A clean energy standard (CES) has been proposed as a tool to provide a more sustainable domestic energy supply, reduce greenhouse gas emissions, and secure the United States as a leader in clean energy technology. Some assert that a CES could contribute to economic growth. Opponents of a CES contend that a CES could raise electricity prices, introduce grid reliability concerns, require significant investment in additional transmission lines, and depend on adopting some technologies not yet established for widespread commercial-scale use. A CES could require certain electricity providers to obtain a portion of their electricity from qualifying clean energy sources. In his 2012 State of the Union address, President Obama challenged Congress to "set a clean energy standard that creates a market for innovation." A CES could be one approach if Congress chooses to act. On March 21, 2011, Senators Bingaman and Murkowski issued a white paper on a clean energy standard that laid out some of the key questions and potential design elements of a CES, in order to solicit input from a broad range of interested parties, to facilitate discussion, and to ascertain whether or not consensus can be achieved. On March 1, 2012, Senator Bingaman introduced the Clean Energy Standard Act of 2012 ( S. 2146 ), which would require large utilities to sell a percentage of their electricity from clean energy sources—at least 24% in 2015 and gradually increasing over time to 84% by 2035. The CES expands on the concept of a national renewable energy standard (RES), an idea that has received significant congressional attention. An RES would encourage the production of electricity from renewable resources. RES legislative discussions date back to at least the 105 th Congress. A CES expands qualifying energy sources to include other "clean" energy sources (e.g., nuclear, natural gas, clean coal) along with renewable energy sources (e.g., wind, solar). Any CES would need to define which energy sources would qualify as "clean." The bulk of electricity generation in 2011 was from coal, natural gas, and nuclear (see Figure 1 ). Renewable sources constituted roughly 12% of total electric power net generation in 2011 (including conventional hydroelectric). Analysis from the Energy Information Administration (EIA) that does not include the addition of a federal RES or CES suggests that electricity generation from renewable sources (including conventional hydroelectric) will grow from 10% in 2010 to 16% in 2035. EIA analysis indicates that most of the growth in renewable electricity generation in the power sector, excluding hydroelectricity, from 2010 to 2035 will consist of generation from wind and biomass facilities. Multiple features of a CES may require congressional action. Eligible energy sources would likely be among the topics at the forefront of a CES debate; at stake is what resources could and could not participate in a CES. Each energy source has advantages and disadvantages, and each brings different natural resource, economic, and technical challenges. For example, the cost to build clean energy projects and operate and maintain them is expected to vary (see Table 1 ). Additionally, sources for supply-side energy options are dependent on regional resources; energy efficiency is a demand-side source that is available everywhere because it is derived from power consumption locations—not from natural resource geography. Congress might consider these factors, among others such as how a federal CES would interact with state power provisions, in determining which energy sources would be eligible for a CES. This report begins with a brief examination of clean energy, renewable energy, and alternative energy. It then presents possible selection criteria Congress could use to determine which sources could be eligible for a CES depending on the goal(s) of the CES. The report provides an overview of the energy sources most commonly discussed as potential CES qualifying sources: biomass, fossil fuels (natural gas combined-cycle and coal-fired power plants with carbon capture and sequestration), geothermal resources, nuclear, solar, water, and wind. The report describes where each source can be found in the United States, the estimated quantity available for electricity generation, technologies used to create electricity from the source, advantages and disadvantages of using the source for electricity generation, and policy implications should the source be included in a CES. The report also contains a section on energy efficiency and its potential inclusion in a CES. Clean energy, renewable energy, alternative energy—these terms have been used interchangeably. However, they are not synonymous, although one term can encompass another. The terms differ with respect to an energy source's point of origin (e.g., fossil or non-fossil), replenishment time frame (e.g., instantaneously or millions of years), supply (e.g., exhaustible or inexhaustible), and environmental impact (e.g., greenhouse gas emissions and other pollutants), among other qualities. In general, a renewable energy source is naturally replenishing but flow-limited. A clean energy source is typically a source that produces little to no air pollution. The term "alternative" usually describes a non-conventional energy source. Most but not