In the past, Congress has regularly acted to extend expired or expiring temporary tax provisions. Collectively, these temporary tax provisions are often referred to as "tax extenders." Of the 33 expiring provisions addressed in the most recent tax extender legislation, included in the Bipartisan Budget Act of 2018 (BBA18; P.L. 115-123 ), 18 were energy-related. Thirteen of the energy tax provisions addressed in BBA18 were provisions that had expired in 2016 and were extended through 2017. Thus, even though these provisions were recently extended, they remain expired. These 13 expired energy tax provisions are reviewed in this report. The expired energy tax provisions are diverse in purpose, providing various types of tax incentives for renewable electricity, alternative and renewable fuels, vehicles, building energy efficiency, and other energy activities. Before BBA18, expired energy tax provisions were extended in the Protecting Americans from Tax Hikes (PATH) Act of 2015, enacted as Division Q of the Consolidated Appropriations Act, 2016 ( P.L. 114-113 ). This law retroactively extended temporary tax provisions that had expired at the end of 2014. Unlike much of the previous tax extender legislation, this law did not temporarily extend most or all expired provisions. Instead, a number of temporary provisions were made permanent, while others were extended through 2016 or 2019. While certain business, individual, and charitable provisions were made permanent or extended through 2019, energy-related provisions were generally extended for two years, through 2016, in the PATH Act. There were longer-term extensions for certain renewables in Division P of P.L. 114-113 . Specifically, the law provided longer-term extensions with scheduled phaseouts of tax credits for wind and solar. The renewable electricity production tax credit (PTC) for wind was extended through 2019, with the phaseout starting for facilities beginning construction in 2017. For wind facilities that begin construction during 2017, the credit is reduced by 20%. The credit is reduced by 40% for facilities that begin construction in 2018, and reduced by 60% for facilities that begin construction in 2019. The 30% investment tax credit (ITC) for business solar was extended through 2019 and the deadline changed from a placed-in-service deadline to a construction start date. The business solar ITC was set to be 26% for facilities beginning construction in 2020 and 22% for facilities beginning construction in 2021, so long as these facilities are placed in service before the end of 2023. The business solar ITC is scheduled to return to 10% in 2022. The tax credit for residential solar was extended through 2021, with a phaseout starting in 2020. Several provisions in BBA18 provided extensions for energy-tax provisions that matched the longer-term extensions that had been enacted as part of the PATH Act. Specifically, the ITC for nonsolar technologies was extended and modified, such that the phaseout and termination for nonsolar technologies matched what had been established for solar in the PATH Act. Five-year cost recovery that is generally available to ITC-eligible property, as well as wind property, was also extended through 2021. The tax credit for residential energy-efficient property was also extended through 2021 for nonsolar technologies, with a phaseout in 2020 and 2021. Further discussion of these provisions is not included in this report, as these provisions did not expire in 2017. Additional energy tax provisions were included in BBA18. Specifically, the law extended the per-barrel excise tax on oil used to fund the oil spill liability trust fund, modified the tax credit for production from advanced nuclear power facilities, and enhanced the carbon dioxide sequestration credit. Again, further discussion of these provisions is not included in this report, as these provisions did not expire in 2017. There are several options for Congress to consider regarding temporary provisions. The 2017 tax revision ( P.L. 115-97 ) adds new context in considering further extension of expired temporary tax provisions. Broadly, the revised tax system became effective in 2018. Thus, one perspective might be that extending expired provisions through 2017 allows these provisions to remain available up until the revised tax system took effect at the beginning of 2018. Whether the temporary tax provisions that expired at the end of 2017 have a place in the new tax system is a question the 115 th Congress may consider. Ultimately, provisions that expired at the end of 2017 could be extended. The extension could be retroactive, such that provisions will be available for all of the 2018 tax year. The extensions also could be short term, long term, or permanent. Another option would be to allow expired provisions to remain expired. Table 1 provides information