Revenue

6 Months Ended

Jun. 30, 2020

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,
	2020		2019		2020		2019
Sales of fuel cell systems	$	41,264	$	38,696	$	55,915	$	41,240
Sale of hydrogen installations and other infrastructure		6,482		—		12,218		—
Services performed on fuel cell systems and related infrastructure		6,236		5,341		12,757		11,684
Power Purchase Agreements		6,654		6,409		13,150		12,519
Fuel delivered to customers		7,372		7,089		14,705		13,671
Other		62		—		138		—
Net revenue	$	68,070	$	57,535	$	108,883	$	79,114

Contract balances

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

	June 30,		December 31,
	2020		2019
Accounts receivable	$	45,522	$	25,448
Contract assets		20,481		13,251
Contract liabilities		50,233		43,480

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). Contract liabilities also include advance consideration received from customers prior to delivery of products. These amounts are included within deferred revenue on the accompanying unaudited interim condensed consolidated interim balance sheets.  

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, 2020
Transferred to receivables from contract assets recognized at the beginning of the period	$	(9,671)
Revenue recognized and not billed as of the end of the period		16,901
Net change in contract assets		7,230

Contract liabilities	Six months ended
	June 30, 2020
Increases due to cash received, net of amounts recognized as revenue during the period	$	24,835
Revenue recognized that was included in the contract liability balance as of the beginning of the period		(18,082)
Net change in contract liabilities	$	6,753

Estimated future revenue

The following table includes estimated revenue expected to be recognized in the future (sales of fuel cell systems and hydrogen installations are expected to be recognized as revenue within one year; sales of services and PPAs are expected to be recognized as revenue over five to seven years) related to performance obligations that are unsatisfied (or partially unsatisfied) at the end of the reporting period, excluding provision for common stock warrants as it is not readily estimable as it depends on the valuation of the common stock warrants when revenue is recognized (in thousands):

	June 30,
	2020
Sales of fuel cell systems	$	71,402
Sale of hydrogen installations and other infrastructure		71,753
Services performed on fuel cell systems and related infrastructure		94,725
Power Purchase Agreements		151,793
Other rental income		4,117
Total estimated future revenue	$	393,790

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, 2020 and December 31, 2019 were $0.5 million and $0.5, respectively. Expense related to the amortization of capitalized contract costs was not significant for the three or six months ended June 30, 2020 and 2019.