our performance of the related maintenance services. Finally, deferred revenue exists in our extended coverage contracts, where the cash is collected prior to the commencement of the coverage period. Deferred revenue is included in our Consolidated Balance Sheets as a component of current liabilities for the amount expected to be recognized in revenue in a period of less than one year and long-term liabilities for the amount expected to be recognized as revenue in a period beyond one year. Deferred revenue is recognized as revenue when control of the underlying product, project or service passes to the customer under the related contract.
We recognize unbilled revenue when the revenue has been earned, but not yet billed. Unbilled revenue is included in our Consolidated Balance Sheets as a component of current assets for those expected to be collected in a period of less than one year and long-term assets for those expected to be collected in a period beyond one year. Unbilled revenue relates to our right to consideration for our completed performance under a contract. Unbilled revenue generally arises from contractual provisions that delay a portion of the billings on genset deliveries until commissioning occurs. Unbilled revenue may also occur when billings trail the provision of service in construction and long-term maintenance contracts. We periodically assess our unbilled revenue for impairment. We did not record any impairment losses on our unbilled revenues during 2018.
The following is a summary of our unbilled and deferred revenue and related activity:
Deferred revenue, primarily extended warranty
Revenue recognized (1)
Relates to year-to-date revenues recognized from amounts included in deferred revenue at the beginning of the year. Revenue recognized in the period from performance obligations satisfied in previous periods was immaterial.
We are required to record an asset for the incremental costs of obtaining a contract with a customer and other costs to fulfill a contract not otherwise required to be immediately expensed when we expect to recover those costs. The only material incremental cost we incur is commission expense, which is generally incurred in the same period as the underlying revenue. Costs to fulfill a contract are generally limited to customer-specific engineering expenses that do not meet the definition of research and development expenses. As a practical expedient, we have elected to recognize these costs of obtaining a contract as an expense when the related contract period is less than one year. When the period exceeds one year, this asset is amortized over the life of the contract. We did not have any material capitalized balances at December 31, 2018.
We sell extended warranty coverage on most of our engines and on certain components. We consider a warranty to be extended coverage in any of the following situations:
When a warranty is sold separately or is optional (extended coverage contracts, for example) or
When a warranty provides additional services.
The consideration collected is initially deferred and is recognized as revenue in proportion to the costs expected to be incurred in performing services over the contract period. We compare the remaining deferred revenue balance quarterly to the estimated amount of future claims under extended warranty programs and provide an additional accrual when the deferred revenue balance is less than expected future costs.
Foreign Currency Transactions and Translation
We translate assets and liabilities of foreign entities to U.S. dollars, where the local currency is the functional currency, at year-end exchange rates. We translate income and expenses to U.S. dollars using weighted-average exchange rates for the year. We record adjustments resulting from translation in a separate component of accumulated other comprehensive loss (AOCL) and include the adjustments in net income only upon sale, loss of controlling financial interest or liquidation of the underlying foreign investment.