. An asset’s or liability’s categorization within the fair value hierarchy is based upon the lowest level of input that is significant to the fair value measurement. Three levels of inputs may be used to measure fair value: • Level 1 - Valuations based on quoted prices in active markets for identical assets or liabilities that the Company is able to access. Since valuations are based on quoted prices that are readily and regularly available in an active market, valuation of such assets or liabilities do not entail a significant degree of judgment. • Level 2 - Valuations based on one or more quoted prices in markets that are not active or for which all significant inputs are observable, either directly or indirectly.
Earnings Per Share	Basic net income per share is computed by dividing net income by the weighted average number of shares of common stock outstanding during the period. Diluted net income per share is computed in a similar manner, but it also includes the effect of potential common shares outstanding during the period, when dilutive. Potential common shares include Stock Options, RSUs, PSUs, shares to be purchased under the Company’s ESPP, the Notes due 2023, Notes due 2024, 2024 Warrants, Notes due 2025, 2025 Warrants, Notes due 2026, 2026 Warrants, the Notes due 2028 and the 2028 Warrants. See Note 8. “Debt” of the notes to the condensed consolidated financial statements included in Part I, Item 1 of this Form 10-Q for additional information. The dilutive effect of potentially dilutive common shares is reflected in diluted earnings per share by application of the treasury stock method for stock options, RSUs, PSUs, the Notes due 2024, the 2024 Warrants, the Notes due 2025, the 2025 Warrants, the Notes due 2026, the 2026 Warrants, the Notes due 2028 and the 2028 Warrants. To the extent these potential common shares are antidilutive, they are excluded from the calculation of diluted net income per share.

REVENUE RECOGNITION (Tables)

REVENUE RECOGNITION (Tables)	3 Months Ended
	Mar. 31, 2021
Revenue from Contract with Customer [Abstract]
Summary of Revenue Disaggregation	Disaggregated revenue by primary geographical market and timing of revenue recognition for the Company’s single product line are as follows: Three Months Ended 2021 2020 (In thousands) Primary geographical markets: U.S. $ 247,782 $ 179,600 International 53,972 25,945 Total $ 301,754 $ 205,545 Timing of revenue recognition: Products delivered at a point in time $ 288,871 $ 194,679 Products and services delivered over time 12,883 10,866 Total $ 301,754 $ 205,545
Summary of Contract Assets and Contract Liabilities, and Changes in Balances from Contracts with Customers	Receivables, and contract assets and contract liabilities from contracts with customers are as follows: March 31, December 31, (In thousands) Receivables $ 236,090 $ 182,165 Short-term contract assets (Prepaid expenses and other assets) 19,338 17,879 Long-term contract assets (Other assets) 57,400 51,986 Short-term contract liabilities (Deferred revenues) 49,118 47,665 Long-term contract liabilities (Deferred revenues) 142,985 125,473 Significant changes in the balances of contract assets (prepaid expenses and other assets) during the period are as follows (in thousands): Contract Assets Contract Assets, beginning of period $ 69,865 Amount recognized (4,955) Increase 11,828 Contract Assets, end of period $ 76,738 Significant changes in the balances of contract liabilities (deferred revenues) during the period are as follows (in thousands): Contract Liabilities Contract Liabilities, beginning of period $ 173,138 Revenue recognized (16,235) Increase due to billings 35,200 Contract Liabilities, end of period $ 192,103
Summary of Estimated Revenue Expected to be Recognized in Future Periods	Estimated revenue expected to be recognized in future periods related to performance obligations that are unsatisfied or partially unsatisfied at the end of the reporting period are as follows: March 31, (In thousands) Fiscal year: 2021 (remaining nine months) $ 38,116 2022 44,067 2023 38,220 2024 32,994 2025 26,021 Thereafter 12,685 Total $ 192,103

OTHER FINANCIAL INFORMATION (Ta

OTHER FINANCIAL INFORMATION (Tables)	3 Months Ended
	Mar. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Inventory	Inventory consist of the following: March 31, December 31, (In thousands) Raw materials $ 10,611 $ 10,140 Finished goods 24,265 31,624 Total inventory $ 34,876 $ 41,764
Schedule of Accrued Liabilities	Accrued liabilities consist of the following: March 31, December 31, (In thousands) Salaries, commissions, incentive compensation and benefits $ 17,466 $ 6,634 Customer rebates and sales incentives 55,397 36,622 Freight 11,691 10,300 Operating lease liabilities, current 4,772 4,542 Liability due to supply agreements 4,310 5,500 Contingent consideration 4,193 — Other 17,343 12,944 Total accrued liabilities $ 115,172 $ 76,542

BUSINESS COMBINATION (Tables)

BUSINESS COMBINATION (Tables)	3 Months Ended
	Mar. 31, 2021
Business Combinations [Abstract]
Summary of consideration transferred for business acquisition	The acquisition date fair value of the purchase price was approximately $35.5 million, which consisted of the following (in thousands): Cash consideration $ 31,988 Fair value of contingent consideration 3,500 Total $ 35,488
Summary of fair values of assets acquired and liabilities assumed	The following table summarizes the preliminary estimated fair values of the assets acquired and liabilities assumed at the acquisition date, which are subject to change within the measurement period as the fair value assessments are finalized (in thousands): Net tangible assets acquired $ 1,441 Intangible assets 9,200 Deferred tax asset 457 Goodwill 24,390 Net assets acquired $ 35,488 The following table summarizes the preliminary estimated fair values of the assets acquired and liabilities assumed at the acquisition date, which are subject to change within the measurement period as the fair value assessments are finalized (in thousands): Net tangible assets acquired $ 1,541 Intangible assets 11,700 Goodwill 11,544 Net assets acquired $ 24,785
Summary of intangible assets acquired	The following table shows the fair value of the separately identifiable intangible assets at the time of acquisition and the period over which each intangible asset will be amortized: Preliminary Fair Value Useful Life (In thousands) (Years) Developed technology $ 6,900 5 Customer relationship 1,800 5 Trade Name 500 5 Total identifiable intangible assets $ 9,200 The following table shows the fair value of the separately identifiable intangible assets at the time of acquisition and the period over which each intangible asset will be amortized: Preliminary Fair Value Useful Life (In thousands) (Years) Customer relationship $ 11,700 5

GOODWILL AND INTANGIBLE ASSETS

GOODWILL AND INTANGIBLE ASSETS (Tables)	3 Months Ended
	Mar. 31, 2021
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Goodwill and Intangible Assets	The Company’s goodwill and purchased intangible assets as of March 31, 2021 and December 31, 2020 are as follows: March 31, 2021 December 31, 2020 Gross Additions Accumulated Amortization Net Gross Additions Accumulated Amortization Net (In thousands) Goodwill $ 24,783 $ 36,255 $ — $ 61,038 $ 24,783 $ — $ — $ 24,783 Intangible assets: Other indefinite-lived intangibles 286 — — 286 286 — — 286 Intangible assets with finite lives: Developed technology 13,100 6,900 (6,075) 13,925 13,100 — (5,276) 7,824 Customer relationships 26,421 13,500 (6,653) 33,268 23,100 3,321 (5,723) 20,698 Trade names — 500 (62) 438 — — — — Total purchased intangible assets $ 39,807 $ 20,900 $ (12,790) $ 47,917 $ 36,486 $ 3,321 $ (10,999) $ 28,808
Schedule of Amortization Expense	Amortization expense related to finite-lived intangible assets are as follows: Three Months Ended 2021 2020 (In thousands) Developed technology $ 799 $ 546 Customer relationships 930 701 Trade names 62 — Total amortization expense $ 1,791 $ 1,247

WARRANTY OBLIGATIONS (Tables)

