The following table summarizes our revenue (revenue amounts in millions):
| | | | | | | | | | | | | | |
| | Three Months Ended September 30, |
| | 2022 | | 2021 |
| | Revenue | | Megawatts | | Units | | Revenue | | Megawatts | | Units |
Microturbine Product | | $ | 10.4 | | 8.7 | | 43 | | $ | 8.4 | | 7.5 | | 41 |
Accessories | | | 0.2 | | | | | | | 0.1 | | | | |
Total Product and Accessories | | | 10.6 | | | | | | | 8.5 | | | | |
| | | | | | | | | | | | | | |
Parts, Service and Rentals | | | 10.2 | | | | | | | 8.7 | | | | |
Total | | $ | 20.8 | | | | | | $ | 17.2 | | | | |
For the three months ended September 30, 2022, revenue from microturbine products and accessories increased $2.1 million, or 25%, to $10.6 million from $8.5 million for the three months ended September 30, 2021. The $2.1 million increase was primarily driven by an increase in megawatts and units shipped during the three months ended September 30, 2022 compared to the same period last year. Megawatts shipped were 8.7 megawatts and 7.5 megawatts during the three months ended September 30, 2022 and 2021, respectively. Average revenue per megawatt shipped was approximately $1.2 million and $1.1 million during the three months ended September 30, 2022 and 2021, respectively. The timing of shipments is variable and based on several factors (including customer deposits, payments, availability of credit and delivery schedule changes), most of which are not within our control and can affect the timing of revenue recognition.
Parts, service and rentals revenue (which are part of our EaaS business line and includes revenue from our parts shipments, FPP contracts, rentals, Distributor Support Subscription fees, and other service revenue) increased $1.5 million, or 17%, to $10.2 million for three months ended September 30, 2022 from $8.7 million for the three months ended September 30, 2021. The $1.5 million increase was primarily driven by an increase in rental revenue as a result of our EaaS business plan during the three months ended September 30, 2022 compared to the same period last year.
Sales to RSP Systems, Capstone Engineered Solutions, Cal Microturbine, and Horizon Power Systems, accounted for 15%, 13%, 12%, and 10% of our revenue for the three months ended September 30, 2022, respectively. Sales to E-Finity and Optimal accounted for 23% and 11% of our revenue for the three months ended September 30, 2021, respectively.
RSP Systems, Capstone Engineered Solutions, Cal Microturbine, and Horizon Power Systems, four of the Company’s domestic distributors, accounted for 15%, 13%, 12%, and 10% of revenue for the three months ended September 30, 2022, respectively. E-Finity Distributed Generation, LLC (“E-Finity), one of the Company’s domestic distributors, and Optimal Group Australia Pty Ltd (“Optimal”), one of the Company’s international distributors, accounted for 23% and 11% of revenue for the three months ended September 30, 2021, respectively. Cal Microturbine accounted for 18% of revenue for the six months ended September 30, 2022. E-Finity and Optimal accounted for 17% and 10% of revenue for the six months ended September 30, 2021, respectively.
Additionally, E-Finity accounted for 11% of net accounts receivable as of September 30, 2022. E-Finity accounted for 28% of net accounts receivable as of March 31, 2022. The Company recorded a net bad debt expense of $0.1 million during the three and six months ended September 30, 2022. The Company recorded a net bad debt expense of zero during three and six months ended September 30, 2021.
Gross Margin Cost of goods sold includes direct material costs, production and service center labor and overhead, inventory charges and provision for estimated product warranty expenses. Gross margin was $2.2 million, or 11% of revenue, for the three months ended September 30, 2022 compared to a gross margin of $2.7 million, or 16% of revenue, for the three months ended September 30, 2021. The decrease of $0.5 million was primarily the result of higher production and service center labor and overhead expense of $1.1 million, increases in inventory charges of $0.2 million and warranty expense of $0.1 million, partially offset by higher direct material costs margin of $0.9 million.
Direct material costs margin, calculated as total revenue less our direct material costs, increased $0.9 million during the second quarter of Fiscal 2023 compared to the second quarter of Fiscal 2022. The increase was due to greater sales volume, partially offset by higher direct material costs due to vendor price increases and supply chain shortages. We are increasing our sales prices effective January 2023 and negotiating with vendors to reduce these costs.
Warranty expense is a combination of a standard warranty provision recorded at the time revenue is recognized and changes, if any, in estimates for reliability repair programs. Reliability repair programs are based upon estimates that