Revenue

6 Months Ended

Jun. 30, 2021

Revenue

Revenue

13. Revenue

Disaggregation of revenue

The following table provides information about disaggregation of revenue (in thousands):

Major products/services lines
	Three months ended June 30,				Six months ended June 30,
	2021		2020		2021		2020
Sales of fuel cell systems	$	59,169	$	40,785	$	85,588	$	55,425

Sale of hydrogen installations and other infrastructure		40,109		6,961		60,462		12,789
Services performed on fuel cell systems and related infrastructure		5,675		6,236		11,720		12,757
Power Purchase Agreements		8,361		6,579		16,187		13,000
Fuel delivered to customers		11,121		7,372		22,248		14,705
Other		122		62		310		138
Net revenue	$	124,557	$	67,995	$	196,515	$	108,814

Contract balances

The following table provides information about receivables, contract assets and contract liabilities from contracts with customers (in thousands):

	June 30,		December 31,
	2021		2020
Accounts receivable	$	91,359	$	43,041
Contract assets		11,549		18,189
Contract liabilities		93,665		76,285

Contract assets relate to contracts for which revenue is recognized on a straight-line basis, however billings escalate over the life of a contract. Contract assets also include amounts recognized as revenue in advance of billings to customers, which are dependent upon the satisfaction of another performance obligation. These amounts are included within prepaid expenses and other current assets on the accompanying unaudited interim condensed consolidated balance sheets.

The contract liabilities relate to the advance consideration received from customers for services that will be recognized over time (primarily fuel cell and related infrastructure services) and advance consideration received from customers prior to delivery of products.  As of June 30, 2021, the amount of contract liabilities included within deferred revenue was $72.3 million and the amount of contract liabilities within other current liabilities was $21.4 million on the accompanying unaudited interim condensed consolidated balance sheets. As of December 31, 2020, the amount of contract liabilities included within deferred revenue was $56.2 million and the amount of contract liabilities within other current liabilities was $20.1 million.  

Significant changes in the contract assets and the contract liabilities balances during the period are as follows (in thousands):

Contract assets	Six months ended
	June 30, 2021
Transferred to receivables from contract assets recognized at the beginning of the period	$	(7,106)
Revenue recognized and not billed as of the end of the period		466
Net change in contract assets		(6,640)

Contract liabilities	Six months ended
	June 30, 2021
Increases due to cash received, net of amounts recognized as revenue during the period	$	49,628
Revenue recognized that was included in the contract liability balance as of the beginning of the period		(32,248)
Net change in contract liabilities	$	17,380

Estimated future revenue

The following table includes estimated revenue included in the backlog expected to be recognized in the future (sales of fuel cell systems and hydrogen installations are expected to be recognized as revenue within one year; sales of services and PPAs are expected to be recognized as revenue over five to seven years) related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period, including provision for common stock warrants (in thousands):

	June 30,
	2021
Sales of fuel cell systems	$	55,348
Sale of hydrogen installations and other infrastructure		52,193
Services performed on fuel cell systems and related infrastructure		103,505
Power Purchase Agreements		196,903
Fuel delivered to customers		63,632
Other rental income		2,504
Total estimated future revenue	$	474,085

Contract costs

Contract costs consist of capitalized commission fees and other expenses related to obtaining or fulfilling a contract.

Capitalized contract costs at June 30, 2021 and December 31, 2020 were $1.2 million and $1.5 million, respectively.