WARRANTY OBLIGATIONS (Tables)	3 Months Ended
	Mar. 31, 2021
Product Warranties Disclosures [Abstract]
Summary of Warranty Activities	The Company’s warranty activities were as follows: Three Months Ended 2021 2020 (In thousands) Warranty obligations, beginning of period $ 45,913 $ 37,098 Accruals for warranties issued during period 3,894 1,524 Changes in estimates 7,655 1,677 Settlements (2,930) (3,270) Increase due to accretion expense 943 774 Other (922) (302) Warranty obligations, end of period 54,553 37,501 Less: current portion (14,303) (9,678) Noncurrent $ 40,250 $ 27,823

FAIR VALUE MEASUREMENTS (Tables

FAIR VALUE MEASUREMENTS (Tables)	3 Months Ended
	Mar. 31, 2021
Fair Value Disclosures [Abstract]
Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis	March 31, 2021 December 31, 2020 (In thousands) Level 1 Level 2 Level 3 Level 1 Level 2 Level 3 Assets: Cash and cash equivalents: Money market funds $ 1,463,750 $ — $ — $ 654,699 $ — $ — Other assets Investment in debt security — — 26,437 — — — Total assets measured at fair value $ 1,463,750 $ — $ 26,437 $ 654,699 $ — $ — Liabilities: Accrued liabilities Contingent consideration $ — $ — $ 3,540 $ — $ — $ — Warranty obligations Current — — 10,163 — — 8,267 Non-current — — 23,156 — — 20,469 Total warranty obligations measured at fair value — — 33,319 — — 28,736 Total liabilities measured at fair value $ — $ — $ 36,859 $ — $ — $ 28,736
Schedule of Changes in Nonfinancial Liabilities Related to Warrant Obligations Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs	The following table reflects the activity for the Company’s contingent consideration liabilities measured at fair value using Level 3 inputs for the three months ended March 31, 2021 (in thousands): Three Months Ended 2021 (In thousands) Balance at acquisition (January 25, 2021) $ 3,500 Fair value adjustments included in other income (expense), net 40 Balance at end of period $ 3,540 The following table provides information regarding changes in nonfinancial liabilities related to the Company’s warranty obligations measured at fair value on a recurring basis using significant unobservable inputs (Level 3) for the periods indicated. Three Months Ended 2021 2020 (In thousands) Balance at beginning of period $ 28,736 $ 19,806 Accruals for warranties issued during period 3,894 1,524 Changes in estimates 2,583 615 Settlements (1,915) (1,993) Increase due to accretion expense 943 774 Other (922) (301) Balance at end of period $ 33,319 $ 20,425
Summary of Significant Unobservable Inputs used in the Fair Value Measurement of Assets Designated as Level 3	These assumption include implied yield and change in estimated term of investment being held-to-maturity. Three Months Ended 2021 (In thousands) Initial investment (January 2021) $ 25,000 Fair value adjustments included in other income (expenses), net 1,437 Balance at end of period $ 26,437
Summary of Significant Unobservable Inputs used in the Fair Value Measurement of Liabilities Designated as Level 3	As of March 31, 2021 and December 31, 2020, the significant unobservable inputs used in the fair value measurement of the Company’s liabilities designated as Level 3 are as follows: Percent Used (Weighted Average) Item Measured at Fair Value Valuation Technique Description of Significant Unobservable Input March 31, December 31, Warranty obligations for microinverters sold since January 1, 2014 Discounted cash flows Profit element and risk premium 15% 15% Credit-adjusted risk-free rate 13% 13%

DEBT (Tables)

DEBT (Tables)	3 Months Ended
	Mar. 31, 2021
Debt Disclosure [Abstract]
Schedule of Debt	The following table provides information regarding the Company’s debt. March 31, December 31, (In thousands) Convertible notes Notes due 2028 $ 575,000 $ — Less: unamortized discount and issuance costs (164,905) — Carrying amount of Notes due 2028 410,095 — Notes due 2026 632,500 — Less: unamortized discount and issuance costs (129,630) — Carrying amount of Notes due 2026 502,870 — Notes due 2025 102,260 320,000 Less: unamortized discount and issuance costs (19,657) (64,979) Carrying amount of Notes due 2025 82,603 255,021 Notes due 2024 1,068 88,140 Less: unamortized discount and issuance costs (217) (19,119) Carrying amount of Notes due 2024 851 69,021 Notes due 2023 5,000 5,000 Less: unamortized issuance costs (92) (102) Carrying amount of Notes due 2023 4,908 4,898 Sale of long-term financing receivable recorded as debt 902 1,925 Total carrying amount of debt 1,002,229 330,865 Less: current portion of convertible notes and long-term financing receivable recorded as debt (84,356) (325,967) Long-term debt $ 917,873 $ 4,898 The following table presents the total amount of interest cost recognized in the statement of operations relating to the Notes due 2028: Three Months Ended 2021 (In thousands) Amortization of debt discount $ 1,611 Amortization of debt issuance costs 79 Total interest cost recognized $ 1,690 The following table presents the total amount of interest cost recognized in the statement of operations relating to the Notes due 2026: Three Months Ended 2021 (In thousands) Amortization of debt discount $ 1,786 Amortization of debt issuance costs 135 Total interest cost recognized $ 1,921 The following table presents the total amount of interest cost recognized relating to the Notes due 2025 (in thousands): Three Months Ended 2021 2020 (In thousands) Contractual interest expense $ 150 $ 49 Amortization of debt discount 2,389 743 Amortization of debt issuance costs 294 87 Total interest cost recognized $ 2,833 $ 879 The following table presents the total amount of interest cost recognized in the statement of operations relating to the Notes due 2024: Three Months Ended 2021 2020 (In thousands) Contractual interest expense $ 3 $ 330 Amortization of debt discount 733 1,562 Amortization of debt issuance costs 71 166 Total interest cost recognized $ 807 $ 2,058 The following table presents the amount of interest cost recognized relating to the contractual interest coupon and the amortization of debt issuance costs of the Notes due 2023. Three Months Ended 2021 2020 (In thousands) Contractual interest expense $ 50 $ 50 Amortization of debt issuance costs 10 10 Total interest costs recognized $ 60 $ 60

COMMITMENTS AND CONTINGENCIES (

COMMITMENTS AND CONTINGENCIES (Tables)	3 Months Ended
	Mar. 31, 2021
Commitments and Contingencies Disclosure [Abstract]
Components of Lease	The components of lease expense are presented as follows: Three Months Ended 2021 2020 (In thousands) Operating lease costs $ 1,631 $ 1,222 The components of lease liabilities are presented as follows: March 31, December 31, (In thousands) Operating lease liabilities, current (Accrued liabilities) $ 4,772 $ 4,542 Operating lease liabilities, noncurrent (Other liabilities) 13,899 15,209 Total operating lease liabilities $ 18,671 $ 19,751 Supplemental lease information: Weighted average remaining lease term 6.3 years 6.4 years Weighted average discount rate 7.6% 7.7% Supplemental cash flow and other information related to operating leases, are as follows: Three Months Ended 2021 2020 (In thousands) Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ 1,361 $ 1,014 Non-cash investing activities: Lease liabilities arising from obtaining right-of-use assets $ — $ 2,941
Schedule of Future Minimum Rental Payments for Operating Leases	Undiscounted cash flows of operating lease liabilities as of March 31, 2021 are as follows: Lease Amounts (In thousands) Year: 2021 (remaining nine months) $ 4,457 2022 4,660 2023 4,038 2024 3,053 2025 2,262 2026 and thereafter 3,957 Total lease payments 22,427 Less: imputed lease interest (3,756) Total lease liabilities $ 18,671