all renewable energy sources qualify as clean energy, depending on how electricity is produced from the source. Some clean energy sources (e.g., nuclear) are not generally considered renewable. Potential CES energy source selection criteria could depend on the goal of a CES. While a CES generally would have the basic goal of producing a significant portion of electricity from clean energy sources, the reasoning behind this goal could influence which sources are selected. In evaluating individual energy sources for possible inclusion, Congress might consider the following criteria: geographic location of the energy source, energy source supply levels, job creation associated with the energy source, implementation time frame, EPA regulations (existing and forthcoming), environmental issues (air quality, water quality, water quantity, wildlife), greenhouse gas emissions, baseload versus non-baseload, energy balance, energy content, land use change, scalability, and cost. This section discusses selected energy resources that are commonly identified as potential qualifying sources for a CES. Each overview describes where the source can be found in the United States, how much is estimated to be available for electricity generation, the technologies used to create electricity from the identified source, advantages and disadvantages of using the source for electricity generation, and policy implications. Potential resources are renewable (e.g., wind) or technology-aided (e.g., fossil fuels). The energy resources may include biomass, clean energy fossil fuels, geothermal resources, nuclear, solar, water, and wind. A number of cross-cutting issues are associated with large-scale electricity production for many or all of the clean energy sources: Technology. For certain energy sources, technology to capture the resource and generate electric power is likely to require significant upgrading and financial investment. Electricity Transmission. In some cases, the energy source is located a considerable distance from where the electricity is needed. Transmission lines may have to be constructed to transmit the electricity from its point of origin to its point of use. This raises concerns not only for capital investment, but also siting issues. Variability. Electricity generated from certain sources (e.g., wind, solar) is variable; it is subject to acts of nature, and is not always available. This may affect grid stability if sufficient back-up generation is not available. Project economics. It may be cost-prohibitive to embark on certain clean electricity investments if the rate-of-return on the investment is not favorable. The rate-of-return is likely to vary across the United States, depending on source availability and regional market prices among other things. Material cost and supply. Certain clean electricity projects may require expensive or scarce amounts of raw materials. Use of these materials for electricity generation may have an impact on other markets that rely on the same materials. Biomass is organic matter that can be converted to energy. Common types of biomass are wood, wood residues (e.g., branches), agricultural biomass (e.g., corn stover), aquatic biomass (e.g., algae), animal manure, and industrial biomass wastes. Relative to other locations in the United States, large quantities of biomass have been identified east of the Rocky Mountains, and in the Northwest, Alaska, and Hawaii. Biomass can be converted to electricity (biopower) by either a thermochemical or a biochemical conversion process. Biopower conversion processes include combustion, co-firing, gasification, pyrolysis, and anaerobic digestion. The technologies are at varying stages of maturity. Approximately 1.4% of electricity generation in 2011 originated from biomass. Biopower was the third-largest renewable energy source for electricity generation (following conventional hydroelectric power and wind). The EIA estimates that electricity generation for all sectors from biomass will grow 5.6% from 2009 to 2035, and its contribution to the total U.S. electric power capacity will be 0.5% in 2035. There are advantages and disadvantages to pursuing biomass for electricity generation under a CES. Proponents of using biomass point out that various types of biomass are available in more than 35 states for electricity generation. In addition, they suggest that a CES would establish another market for biomass producers to sell their product, and that it would serve as an economic incentive to remove and use pest-infested woody biomass for electricity generation. Also, some biomass feedstocks can be used for baseload power production (minimum amount of electric power delivered or required over a given period of time at a steady rate), unlike wind or solar, which are variable resources. Some suggest that biomass use may be unsustainable, or that biomass combustion could increase carbon emissions. If it is economically advantageous to use biomass for electricity generation, some contend, less biomass will be available for traditional purposes. In particular, some are concerned there may not be enough biomass to meet both liquid transportation fuels and electricity needs under a CES. The natural resource implications of using biomass will depend on management of the sources from which it originates.