4.  Customer Concentrations and Accounts Receivable

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. Sales to E-Quad Power Systems GmbH (“E-Quad”), one of the Company’s international distributors, accounted for 10% of revenue for the three months ended September 30, 2020. E-Finity and Optimal accounted for 17% and 10% of revenue for the six months ended September 30, 2021, respectively. E-Finity and Cal Microturbine, one of the Company’s domestic distributors, each accounted for 10% of revenue for the six months ended September 30, 2020.

Additionally, E-Finity accounted for 20% and 13% of net accounts receivable as of September 30, 2021 and March 31, 2021, respectively. The Company recorded a net bad debt expense of zero during three and six months ended September 30, 2021. The Company recorded a net bad debt recovery of approximately $0.1 million during the three and six months ended September 30, 2020.