STOCK-BASED COMPENSATION (Table

STOCK-BASED COMPENSATION (Tables)	3 Months Ended
	Mar. 31, 2021
Share-based Payment Arrangement [Abstract]
Summary of the Components of Total Stock-Based Compensation Expense	The following table summarizes the components of total stock-based compensation expense included in the condensed consolidated statements of operations for the periods presented. Three Months Ended 2021 2020 (In thousands) Cost of revenues $ 982 $ 606 Research and development 5,749 1,919 Sales and marketing 3,537 1,942 General and administrative 4,576 3,048 Total $ 14,844 $ 7,515
Summary of Stock-Based Compensation Associated with Each Type of Award	The following table summarizes the various types of stock-based compensation expense for the periods presented. Three Months Ended 2021 2020 (In thousands) Stock options, RSUs, and PSUs $ 13,840 $ 7,077 Employee stock purchase plan 1,004 438 Total $ 14,844 $ 7,515
Summary of Stock Option Activity	The following is a summary of stock option activity. Number of Weighted- Weighted- Aggregate (1) (In thousands) (Years) (In thousands) Outstanding at December 31, 2020 2,532 $ 1.96 Granted — — Exercised (55) 3.91 $ 9,882 Canceled (1) 0.83 Outstanding at March 31, 2021 2,476 $ 1.92 3.5 $ 396,797 Vested and expected to vest at March 31, 2021 2,476 $ 1.92 3.5 $ 396,797 Exercisable at March 31, 2021 2,248 $ 1.92 3.5 $ 360,300 (1) The intrinsic value of options exercised is based upon the value of the Company’s stock at exercise. The intrinsic value of options outstanding, vested and expected to vest, and exercisable as of March 31, 2021 is based on the closing price of the last trading day during the period ended March 31, 2021. The Company’s stock fair value used in this computation was $162.16 per share.
Summary of Stock Option Outstanding	The following table summarizes information about stock options outstanding at March 31, 2021. Options Outstanding Options Exercisable Range of Exercise Prices Number of Weighted- Weighted- Number of Weighted- (In thousands) (Years) (In thousands) $0.70 —– $1.11 533 4.0 $ 0.85 496 $ 0.85 $1.29 —– $1.29 1,000 3.5 1.29 875 1.29 $1.31 —– $1.31 670 3.0 1.31 649 1.31 $1.39 —– $14.58 262 3.7 5.52 219 6.12 $64.17 —– $64.17 11 6.1 64.17 9 64.17 Total 2,476 3.5 $ 1.92 2,248 $ 1.92
Summary of Restricted Stock Unit Activity	The following is a summary of RSU activity. Number of Weighted- Weighted- Aggregate (1) (In thousands) (Years) (In thousands) Outstanding at December 31, 2020 3,588 $ 27.61 Granted 168 183.78 Vested (847) 17.49 $ 163,030 Canceled (12) 88.18 Outstanding at March 31, 2021 2,897 $ 39.37 1.17 $ 469,766 Expected to vest at March 31, 2021 2,897 $ 39.37 1.17 $ 469,718 (1) The intrinsic value of RSUs vested is based upon the value of the Company’s stock when vested. The intrinsic value of RSUs outstanding and expected to vest as of March 31, 2021 is based on the closing price of the last trading day during the period ended March 31, 2021. The Company’s stock fair value used in this computation was $162.16 per share.
Share-based Compensation, Performance Shares Award Outstanding Activity	The following is a summary of PSU activity. Number of Weighted- Weighted- Aggregate (1) (In thousands) (Years) (In thousands) Outstanding at December 31, 2020 494 $ 51.10 Granted 262 66.89 Vested (494) 59.19 $ 91,803 Canceled (247) 42.00 Outstanding at March 31, 2021 15 $ 193.74 0.4 $ 2,432 (1) The intrinsic value of PSUs vested is based upon the value of the Company’s stock when vested. The intrinsic value of PSUs outstanding and expected to vest as of March 31, 2021 is based on the closing price of the last trading day during the period ended March 31, 2021. The Company’s stock fair value used in this computation was $162.16 per share.

NET INCOME PER SHARE (Tables)

NET INCOME PER SHARE (Tables)	3 Months Ended
	Mar. 31, 2021
Earnings Per Share [Abstract]
Schedule of Computation of Basic and Diluted Net Income Per Share	The following table presents the computation of basic and diluted net income per share for the periods presented. Three Months Ended 2021 2020 (In thousands, except per share data) Numerator: Net income $ 31,698 $ 68,936 Notes due 2023 interest and financing costs, net 44 44 Adjusted net income $ 31,742 $ 68,980 Denominator: Shares used in basic per share amounts: Weighted average common shares outstanding 131,303 123,531 Shares used in diluted per share amounts: Weighted average common shares outstanding 131,303 123,531 Effect of dilutive securities: Employee stock-based awards 5,735 8,609 Notes due 2023 900 900 Notes due 2024 2,984 2,936 2024 Warrants 2,506 2,128 Notes due 2025 1,713 — 2025 Warrants 1,301 — Weighted average common shares outstanding for diluted calculation 146,442 138,104 Basic and diluted net income per share Net income per share, basic $ 0.24 $ 0.56 Net income per share, diluted $ 0.22 $ 0.50
Schedule of Potentially Dilutive Securities Excluded from the Computation of Diluted Net Income Per Share	The following outstanding shares of common stock equivalents were excluded from the calculation of the diluted net income per share attributable to common stockholders because their effect would have been antidilutive. Three Months Ended 2021 2020 (In thousands) Employee stock-based awards 54 — Notes due 2026 682 — 2026 Warrants 1,070 — Notes due 2028 569 — 2028 Warrants 1,070 — Total 3,445 —

REVENUE RECOGNITION - Summary o

REVENUE RECOGNITION - Summary of Disaggregated Revenue by Primary Geographical Market and Timing of Revenue Recognition (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	$ 301,754	$ 205,545
Products delivered at a point in time
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	288,871	194,679
Products and services delivered over time
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	12,883	10,866
U.S.
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	247,782	179,600
International
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net revenues	$ 53,972	$ 25,945

REVENUE RECOGNITION - Summary_2

REVENUE RECOGNITION - Summary of Contract Assets and Contract Liabilities from Contracts with Customers (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Receivables	$ 236,090	$ 182,165
Short-term contract assets (Prepaid expenses and other assets)	19,338	17,879
Long-term contract assets (Other assets)	57,400	51,986
Short-term contract liabilities (Deferred revenues)	49,118	47,665
Long-term contract liabilities (Deferred revenues)	$ 142,985	$ 125,473

REVENUE RECOGNITION - Narrative

REVENUE RECOGNITION - Narrative (Details)	3 Months Ended
	Mar. 31, 2021USD ($)
Revenue from Contract with Customer [Abstract]
Contract asset impairment charges	$ 0

REVENUE RECOGNITION - Summary_3

REVENUE RECOGNITION - Summary of Significant Changes in the Balances of Contract Liabilities and Assets (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Contract Assets
Balance, beginning of period	$ 69,865
Amount recognized	(4,955)
Increase	11,828
Balance, end of period	76,738
Contract Liabilities
Balance, beginning of period	173,138
Revenue recognized	(16,235)
Increase due to billings	35,200
Balance, end of period	$ 192,103

REVENUE RECOGNITION - Summary_4

REVENUE RECOGNITION - Summary of Estimated Revenue Expected to be Recognized in Future Periods (Details) $ in Thousands	Mar. 31, 2021USD ($)
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 192,103
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-04-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 38,116
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing	9 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 44,067
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 38,220
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2024-01-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 32,994
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-01-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 26,021
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue from Contract with Customer [Abstract]
Total estimated revenue expected to be recognized in future periods	$ 12,685
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Total estimated revenue expected to be recognized in future periods, expected timing

OTHER FINANCIAL INFORMATION - I

OTHER FINANCIAL INFORMATION - Inventory (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Raw materials	$ 10,611	$ 10,140
Finished goods	24,265	31,624
Total inventory	$ 34,876	$ 41,764

OTHER FINANCIAL INFORMATION - A

OTHER FINANCIAL INFORMATION - Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Salaries, commissions, incentive compensation and benefits	$ 17,466	$ 6,634
Customer rebates and sales incentives	55,397	36,622
Freight	11,691	10,300
Operating lease liabilities, current	4,772	4,542
Liability due to supply agreements	4,310	5,500
Contingent consideration	4,193	0
Other	17,343	12,944
Total accrued liabilities	$ 115,172	$ 76,542

