6. Revenue

Revenue Recognition

The majority of our revenues are derived from short-term contracts for fixed consideration. Product sales generally do not include right of return or other significant post-delivery obligations. Revenues are recognized when we satisfy a performance obligation by transferring control of the promised goods or providing services to our customers at a point in time, in an amount specified in the contract with our customer and that reflects the consideration we expect to be entitled to in exchange for those goods or services. We also assess our customers’ ability and intention to pay, which is based on a variety of factors including our customers’ historical payment experience and financial condition. We do not adjust the amount of consideration per the contract for the effects of a significant financing component when we expect, at contract inception, that the period between the transfer of a promised good or service to a customer and when the customer pays for that good or service will be one year or less, which is in substantially all cases. Payment terms and conditions vary, although terms generally include a requirement of payment within 30 to 45 days. Revenues are recognized net of any taxes collected from customers, which are subsequently remitted to governmental authorities. We treat shipping and handling associated with outbound freight as a fulfillment cost instead of as a separate performance obligation. We recognize the cost for shipping and handling when incurred as an expense in cost of sales.

Performance Obligations

A performance obligation is a promise in a contract to transfer a distinct good or service to the customer. A contract’s transaction price is allocated to each distinct performance obligation and recognized as revenue when, or as, the performance obligation is satisfied. The majority of our contracts with customers contain a single performance obligation to provide agreed-upon products or services. For contracts with multiple performance obligations, we allocate revenue to each performance obligation based on its relative standalone selling price. We do not assess whether promised goods or services are performance obligations if they are immaterial in the context of the contract with the customer.

We do not disclose the value of unsatisfied performance obligations for (i) contracts with an original expected length of one year or less and (ii) contracts for which we recognize revenue at the amount to which we have the right to invoice for services performed.

Disaggregation of Revenue

We disaggregate revenue from contracts with customers into three revenue categories: (i) product revenues, (ii) rental revenues and (iii) field service and other revenues. We have predominately domestic operations, with a small amount of sales being generated in Australia. For the nine months ended September 30, 2019, we derived 56% of total revenues from the sale of our products, 23% of total revenues from rental and 21% of total revenues from field service and other. This compares to 52% of total revenues from the sale of our products, 25% of total revenues from rental and 23% of total revenues from field service and other for the nine months ended September 30, 2018. The following table presents our revenues disaggregated by category:


	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Product revenue	$	92,582	$	79,388	$	273,716	$	211,595
Rental revenue		35,528		38,135		113,601		102,224
Field service and other revenue		32,698		33,135		100,859		90,492
Total revenue	$	160,808	$	150,658	$	488,176	$	404,311

Contract Balances

The timing of our performance can differ from the timing of invoicing and our customers’ payments, which can result in the recording of unbilled revenue within accounts receivable, net, and deferred revenue within accrued expenses and other current liabilities. Balances related to contracts with customers consisted of the following:


	September 30,		December 31,
	2019		2018
Unbilled revenue	$	22,027	$	26,804
Deferred revenue		1,492		1,110

Contract Costs

We do not incur any material costs of obtaining contracts.