The Company recognizes revenue when control of the promised goods or services is transferred to its customers, in an amount that reflects the consideration to which it expects to be entitled in exchange for the goods or services. To achieve that core principle, a five-step approach is applied: (1) identify the contract with a customer, (2) identify the performance obligations in the contract, (3) determine the transaction price, (4) allocate the transaction price to the performance obligations in the contract, and (5) recognize revenue allocated to each performance obligation when the Company satisfies the performance obligation. A performance obligation is a promise in a contract to transfer a distinct good or service to the customer, and is the unit of account for revenue recognition.

The Company is generally the principal in its customer contracts because it has control over the goods and services prior to transfer to the customer, and as such, revenue is recognized on a gross basis. Sales and usage-based taxes are excluded from revenues. Revenue is recognized net of allowances for returns and any taxes collected from customers, which are subsequently remitted to governmental authorities. The table below presents the Company’s revenue disaggregated by revenue source (in thousands):

(1)

Includes changes in fair value of derivative instruments related to the Company’s commodity swap and customer fueling contracts associated with the Company’s Zero Now truck financing program. The amounts are classified as revenue because the Company’s commodity swap contracts are used to economically offset the risk associated with the diesel-to-natural gas price spread resulting from customer fueling contracts under the Company’s Zero Now truck financing program. See Note 6 for more information about these derivative instruments. For the three and six months ended June 30, 2019, changes in the fair value of commodity swaps and customer fueling contracts amounted to a gain of $0.6 million and a loss of $4.4 million, respectively. For the three and six months ended June 30, 2020, changes in the fair value of commodity swaps and customer fueling contracts amounted to a loss of $1.5 million and a gain of $4.2 million, respectively.

(2)	Represents the federal alternative fuel excise tax credit that we refer to as “AFTC,” which had previously expired but on December 20, 2019 was retroactively extended for vehicle fuel sales made beginning January 1, 2018 through December 31, 2020. See Note 19 for more information.

Remaining Performance Obligations

Remaining performance obligations represent the transaction price of customer orders for which the work has not been performed. As of June 30, 2020, the aggregate amount of the transaction price allocated to remaining performance obligations was $15.3 million, which related to the Company’s station construction sale contracts. The Company expects to recognize revenue on the remaining performance obligations under these contracts over the next 12 to 24 months.

For volume-related revenue, the Company has elected to apply an optional exemption, which waives the requirement to disclose the remaining performance obligation for revenue recognized through the ‘right to invoice’ practical expedient.

Contract Balances

The timing of revenue recognition, billings and cash collections results in billed accounts receivable, unbilled receivables (contract assets), and customer advances and deposits (contract liabilities) in the accompanying condensed consolidated balance sheets.

As of December 31, 2019 and June 30, 2020, the Company’s contract balances were as follows (in thousands):


	December 31,		June 30,
	2019		2020
Accounts receivable, net	$	61,760	$	48,141

Contract assets - current	$	455	$	1,537
Contract assets - non-current		3,777		3,829
Contract assets - total	$	4,232	$	5,366

Contract liabilities - current	$	5,329	$	2,044
Contract liabilities - non-current		6,339		—
Contract liabilities - total	$	11,668	$	2,044

Accounts Receivable, Net

"Accounts receivable, net" in the accompanying condensed consolidated balance sheets include amounts billed and currently due from customers. The amounts due are stated at their net estimated realizable value. The Company maintains an allowance to provide for the estimated amount of receivables that will not be collected. The allowance is based upon an assessment of customer creditworthiness, historical payment experience, the age of outstanding receivables, and economic conditions that may affect a customer’s ability to pay.

Contract Assets

Contract assets include unbilled amounts typically resulting from the Company’s station construction sale contracts, when the cost-to-cost method of revenue recognition is utilized and revenue recognized exceeds the amount billed to the customer, and right to payment is not just subject to the passage of time. Amounts may not exceed their net realizable value. Contract assets are classified as current or noncurrent based on the timing of billings. The current portion is included in “Prepaid expenses and other current assets” and the noncurrent portion is included in “Notes receivable and other long-term assets, net” in the accompanying condensed consolidated balance sheets.

Contract Liabilities

Contract liabilities consist of billings in excess of revenue recognized from the Company’s station construction sale contracts and payments received primarily from a customer of NG Advantage, LLC (“NG Advantage”) in advance of the performance obligations and are classified as current or noncurrent based on when the related revenue is expected to

be recognized. The current portion and noncurrent portion of contract liabilities are included in “Deferred revenue” and “Other long-term liabilities,” respectively, in the accompanying condensed consolidated balance sheets.

Revenue recognized during the six months ended June 30, 2019 related to the Company’s contract liability balances as of December 31, 2018 was $4.4 million. Revenue recognized during the six months ended June 30, 2020 related to the Company’s contract liability balances as of December 31, 2019 was $4.2 million. In June 2020, the Company entered into a termination letter between NG Advantage and BP (as defined in Note 3) and paid $7.8 million in satisfaction of the remaining performance obligations, resulting in a reduction of the Company’s contract liability balance as of December 31, 2019 of $7.0 million, and a net revenue impact of $(0.8) million. Contract liabilities increased during the six months ended June 30, 2020 as a result of billings in excess of revenue recognized and advance payments received from customers.