BUSINESS COMBINATION - Narrativ

BUSINESS COMBINATION - Narrative (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Jan. 25, 2021	Mar. 31, 2021	Mar. 31, 2020
Business Acquisition [Line Items]
Contingent consideration in connection with the acquisition			$ 3,500	$ 0
Sofdesk
Business Acquisition [Line Items]
Voting interests acquired		100.00%
Purchase price		$ 31,988
Contingency payable, maximum		3,700
Contingent consideration in connection with the acquisition		3,500
Consideration transferred for business acquisition		$ 35,488
Total acquisition related costs	$ 1,700		1,700
DIN's
Business Acquisition [Line Items]
Purchase price	24,800
Total acquisition related costs	$ 1,400		1,400
Timing of monthly installment payments	1 year
DIN's | Additional Consideration, Equal Monthly Installments
Business Acquisition [Line Items]
Contingency payable, maximum	$ 5,000		5,000
Timing of monthly installment payments	1 year
DIN's | Additional Consideration, Payable One Year Anniversary
Business Acquisition [Line Items]
Contingency payable, maximum	$ 5,000		$ 5,000

BUSINESS COMBINATION - Summary

BUSINESS COMBINATION - Summary of Consideration Transferred for Business Acquisition (Details) - USD ($) $ in Thousands	Jan. 25, 2021	Mar. 31, 2021	Mar. 31, 2020
Business Acquisition [Line Items]
Fair value of contingent consideration		$ 3,500	$ 0
Sofdesk
Business Acquisition [Line Items]
Cash consideration	$ 31,988
Fair value of contingent consideration	3,500
Total	$ 35,488

BUSINESS COMBINATION - Summar_2

BUSINESS COMBINATION - Summary of Fair Values of Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Jan. 25, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Goodwill	$ 61,038		$ 24,783
Sofdesk
Business Acquisition [Line Items]
Net tangible assets acquired		$ 1,441
Intangible assets		9,200
Deferred tax asset		457
Goodwill		24,390
Net assets acquired		35,488
Sofdesk | Customer relationship
Business Acquisition [Line Items]
Intangible assets		$ 1,800
DIN's
Business Acquisition [Line Items]
Net tangible assets acquired	1,541
Intangible assets	11,700
Goodwill	11,544
Net assets acquired	24,785
DIN's | Customer relationship
Business Acquisition [Line Items]
Intangible assets	$ 11,700

BUSINESS COMBINATION - Summar_3

BUSINESS COMBINATION - Summary of Identifiable Intangible Assets Acquired (Details) - USD ($) $ in Thousands	Jan. 25, 2021	Mar. 31, 2021
Sofdesk
Business Acquisition [Line Items]
Intangible assets	$ 9,200
Sofdesk | Developed technology
Business Acquisition [Line Items]
Intangible assets	$ 6,900
Useful Life	5 years
Sofdesk | Customer relationship
Business Acquisition [Line Items]
Intangible assets	$ 1,800
Useful Life	5 years
Sofdesk | Trade Name
Business Acquisition [Line Items]
Intangible assets	$ 500
Useful Life	5 years
DIN's
Business Acquisition [Line Items]
Intangible assets		$ 11,700
DIN's | Customer relationship
Business Acquisition [Line Items]
Intangible assets		$ 11,700
Useful Life		5 years

GOODWILL AND INTANGIBLE ASSET_2

GOODWILL AND INTANGIBLE ASSETS - Schedule of Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Goodwill, Gross	$ 24,783	$ 24,783
Goodwill, Additions	36,255
Goodwill	61,038	24,783
Intangible assets with finite lives:
Other indefinite-lived intangibles	286	286
Additions	20,900	3,321
Accumulated Amortization	(12,790)	(10,999)
Total purchased intangible assets, Gross	39,807	36,486
Total purchased intangible assets, Net	47,917	28,808
Developed technology
Intangible assets with finite lives:
Gross	13,100	13,100
Additions	6,900	0
Accumulated Amortization	(6,075)	(5,276)
Net	13,925	7,824
Customer relationship
Intangible assets with finite lives:
Gross	26,421	23,100
Additions	13,500	3,321
Accumulated Amortization	(6,653)	(5,723)
Net	33,268	20,698
Trade names
Intangible assets with finite lives:
Gross	0	0
Additions	500	0
Accumulated Amortization	(62)	0
Net	$ 438	$ 0

GOODWILL AND INTANGIBLE ASSET_3

GOODWILL AND INTANGIBLE ASSETS - Amortization Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 1,791	$ 1,247
Developed technology
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	799	546
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	930	701
Trade names
Finite-Lived Intangible Assets [Line Items]
Amortization of intangible assets	$ 62	$ 0

WARRANTY OBLIGATIONS - Summary

WARRANTY OBLIGATIONS - Summary of Warranty Activities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Changes in the Company's product warranty liability
Warranty obligations, beginning of period	$ 45,913	$ 37,098
Accruals for warranties issued during period	3,894	1,524
Changes in estimates	7,655	1,677
Settlements	(2,930)	(3,270)
Increase due to accretion expense	943	774
Other	(922)	(302)
Warranty obligations, end of period	54,553	37,501
Less: current portion	(14,303)	(9,678)	$ (11,260)
Noncurrent	$ 40,250	$ 27,823	$ 34,653

WARRANTY OBLIGATIONS - Narrativ

WARRANTY OBLIGATIONS - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Product Warranty Liability [Line Items]
Additional warranty expense	$ 7,655	$ 1,677
Field Performance Data And Diagnostic Root-Cause Failure Analysis
Product Warranty Liability [Line Items]
Additional warranty expense	6,300	1,500
Unit Costs For Prior Generation Microinverter Replacement Driven By Tariffs
Product Warranty Liability [Line Items]
Additional warranty expense	$ 1,300	$ 200

FAIR VALUE MEASUREMENTS - Sched

FAIR VALUE MEASUREMENTS - Schedule of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Jan. 31, 2021	Dec. 31, 2020
Liabilities:
Contingent consideration	$ 4,193		$ 0
Level 3
Assets [Abstract]
Investment in debt security		$ 25,000
Recurring | Level 1
Assets [Abstract]
Investment in debt security	0		0
Total assets measured at fair value	1,463,750		654,699
Liabilities:
Contingent consideration	0		0
Warranty obligations
Warranty obligations, current	0		0
Warranty obligations, non-current	0		0
Total warranty obligations measured at fair value	0		0
Total liabilities measured at fair value	0		0
Recurring | Level 1 | Money market funds
Assets [Abstract]
Cash and cash equivalents	1,463,750		654,699
Recurring | Level 2
Assets [Abstract]
Investment in debt security	0		0
Total assets measured at fair value	0		0
Liabilities:
Contingent consideration	0		0
Warranty obligations
Warranty obligations, current	0		0
Warranty obligations, non-current	0		0
Total warranty obligations measured at fair value	0		0
Total liabilities measured at fair value	0		0
Recurring | Level 2 | Money market funds
Assets [Abstract]
Cash and cash equivalents	0		0
Recurring | Level 3
Assets [Abstract]
Investment in debt security	26,437		0
Total assets measured at fair value	26,437		0
Liabilities:
Contingent consideration	3,540		0
Warranty obligations
Warranty obligations, current	10,163		8,267
Warranty obligations, non-current	23,156		20,469
Total warranty obligations measured at fair value	33,319		28,736
Total liabilities measured at fair value	36,859		28,736
Recurring | Level 3 | Money market funds
Assets [Abstract]
Cash and cash equivalents	$ 0		$ 0