on (1) the cost of the one-year extension of expired energy tax provisions in BBA18, over the 2018 to 2027 budget window; and (2) the estimated reduction in federal revenues that would result from making expired energy tax provisions permanent. The renewable electricity PTC is a per-kilowatt-hour (kWh) credit for electricity produced by a qualified energy resource. Eligible facilities can claim the tax credit for the first 10 years of qualified production. Resources that qualify for the full credit amount, 2.4 cents per kWh in 2017, include wind, closed-loop biomass, and geothermal. Other resources are eligible for a half credit amount, including open-loop biomass, small irrigation power, municipal solid waste, qualified hydropower, and marine and hydrokinetic resources. Under current law, nonwind facilities must have begun construction before January 1, 2017, to be tax-credit eligible. The PTC was enacted in 1992 as part of the Energy Policy Act (EPACT92; P.L. 102-486 ). When first enacted, the PTC was available for electricity generated using wind or closed-loop biomass systems. The credit was initially set to expire on June 30, 1999. Since 1999, the PTC has regularly been extended, often as part of "tax extenders" legislation. The credit has also been expanded to include additional qualifying resources. At several points in time, the PTC was allowed to lapse before a retroactive extension was enacted. Taxpayers can elect to receive a 30% investment tax credit in lieu of the PTC. The ITC in lieu of PTC election was enacted in 2009 alongside the Section 1603 grants in lieu of tax credits that expired at the end of 2011. In recent years, the option to elect the ITC in lieu of the PTC has been extended alongside the PTC in "tax extenders" legislation. The PTC was enacted in 1992 to promote the "development and utilization of certain renewable energy sources." The 199 9 sunset was included to provide an "opportunity to assess the effectiveness of the credit." When the PTC was extended as part of a "tax extenders" package in 1999, Congress noted that the PTC had been important to the development of environmentally friendly renewable power, and extended the credit to promote further development of wind (and other) resources. Subsequent extensions of the PTC reflected a belief that the tax incentive contributed to the development of renewable-energy infrastructure, which advanced environmental and energy policy goals. Most recently, the PTC was extended for one year for qualifying nonwind technologies, from 2016 through 2017, in BBA18. Previously, a two-year extension of the PTC for nonwind technologies was included in the Protecting Americans from Tax Hikes (PATH) Act of 2015, enacted as Division Q of P.L. 114-113 . Division P of the Consolidated Appropriations Act, 2016 ( P.L. 114-113 ) included a two-year extension of the PTC for wind (through 2016). The PTC for wind was also extended beyond 2016, through 2019, at reduced rates. One policy rationale for supporting renewable electricity resources is to promote development of renewable-energy infrastructure that may not be technologically mature. With this policy rationale in mind, one question is when technologies have reached the point of maturity such that tax-related federal financial support can be eliminated. Environmental considerations provide another policy rationale often used to support tax incentives for renewable electricity production. Some suggest that ongoing support for renewables can help address inefficiencies and market failures in energy production markets, where fossil-fuels-based electricity production and the associated pollution effects generate negative externalities. A more direct approach, however, would be to impose a price on pollution, as opposed to subsidizing a nonpolluting alternative. For more information, see CRS Report R43453, The Renewable Electricity Production Tax Credit: In Brief , by [author name scrubbed]. There are three tax credits for biodiesel: the biodiesel mixture credit, the biodiesel credit, and the small agri-biodiesel producer credit. Each gallon of biodiesel, including agri-biodiesel (biodiesel made from virgin oils), may be eligible for a $1.00 tax credit. The mixtures tax credit may be claimed as an instant excise tax credit against the blender's motor and aviation fuels excise taxes. Credits in excess of excise tax liability may be refunded. The biodiesel and small agri-biodiesel credits may be claimed as income tax credits. The mixtures credit is proportionate to the fraction of biodiesel in the mixture—a blend of 80% diesel with 20% virgin biodiesel would qualify for a 20-cent-per-gallon tax credit. The tax credits for biodiesel expired on December 31, 2017. Additionally, an eligible small agri-biodiesel producer credit of 10 cents is available for each gallon of "qualified agri-biodiesel production."