14. Revenue

Disaggregation of revenue

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


Major products/services lines
	Year ended December 31,
	2019		2018		2017
Sales of fuel cell systems	$	130,721	$	75,146	$	49,206
Sale of hydrogen installations and other infrastructure		19,163		32,146		13,425
Services performed on fuel cell systems and related infrastructure		25,217		22,002		16,202
Power Purchase Agreements		25,853		22,869		12,869
Fuel delivered to customers		29,099		22,469		8,167
Other		186		—		284
Net revenue	$	230,239	$	174,632	$	100,153

Contract balances

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


	2019		2018
Accounts receivable	$	25,448	$	37,347
Contract assets		13,251		3,328
Contract liabilities		43,480		40,476

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 in prepaid expenses and other assets on the consolidated balance sheet.

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). These amounts are included within deferred revenue on the consolidated balance sheet. Contract liabilities also include advance consideration received from customers prior to delivery of products. These amounts are included in other current liabilities on the consolidated balance sheet.

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



Contract assets	Year ended
	December 31, 2019
Transferred to receivables from contract assets recognized at the beginning of the period	$	(1,252)
Revenue recognized and not billed as of the end of the period		11,175
Net change in contract assets	$	9,923


Contract liabilities	Year ended
	December 31, 2019
Revenue recognized that was included in the contract liability balance as of the beginning of the period	$	(10,172)
Increases due to cash received, net of amounts recognized as revenue during the period		13,175
Net change in contract liabilities	$	3,003

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):


	Year ended December 31,
	2019		2018
Sales of fuel cell systems	$	73,491	$	17,318
Sale of hydrogen installations and other infrastructure		72,862		9,141
Services performed on fuel cell systems and related infrastructure		87,020		73,381
Power Purchase Agreements		133,475		111,533
Other rental income		4,993		6,633
Total estimated future revenue	$	371,841	$	218,006

Contract costs

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

Capitalized contract costs at December 31, 2019 and 2018 were $0.5 million and $0.2, respectively. Expense related to the amortization of capitalized contract costs was not significant for the year ended December 31, 2019.