FAIR VALUE MEASUREMENTS - Narra

FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Jan. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase in liability as a result of increasing the profit element and risk premium input by 100 basis points	$ 300
Decrease in liability as a result of decreasing the profit element and risk premium input by 100 basis points	300
Increase in liability as a result of decreasing the discount rate by 100 basis points	1,500
Decrease in liability as a result of increasing the discount rate by 100 basis points	1,700
Level 2 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities	0		$ 0
Level 2 | Recurring | Convertible Notes | Convertible Senior Notes Due 2028
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable fair value	538,300
Level 2 | Recurring | Convertible Notes | Convertible Senior Notes Due 2026
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable fair value	596,100
Level 2 | Recurring | Convertible Notes | Convertible Senior Notes Due 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable fair value	218,700
Level 2 | Recurring | Convertible Notes | Convertible Senior Notes Due 2024
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable fair value	9,100
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities		$ 25,000
Level 3 | Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities	$ 26,437		$ 0

FAIR VALUE MEASUREMENTS - Debt

FAIR VALUE MEASUREMENTS - Debt Securities Schedule of Fair Value (Details) $ in Thousands	2 Months Ended
	Mar. 31, 2021USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Initial investment (January 2021)	$ 25,000
Balance at end of period	26,437
Investment in debt security
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair value adjustments included in other income (expenses), net	$ 1,437

FAIR VALUE MEASUREMENTS - Conti

FAIR VALUE MEASUREMENTS - Contingent Consideration Schedule of Fair Value (Details) $ in Thousands	2 Months Ended
	Mar. 31, 2021USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 3,500
Balance at end of period	3,540
Contingent consideration
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Contingent consideration liability	$ 40

FAIR VALUE MEASUREMENTS - Sch_2

FAIR VALUE MEASUREMENTS - Schedule of Changes in Nonfinancial Liabilities Related to Warrant Obligations Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Details) - USD ($) $ in Thousands	2 Months Ended	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2021	Mar. 31, 2020
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 3,500
Balance at end of period	3,540	$ 3,540
Recurring | Total warranty obligations measured at fair value
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period		28,736	$ 19,806
Accruals for warranties issued during period		3,894	1,524
Changes in estimates		2,583	615
Settlements		(1,915)	(1,993)
Increase due to accretion expense		943	774
Other		(922)	(301)
Balance at end of period	$ 33,319	$ 33,319	$ 20,425

FAIR VALUE MEASUREMENTS - Summa

FAIR VALUE MEASUREMENTS - Summary of Significant Unobservable Inputs used in the Fair Value Measurement of Liabilities Designated as Level 3 (Details) - Recurring - Level 3 - Warranty obligations for microinverters sold since January 1, 2014	Mar. 31, 2021	Dec. 31, 2020
Profit element and risk premium
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warranty obligations, measurement input	15.00%	15.00%
Credit-adjusted risk-free rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warranty obligations, measurement input	13.00%	13.00%

DEBT - Long-term debt (Details)

DEBT - Long-term debt (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Total carrying amount of debt	$ 1,002,229	$ 330,865
Less: current portion of convertible notes and long-term financing receivable recorded as debt	(84,356)	(325,967)
Long-term debt	917,873	4,898
Convertible Notes | Convertible Senior Notes Due 2028
Debt Instrument [Line Items]
Long-term debt, gross	575,000	0
Less unamortized discount and issuance costs	(164,905)	0
Total carrying amount of debt	410,095	0
Convertible Notes | Convertible Senior Notes Due 2026
Debt Instrument [Line Items]
Long-term debt, gross	632,500	0
Less unamortized discount and issuance costs	(129,630)	0
Total carrying amount of debt	502,870	0
Convertible Notes | Convertible Senior Notes Due 2025
Debt Instrument [Line Items]
Long-term debt, gross	102,260	320,000
Less unamortized discount and issuance costs	(19,657)	(64,979)
Total carrying amount of debt	82,603	255,021
Less: current portion of convertible notes and long-term financing receivable recorded as debt	(102,300)
Convertible Notes | Convertible Senior Notes Due 2024
Debt Instrument [Line Items]
Long-term debt, gross	1,068	88,140
Less unamortized discount and issuance costs	(217)	(19,119)
Total carrying amount of debt	851	69,021
Less: current portion of convertible notes and long-term financing receivable recorded as debt	(900)
Convertible Notes | Convertible Senior Notes Due 2023
Debt Instrument [Line Items]
Long-term debt, gross	5,000	5,000
Less unamortized discount and issuance costs	(92)	(102)
Total carrying amount of debt	4,908	4,898
Financing Receivable | Financing Receivable Recorded as Debt
Debt Instrument [Line Items]
Long-term debt, gross	$ 902	$ 1,925

DEBT - Convertible Senior Notes

DEBT - Convertible Senior Notes due in 2028 Narrative (Details) $ / shares in Units, shares in Millions	Mar. 01, 2021USD ($)trading_day$ / sharesshares	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)
Debt Instrument [Line Items]
Payment for bonds hedge		$ 286,235,000	$ 89,056,000
Proceeds from sale of warrants		220,800,000	$ 71,552,000
Convertible Notes | Convertible Senior Notes Due 2028
Debt Instrument [Line Items]
Debt instrument face amount	$ 575,000,000
Interest rate	0.00%
Proceeds from convertible debt	$ 566,400,000
Conversion ratio	0.0035104
Debt conversion price (in USD per share) | $ / shares	$ 284.87
Convertible note, liability component	$ 415,000,000
Convertible note, equity component	160,000,000
Debt issuance costs	9,100,000
Unamortized debt issuance costs	6,600,000	6,500,000
Debt issuance costs, allocated to capital	$ 2,500,000
Unamortized discount		$ 158,400,000
Remaining discount amortization period		6 years 10 months 24 days
Conversion shares (in shares) | shares	2
Payment for bonds hedge	$ 161,600,000
Warrants issued, strike price (in USD per share) | $ / shares	$ 397.91
Proceeds from sale of warrants	$ 123,400,000
Convertible Notes | Convertible Senior Notes Due 2028 | Discount rate
Debt Instrument [Line Items]
Measurement input		0.0477
Period One | Convertible Notes | Convertible Senior Notes Due 2028
Debt Instrument [Line Items]
Threshold percentage	130.00%
Stock trigger price (in USD per share) | $ / shares	$ 370.33
Number of threshold trading days | trading_day	20
Number of consecutive trading days | trading_day	30
Measurement period percentage of stock price trigger	98.00%
Period Two | Convertible Notes | Convertible Senior Notes Due 2028
Debt Instrument [Line Items]
Threshold percentage	100.00%
Number of consecutive trading days | trading_day	5

DEBT - Schedule of Convertible

DEBT - Schedule of Convertible Senior Notes due in 2028 (Details) - Convertible Notes - Convertible Senior Notes Due 2028 $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Debt Instrument [Line Items]
Amortization of debt discount	$ 1,611
Amortization of debt issuance costs	79
Total interest cost recognized	$ 1,690

DEBT - Convertible Senior Not_2

DEBT - Convertible Senior Notes due in 2026 Narrative (Details) $ / shares in Units, shares in Millions	Mar. 01, 2021USD ($)trading_day$ / sharesshares	Mar. 09, 2020trading_day	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)	Mar. 12, 2021USD ($)
Debt Instrument [Line Items]
Payment for bonds hedge			$ 286,235,000	$ 89,056,000
Proceeds from sale of warrants			220,800,000	$ 71,552,000
Convertible Notes | Convertible Senior Notes Due 2026
Debt Instrument [Line Items]
Debt instrument face amount	$ 575,000,000				$ 57,500,000
Interest rate	0.00%
Proceeds from convertible debt	$ 623,000,000
Conversion ratio	0.0032523
Debt conversion price (in USD per share) | $ / shares	$ 307.47
Convertible note, liability component	$ 509,000,000
Convertible note, equity component	123,500,000
Debt issuance costs	10,000,000
Unamortized debt issuance costs	8,100,000		$ 7,900,000
Debt issuance costs, allocated to capital	$ 2,000,000
Effective percentage rate			4.44%
Unamortized discount			$ 121,700,000
Remaining discount amortization period			4 years 10 months 24 days
Conversion shares (in shares) | shares	2.1
Payment for bonds hedge	$ 124,600,000
Proceeds from sale of warrants	$ 97,400,000
Warrants issued, strike price (in USD per share) | $ / shares	$ 397.91
Convertible Notes | Convertible Senior Notes Due 2026 | Discount rate
Debt Instrument [Line Items]
Measurement input	0.0444
Period One | Convertible Notes | Convertible Senior Notes Due 2026
Debt Instrument [Line Items]
Threshold percentage	130.00%
Stock trigger price (in USD per share) | $ / shares	$ 399.71
Number of threshold trading days | trading_day	20
Number of consecutive trading days | trading_day	30
Measurement period percentage of stock price trigger	98.00%
Period Two | Convertible Notes | Convertible Senior Notes Due 2026
Debt Instrument [Line Items]
Threshold percentage	100.00%
Number of consecutive trading days | trading_day		5

