We capitalize incremental costs to obtain a contract with a customer if we expect to recover those costs. Capitalized costs include commissions paid to our sales force to obtain contract operations contracts. We expense commissions paid for sales of service contracts and OTC parts and components within our aftermarket services segment, as the amortization period is less than one year. We had contract costs of $4.8 million and $4.2 million associated with sales commissions recorded in our consolidated balance sheets at December 31, 2019 and 2018, respectively.

We capitalize costs incurred to fulfill a contract if those costs relate directly to a contract, enhance resources that we will use in satisfying performance obligations and if we expect to recover those costs. Capitalized costs incurred to fulfill our customer contracts include freight charges to transport compression assets before transferring services to the customer and mobilization activities associated with our contract operations services. We had contract costs of $38.1 million and $34.8 million associated with freight and mobilization recorded in our consolidated balance sheets at December 31, 2019 and 2018, respectively.

Contract operations costs are amortized based on the transfer of service to which the assets relate, which is estimated to be 38 months based on average contract term, including anticipated renewals. We assess periodically whether the 38-month estimate fairly represents the average contract term and adjust as appropriate. Aftermarket services fulfillment costs are recognized based on the percentage-of-completion method applicable to the customer contract. Contract costs associated with commissions are amortized to SG&A. Contract costs associated with freight and mobilization are amortized to cost of sales (excluding depreciation and amortization). During the years ended December 31, 2019 and 2018, we amortized $2.6 million and $1.5 million, respectively, related to sales commissions and $20.7 million and $13.4 million, respectively, related to freight and mobilization. No impairment losses were recorded related to our capitalized contract costs during the years ended December 31, 2019 and 2018.

Revenue is recognized when control of the promised goods or services is transferred to our customers, in an amount that reflects the consideration we are entitled to receive in exchange for those goods or services. Sales and usage-based taxes that are collected from the customer are excluded from revenue.



	Year Ended December 31,
	2019		2018
Contract operations ⁽¹⁾:
0 - 1,000 horsepower per unit	$	259,985	$	241,810
1,001 - 1,500 horsepower per unit	316,082		276,775
Over 1,500 horsepower per unit	191,510		149,783
Other ⁽²⁾	3,962		4,168
Total contract operations ⁽³⁾	771,539		672,536

Aftermarket services ⁽¹⁾:
Services ⁽⁴⁾	122,076		142,476
OTC parts and components sales	71,870		89,429
Total aftermarket services ⁽⁵⁾	193,946		231,905

Total revenue	$	965,485	$	904,441

Natural gas compression services are generally satisfied over time, as the customer simultaneously receives and consumes the benefits provided by these services. Our performance obligation is a series in which the unit of service is one month, as the customer receives substantially the same benefit each month from the services regardless of the type of service activity performed, which may vary. If the transaction price is based on a fixed fee, revenue is recognized monthly on a straight-line basis over the period that we are providing services to the customer. Amounts invoiced to customers for costs associated with moving our compression assets to a customer site are also included in the transaction price and are amortized over the initial contract term. We do not consider the effects of the time value of money, as the expected time between the transfer of services and payment for such services is less than one year.

Variable consideration exists if customers are billed at a lesser standby rate when a unit is not running. We recognize revenue for such variable consideration monthly, as the invoice corresponds directly to the value transferred to the customer based on our performance completed to date. The rate for standby service is lower to reflect the decrease in costs and effort required to provide standby service when a unit is not running.

We also perform billable maintenance service on our natural gas compression equipment at the customer’s request on an as-needed basis. The performance obligation is satisfied and revenue is recognized at the agreed-upon transaction price at the point in time when service is complete and the customer has accepted the work performed and can obtain the remaining benefits of the service that the unit will provide.

As of December 31, 2019, we had $469.6 million of remaining performance obligations related to our contract operations segment. The remaining performance obligations will be recognized through 2024 as follows (in thousands):

For sales of OTC parts and components, the performance obligation is generally satisfied at the point in time when delivery takes place and the customer obtains control of the part or component. The transaction price is the fixed sales price for the part stated in the contract. Revenue is recognized upon delivery, as we have a present right to payment and the customer has legal title.

For our service activities, the performance obligation is satisfied over time, as the work performed enhances the customer-controlled asset and another entity would not have to substantially re-perform the work we completed if they were to fulfill the remaining performance obligation. The transaction price may be a fixed monthly service fee, a fixed quoted fee or entirely variable, calculated on a time and materials basis.

For service provided based on a fixed monthly fee, the performance obligation is a series in which the unit of service is one month. The customer receives substantially the same benefit each month from the service, regardless of the type of service activity performed, which may vary. As the progress towards satisfaction of the performance obligation is measured based on the passage of time, revenue is recognized monthly based on the fixed fee provided for in the contract.

For service provided based on a quoted fixed fee, progress towards satisfaction of the performance obligation is measured using an input method based on the actual amount of labor and material costs incurred. The amount of the transaction price recognized as revenue each reporting period is determined by multiplying the transaction price by the ratio of actual costs incurred to date to total estimated costs expected for the service. Significant judgment is involved in the estimation of the progress to completion. Any adjustments to the measure of the progress to completion will be accounted for on a prospective basis. Changes to the scope of service is recognized as an adjustment to the transaction price in the period in which the change occurs.

Service provided based on time and materials are generally short-term in nature and labor rates and parts pricing is agreed upon prior to commencing the service. We have elected to use the right-to-invoice practical expedient using an estimated gross margin percentage applied to actual costs incurred. The estimated gross margin percentage is fixed based on historical time and materials-based service. We evaluate the estimated gross margin percentage at the end of each reporting period and adjust the transaction price as appropriate.

We believe these fee-based and cost-based inputs fairly depict our efforts to provide aftermarket services and the amount of revenue recognized is representative of the transfer of service and value that the customer will have received as of the reporting date. As of December 31, 2019 we do not disclose the aggregate transaction price for the remaining performance obligations for aftermarket services, as there are no contracts with customers with an original contract term that is greater than one year.

Contract operations services are generally billed monthly at the beginning of the month in which service is being provided. For aftermarket services, billings will typically occur when parts are delivered or when service is complete; however, milestone billings may be used in longer-term projects. We recognize a contract asset when we have the right to consideration in exchange for goods or services transferred to a customer when the right is conditioned on something other than the passage of time. We recognize a contract liability when we have an obligation to transfer goods or services to a customer for which we have already received consideration.

Freight billings to customers for the transport of compression assets, customer-specified modifications of compression assets and milestone billings on aftermarket services often result in a contract liability. As of December 31, 2019 and 2018, our contract liabilities were $11.4 million and $17.1 million, respectively, which were included in deferred revenue and other liabilities in our consolidated balance sheets. The decrease in the contract liability balance during the year ended December 31, 2019 was due to the deferral of $36.6 million, partially offset by $42.3 million recognized as revenue during the period, each primarily related to freight billings and aftermarket services.