DEBT - Schedule of Convertibl_2

DEBT - Schedule of Convertible Senior Notes due in 2026 (Details) - Convertible Notes - Convertible Senior Notes Due 2026 $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Debt Instrument [Line Items]
Amortization of debt discount	$ 1,786
Amortization of debt issuance costs	135
Total interest cost recognized	$ 1,921

DEBT - Convertible Senior Not_3

DEBT - Convertible Senior Notes due in 2025 Narrative (Details)	Mar. 01, 2021USD ($)	Mar. 09, 2020USD ($)trading_day$ / sharesshares	Apr. 27, 2021USD ($)	Mar. 31, 2021USD ($)$ / sharesshares	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)$ / sharesshares	May 20, 2020USD ($)$ / sharesshares	May 19, 2020shares
Debt Instrument [Line Items]
Debt, current				$ 84,356,000		$ 325,967,000
Common stock, par value (in usd per share) | $ / shares				$ 0.00001		$ 0.00001	$ 0.00001
Common stock, shares authorized (in shares) | shares				200,000,000		200,000,000	200,000,000	150,000,000
Deferred tax assets, net				$ 132,231,000		$ 92,904,000
Partial repurchase of convertible notes				289,233,000	$ 0
Loss on partial repurchase of convertible notes				56,369,000	0
Payment for bonds hedge				286,235,000	89,056,000
Proceeds from sale of warrants				220,800,000	$ 71,552,000
Warrants obligations measured at fair value							$ 96,400,000
Convertible Notes
Debt Instrument [Line Items]
Convertible notes embedded derivative							117,100,000
Convertible Notes | Convertible Senior Notes Due 2025
Debt Instrument [Line Items]
Debt instrument face amount		$ 320,000,000
Interest rate		0.25%
Debt conversion price (in USD per share) | $ / shares		$ 81.54
Proceeds from convertible debt		$ 313,000,000
Conversion ratio		0.0122637
Debt, current				102,300,000
Convertible notes embedded derivative		$ 68,700,000					$ 116,300,000
Embedded derivative, host contract		251,300,000
Deferred tax assets, net				$ 200,000
Debt issuance costs		$ 7,600,000
Partial repurchase of convertible notes	$ 217,700,000
Exercise of warrants (shares) | shares				1,670,000
Equity component of convertible notes due 2025, net	302,700,000
Convertible note, liability component	$ 184,500,000
Residual				$ 4,300,000
Induced conversion of convertible debt expense				$ 37,500,000
Remaining expected life				4 years 1 month 6 days
Write off of debt discount				$ 38,500,000
Write off of deferred debt issuance cost				4,100,000
Loss on partial repurchase of convertible notes				$ 9,400,000
Effective percentage rate				5.18%
Unamortized discount				$ 17,700,000		$ 58,600,000
Remaining discount amortization period				3 years 10 months 24 days
Conversion shares (in shares) | shares		3,900,000
Payment for bonds hedge		$ 89,100,000
Warrants issued, strike price (in USD per share) | $ / shares		$ 106.94
Proceeds from sale of warrants		$ 71,600,000
Convertible notes hedge settlements, shares received (in shares) | shares				1,900,000
Warrants unwound, shares issued (in shares) | shares				1,800,000
Convertible notes hedge transaction, options outstanding (in shares) | shares				1,300,000
Warrants outstanding (in shares) | shares				1,300,000
Convertible Notes | Convertible Senior Notes Due 2025 | Subsequent Event
Debt Instrument [Line Items]
Amount to be converted			$ 100,000
Convertible Notes | Convertible Senior Notes Due 2025 | Discount rate
Debt Instrument [Line Items]
Measurement input				0.0435
Period One | Convertible Notes | Convertible Senior Notes Due 2025
Debt Instrument [Line Items]
Number of threshold trading days | trading_day		20
Number of consecutive trading days | trading_day		30
Threshold percentage		130.00%
Stock trigger price (in USD per share) | $ / shares				$ 106
Period Two | Convertible Notes | Convertible Senior Notes Due 2025
Debt Instrument [Line Items]
Number of consecutive trading days | trading_day		5
Threshold percentage		100.00%
Measurement period percentage of stock price trigger		98.00%

DEBT - Schedule of Convertibl_3

DEBT - Schedule of Convertible Senior Notes due in 2025 (Details) - Convertible Notes - Convertible Senior Notes Due 2025 - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Debt Instrument [Line Items]
Contractual interest expense	$ 150	$ 49
Amortization of debt discount	2,389	743
Amortization of debt issuance costs	294	87
Total interest cost recognized	$ 2,833	$ 879

DEBT - Convertible Senior Not_4

DEBT - Convertible Senior Notes due 2024 Narrative (Details) $ / shares in Units, shares in Millions	Mar. 01, 2021trading_day	Jun. 05, 2019USD ($)trading_day$ / sharesshares	Mar. 31, 2021USD ($)trading_day$ / sharesshares	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)
Debt Instrument [Line Items]
Debt, current			$ 84,356,000		$ 325,967,000
Deferred tax assets, net			132,231,000		92,904,000
Loss on partial repurchase of convertible notes			56,369,000	$ 0
Payment for bonds hedge			286,235,000	89,056,000
Proceeds from sale of warrants			220,800,000	$ 71,552,000
Convertible Notes | Convertible Senior Notes Due 2024
Debt Instrument [Line Items]
Debt instrument face amount		$ 132,000,000	1,100,000
Interest rate		1.00%
Debt conversion price (in USD per share) | $ / shares		$ 20.50
Proceeds from convertible debt		$ 128,000,000
Conversion ratio		0.0487781
Debt, current			900,000
Conversion of debt			87,100,000
Convertible note, liability component		$ 95,600,000	78,400,000
Convertible note, equity component		36,400,000	8,600,000
Deferred tax assets, net		300,000
Debt issuance costs		4,600,000
Unamortized debt issuance costs		3,300,000	100,000		1,500,000
Debt issuance costs, allocated to capital		$ 1,300,000
Repurchase amount			$ 25,500,000
Exercise of warrants (shares) | shares			3.8
Equity component of convertible notes due 2025, net			$ 659,400,000
Remaining expected life			3 years 3 months 18 days
Write off of debt discount			$ 16,700,000
Write off of deferred debt issuance cost			1,400,000
Loss on partial repurchase of convertible notes			$ 9,500,000
Effective percentage rate			7.75%
Unamortized discount			$ 200,000		$ 17,600,000
Remaining discount amortization period			3 years 2 months 12 days
Conversion shares (in shares) | shares		6.4
Payment for bonds hedge		$ 36,300,000
Warrants sold (in shares) | shares		6.4
Warrants issued, strike price (in USD per share) | $ / shares		$ 25.23
Proceeds from sale of warrants		$ 29,800,000
Convertible notes hedge transaction, shares received (in shares) | shares			1.1
Convertible notes hedge settlements, shares received (in shares) | shares			2.7
Debt converted			$ 61,500,000
Warrants unwound, shares issued (in shares) | shares			3.8
Warrants outstanding (in shares) | shares			0.1
Convertible notes hedge transaction, options outstanding (in shares) | shares			0.1
Convertible Notes | Convertible Senior Notes Due 2024 | Discount rate
Debt Instrument [Line Items]
Measurement input		0.0775	0.0425
Period One | Convertible Notes | Convertible Senior Notes Due 2024
Debt Instrument [Line Items]
Number of threshold trading days | trading_day		20	20
Number of consecutive trading days | trading_day			30
Stock trigger price (in USD per share) | $ / shares			$ 26.65
Threshold percentage			130.00%
Measurement period percentage of stock price trigger		98.00%
Period Two | Convertible Notes | Convertible Senior Notes Due 2024
Debt Instrument [Line Items]
Number of consecutive trading days | trading_day	5
Threshold percentage		100.00%

DEBT - Schedule of Convertibl_4

DEBT - Schedule of Convertible Senior Notes due 2024 (Details) - Convertible Notes - Convertible Senior Notes Due 2024 - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Debt Instrument [Line Items]
Contractual interest expense	$ 3	$ 330
Amortization of debt discount	733	1,562
Amortization of debt issuance costs	71	166
Total interest cost recognized	$ 807	$ 2,058

DEBT - Convertible Senior Not_5

DEBT - Convertible Senior Notes due 2023 Narrative (Details) - Convertible Notes - Convertible Senior Notes Due 2023	Jun. 05, 2019USD ($)shares	Aug. 31, 2018USD ($)$ / shares	Mar. 31, 2021USD ($)	Dec. 31, 2020USD ($)
Debt Instrument [Line Items]
Debt instrument face amount		$ 65,000,000	$ 5,000,000	$ 5,000,000
Interest rate		4.00%
Debt converted	$ 60,000,000
Conversion shares (in shares) | shares	10,801,080
Fees paid for repurchase and exchange of convertible notes	$ 6,000,000
Debt conversion price (in USD per share) | $ / shares		$ 5.56
Conversion ratio		0.1800180
Redemption price percentage		100.00%

DEBT - Schedule of Convertibl_5

DEBT - Schedule of Convertible Senior Notes due 2023 (Details) - Convertible Notes - Convertible Senior Notes Due 2023 - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Debt Instrument [Line Items]
Contractual interest expense	$ 50	$ 50
Amortization of debt issuance costs	10	10
Total interest cost recognized	$ 60	$ 60

DEBT - Sale of Long - Term Fina

DEBT - Sale of Long - Term Financing Receivables (Details) - Financing Receivable - Financing Receivable Recorded As Debt - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Mar. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Proceeds from debt, net of issuance costs	$ 2,800	$ 5,600
Long-term debt, gross			$ 902	$ 1,925

COMMITMENTS AND CONTINGENCIES -

COMMITMENTS AND CONTINGENCIES - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 26, 2020
Line of Credit Facility [Line Items]
Term of lease contract, maximum renewal term	12 years
Purchase obligation	$ 177,200,000
Section 301 Tariff Refunds
Line of Credit Facility [Line Items]
Potential positive impact of approval of requested Tariff refunds			$ 38,900,000
Tariff refund	14,700,000	$ 24,800,000
Tariff refund receivable	$ 0	14,700,000
Recognized in current period		38,900,000
Section 301 Tariff Refunds, Accrued Interest
Line of Credit Facility [Line Items]
Potential positive impact of approval of requested Tariff refunds			$ 600,000
Recognized in current period		$ 600,000

COMMITMENTS AND CONTINGENCIES_2

COMMITMENTS AND CONTINGENCIES - Lease Expense Components (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Operating lease costs	$ 1,631	$ 1,222

COMMITMENTS AND CONTINGENCIES_3

COMMITMENTS AND CONTINGENCIES - Lease Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Operating lease liabilities, current (Accrued liabilities)	$ 4,772	$ 4,542
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	us-gaap:AccruedLiabilitiesCurrent	us-gaap:AccruedLiabilitiesCurrent
Operating lease liabilities, noncurrent (Other liabilities)	$ 13,899	$ 15,209
Operating Lease, Liability, Noncurrent, Statement of Financial Position [Extensible List]	us-gaap:OtherLiabilitiesNoncurrent	us-gaap:OtherLiabilitiesNoncurrent
Total operating lease liabilities	$ 18,671	$ 19,751
Weighted average remaining lease term	6 years 3 months 18 days	6 years 4 months 24 days
Weighted average discount rate	7.60%	7.70%

COMMITMENTS AND CONTINGENCIES_4

COMMITMENTS AND CONTINGENCIES - Supplemental Cash Flow and Other Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Operating cash flows from operating leases	$ 1,361	$ 1,014
Lease liabilities arising from obtaining right-of-use assets	$ 0	$ 2,941

COMMITMENTS AND CONTINGENCIES_5

COMMITMENTS AND CONTINGENCIES - Schedule of Minimum Lease Payments Under Noncancelable Operating Leases (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2021 (remaining nine months)	$ 4,457
2022	4,660
2023	4,038
2024	3,053
2025	2,262
2026 and thereafter	3,957
Total lease payments	22,427
Less: imputed lease interest	(3,756)
Total lease liabilities	$ 18,671	$ 19,751

STOCK-BASED COMPENSATION Narrat

STOCK-BASED COMPENSATION Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)
Share-based Payment Arrangement [Abstract]
Total unrecognized compensation cost	$ 107.4
Weighted-average recognition period for unrecognized compensation cost	3 years

STOCK-BASED COMPENSATION - Summ

STOCK-BASED COMPENSATION - Summary of Stock-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 14,844	$ 7,515
Cost of revenues
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	982	606
Research and development
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	5,749	1,919
Sales and marketing
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	3,537	1,942
General and administrative
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 4,576	$ 3,048

STOCK-BASED COMPENSATION - Su_2

STOCK-BASED COMPENSATION - Summary of Stock-Based Compensation Expense Associated with Each Type of Award (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 14,844	$ 7,515
Stock options, RSUs, and PSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	13,840	7,077
Employee stock purchase plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense	$ 1,004	$ 438

STOCK-BASED COMPENSATION - Su_3

STOCK-BASED COMPENSATION - Summary of Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)$ / sharesshares
Number of Shares Outstanding
Outstanding, beginning balance (in shares) | shares	2,532
Granted (in shares) | shares	0
Exercised (in shares) | shares	(55)
Canceled (in shares) | shares	(1)
Outstanding, ending balance (in shares) | shares	2,476
Shares outstanding, vested and expected to vest (in shares) | shares	2,476
Shares outstanding, exercisable (in shares) | shares	2,248
Weighted- Average Exercise Price per Share
Outstanding, beginning balance (in usd per share)	$ 1.96
Granted (in usd per share)	0
Exercised (in usd per share)	3.91
Canceled (in usd per share)	0.83
Outstanding, ending balance (in usd per share)	1.92
Weighted-average exercise price, vested and expected (in usd per share)	1.92
Weighted-average exercise price, exercisable (in usd per share)	$ 1.92
Weighted-Average Remaining Contractual Term
Outstanding	3 years 6 months
Vested and expected to vest	3 years 6 months
Exercisable	3 years 6 months
Aggregate Intrinsic Value
Exercised | $	$ 9,882
Outstanding | $	396,797
Vested and expected to vest | $	396,797
Exercisable | $	$ 360,300
Share price (in usd per share)	$ 162.16

STOCK-BASED COMPENSATION - Su_4

STOCK-BASED COMPENSATION - Summary of Stock Options Outstanding (Details) shares in Thousands	3 Months Ended
	Mar. 31, 2021$ / sharesshares
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Options outstanding, number of shares (shares) | shares	2,476
Options outstanding - weighted- average remaining life	3 years 6 months
Options outstanding - weighted- average exercise price (usd per share)	$ 1.92
Options exercisable - number of shares exercisable (shares) | shares	2,248
Options Exercisable - weighted-average exercise price (usd per share)	$ 1.92
$0.70 —– $1.11
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price range, lower limit (usd per share)	0.70
Exercise price range, upper limit (usd per share)	$ 1.11
Options outstanding, number of shares (shares) | shares	533
Options outstanding - weighted- average remaining life	4 years
Options outstanding - weighted- average exercise price (usd per share)	$ 0.85
Options exercisable - number of shares exercisable (shares) | shares	496
Options Exercisable - weighted-average exercise price (usd per share)	$ 0.85
$1.29 —– $1.29
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price range, lower limit (usd per share)	1.29
Exercise price range, upper limit (usd per share)	$ 1.29
Options outstanding, number of shares (shares) | shares	1,000
Options outstanding - weighted- average remaining life	3 years 6 months
Options outstanding - weighted- average exercise price (usd per share)	$ 1.29
Options exercisable - number of shares exercisable (shares) | shares	875
Options Exercisable - weighted-average exercise price (usd per share)	$ 1.29
$1.31 —– $1.31
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price range, lower limit (usd per share)	1.31
Exercise price range, upper limit (usd per share)	$ 1.31
Options outstanding, number of shares (shares) | shares	670
Options outstanding - weighted- average remaining life	3 years
Options outstanding - weighted- average exercise price (usd per share)	$ 1.31
Options exercisable - number of shares exercisable (shares) | shares	649
Options Exercisable - weighted-average exercise price (usd per share)	$ 1.31
$1.39 —– $14.58
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price range, lower limit (usd per share)	1.39
Exercise price range, upper limit (usd per share)	$ 14.58
Options outstanding, number of shares (shares) | shares	262
Options outstanding - weighted- average remaining life	3 years 8 months 12 days
Options outstanding - weighted- average exercise price (usd per share)	$ 5.52
Options exercisable - number of shares exercisable (shares) | shares	219
Options Exercisable - weighted-average exercise price (usd per share)	$ 6.12
$64.17 —– $64.17
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Exercise price range, lower limit (usd per share)	64.17
Exercise price range, upper limit (usd per share)	$ 64.17
Options outstanding, number of shares (shares) | shares	11
Options outstanding - weighted- average remaining life	6 years 1 month 6 days
Options outstanding - weighted- average exercise price (usd per share)	$ 64.17
Options exercisable - number of shares exercisable (shares) | shares	9
Options Exercisable - weighted-average exercise price (usd per share)	$ 64.17

STOCK-BASED COMPENSATION - Su_5

STOCK-BASED COMPENSATION - Summary of Restricted Stock Unit Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)$ / sharesshares
Aggregate Intrinsic Value
Share price (in usd per share)	$ 162.16
Restricted stock units
Number of Shares Outstanding
Outstanding, beginning balance (in shares) | shares	3,588
Granted (in shares) | shares	168
Vested (in shares) | shares	(847)
Canceled (in shares) | shares	(12)
Outstanding, ending balance (in shares) | shares	2,897
Number of shares outstanding, expected to vest (in shares) | shares	2,897
Weighted Average Fair Value per Share at Grant Date
Outstanding, beginning balance (in usd per share)	$ 27.61
Granted (in usd per share)	183.78
Vested (in usd per share)	17.49
Canceled (in usd per share)	88.18
Outstanding, ending balance (in usd per share)	39.37
Weighted-Average Fair Value per Share at Grant Date, Expected to vest (in usd per share)	$ 39.37
Weighted-Average Remaining Contractual Term
Outstanding	1 year 2 months 1 day
Expected to vest	1 year 2 months 1 day
Aggregate Intrinsic Value
Vested | $	$ 163,030
Outstanding | $	469,766
Aggregate intrinsic value, expected to vest | $	$ 469,718

STOCK-BASED COMPENSATION - Su_6

STOCK-BASED COMPENSATION - Summary of Performance Stock Unit Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)$ / sharesshares
Aggregate Intrinsic Value
Share price (in usd per share)	$ 162.16
Performance shares
Number of Shares Outstanding
Outstanding, beginning balance (in shares) | shares	494
Granted (in shares) | shares	262
Vested (in shares) | shares	(494)
Canceled (in shares) | shares	(247)
Outstanding, ending balance (in shares) | shares	15
Weighted Average Fair Value per Share at Grant Date
Outstanding, beginning balance (in usd per share)	$ 51.10
Granted (in usd per share)	66.89
Vested (in usd per share)	59.19
Canceled (in usd per share)	42
Outstanding, ending balance (in usd per share)	$ 193.74
Weighted-Average Remaining Contractual Term
Weighted average remaining contractual term	4 months 24 days
Aggregate Intrinsic Value
Vested | $	$ 91,803
Outstanding | $	$ 2,432

INCOME TAXES - Narrative (Detai

INCOME TAXES - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Income tax benefit	$ 33,364	$ 11,868
Income (loss) before income taxes	$ (1,666)	$ 57,068

NET INCOME PER SHARE - Schedule

NET INCOME PER SHARE - Schedule of Computation of Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net income	$ 31,698	$ 68,936
Notes due 2023 interest and financing costs, net	44	44
Adjusted net income	$ 31,742	$ 68,980
Denominator:
Weighted average common shares outstanding (in shares)	131,303	123,531
Employee stock-based awards (in shares)	5,735	8,609
Weighted average common shares outstanding for diluted calculation (in shares)	146,442	138,104
Net income per share, basic (in USD per share)	$ 0.24	$ 0.56
Net income per share, diluted (in USD per share)	$ 0.22	$ 0.50
Convertible Senior Notes Due 2024
Denominator:
Warrants (issued in conjunction with Notes) (in shares)	2,506	2,128
Convertible Senior Notes Due 2025
Denominator:
Warrants (issued in conjunction with Notes) (in shares)	1,301	0
Convertible Notes | Convertible Senior Notes Due 2023
Denominator:
Notes due (in shares)	900	900
Convertible Notes | Convertible Senior Notes Due 2024
Denominator:
Notes due (in shares)	2,984	2,936
Convertible Notes | Convertible Senior Notes Due 2025
Denominator:
Notes due (in shares)	1,713	0

NET INCOME PER SHARE - Schedu_2

NET INCOME PER SHARE - Schedule of Potentially Dilutive Securities Excluded from the Computation of Diluted Net Income Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	3,445,000	0
Employee stock-based awards
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	54,000	0
Warrants | Convertible Senior Notes Due 2026
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	1,070,000	0
Warrants | Convertible Senior Notes Due 2028
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	1,070,000	0
Notes due | Convertible Senior Notes Due 2026
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	682,000	0
Notes due | Convertible Senior Notes Due 2028
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	569,000	0

NET INCOME PER SHARE - Narrativ

NET INCOME PER SHARE - Narrative (Details) - $ / shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 01, 2021	Mar. 09, 2020	Jun. 05, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities (in shares)	3,445,000	0
Convertible Senior Notes Due 2024 | Convertible Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Debt conversion price (in USD per share)					$ 20.50
Convertible Senior Notes Due 2025 | Convertible Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Debt conversion price (in USD per share)				$ 81.54
Convertible Senior Notes Due 2026 | Convertible Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Debt conversion price (in USD per share)			$ 307.47
Convertible Senior Notes Due 2028 | Convertible Notes
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Debt conversion price (in USD per share)			$ 284.87

RELATED PARTY - Narrative (Deta

RELATED PARTY - Narrative (Details) - Convertible Notes - Convertible Senior Notes Due 2023 - USD ($)	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2018	Aug. 31, 2018
Related Party Transaction [Line Items]
Debt instrument face amount	$ 5,000,000	$ 5,000,000		$ 65,000,000
Thurman John Rodgers
Related Party Transaction [Line Items]
Debt instrument face amount	$ 5,000,000	$ 5,000,000	$ 5,000,000

Uncategorized Items - enph-2021

Label	Element	Value
Restricted Cash	us-gaap_RestrictedCash	$ 0
Restricted Cash	us-gaap_RestrictedCash	$ 44,700,000