Electricity Emissions in the United States
The electricity sector is responsible for about one-third of all U.S. greenhouse gas emissions (see Figure 1) and 38 percent of total carbon dioxide (CO2) emissions.
Figure 1: U.S. Greenhouse Gas Emissions by Sector (2011)
Source: U.S. Environmental Protection Agency (EPA), Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011, Table ES-7, 2013. http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html.
A snapshot of the fuels used in the United States for electricity shows that coal-fueled generation provides a little more than 37 percent of all electricity, down from nearly 50 percent in 2006. Filling this gap, natural gas now provides 30 percent of all electricity, and renewables, including wind and large hydroelectric power, provide about 13 percent. Nuclear power continues to provide around one-fifth of net generation (see Figure 2).
Figure 2: U.S. Net Electricity Generation by Energy Source (2012)
Source: Energy Information Administration (EIA), Monthly Energy Review, May 2013, Table 7.2a, 2013. http://www.eia.gov/totalenergy/data/monthly/#electricity.
The greenhouse gas emissions associated with different sources of electricity vary significantly, depending on the carbon content of the fuel being used. Carbon dioxide makes up almost 99 percent of the greenhouse gas emissions from electricity generation, and carbon dioxide emissions from coal combustion account for almost 80 percent of total electricity generation-related emissions. The combustion of natural gas and petroleum account for most of the remaining carbon dioxide emissions (see Figure 3). Due to the continuing shift from coal- to natural gas-fired electricity generation, the percentage of emissions from coal is continuing to decline. Electricity generation-related greenhouse gas emissions have decreased more than six percent since 2007.
Figure 3: Electricity Generation-Related GHG Emissions (2011)
Source: EPA, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2011, Table 2-13, 2013. http://www.epa.gov/climatechange/ghgemissions/usinventoryreport.html.
Key Generation Technologies in Use
Coal-fueled electricity is generated almost exclusively by pulverized coal (PC) power plants. These plants crush coal into a fine powder and then burn it in a boiler to heat water and produce steam. The steam is then used to spin one or more turbines to generate electricity. Natural gas, oil or biomass can be used as a fuel in conjunction with steam turbine technology. Similarly, in a nuclear reactor, fission is used to heat water, which directly or indirectly produces steam to drive a turbine and generate electricity. Large coal and nuclear steam units on the order of 500 – 1000 MW or greater are typically used to provide baseload  generation, meaning that they supply electricity nearly continuously.
Figure 4: Steam Turbine
Source: ONCOR. http://www.oncor.com/community/knowledgecollege/energy_library/generatin....
Combustion (or single-cycle) turbines are another widespread power generation technology. In a combustion turbine, compressed air and burning fuel (diesel, natural gas, propane, kerosene, biogas, etc) are ignited in a combustion chamber. The resulting high temperature, high velocity gas flow is directed at turbine blades that spin the turbine and common shaft, which drives the air compressor and the electric power generator. Combustion turbine plants are typically operated to meet peak  load demand, as they are able to be switched on relatively quickly. Newer turbines are able to be switched on and off frequently, so they can provide a firm backup to intermittent wind and solar on the power grid if needed. The typical size is 100 – 400 MW.
Figure 5: Combustion Turbine
Source: Duke Energy. http://www.duke-energy.com/about-energy/generating-electricity/oil-gas-f....
A basic combined cycle power plant combines a single gas (combustion) turbine and a single steam unit all in one, although there are other possible configurations. As combustion turbines became more advanced in the 1950s, they began to operate at ever higher temperatures, which created a significant amount of exhaust heat. In a combined-cycle power plant, this waste heat is captured and used to boil water for a steam turbine generator, thereby creating additional generation capacity. Historically, they have been used as intermediate  power plants, generally supporting higher electricity use during daytime hours. However, newer natural gas combined cycle plants are now providing baseload support.
Figure 6: Combined-Cycle Turbine
Source: Global-Greenhouse-Warming.com. http://www.global-greenhouse-warming.com/gas-vs-coal.html.
Select these links to find out more about nuclear power, hydropower, wind, and solar generation technologies.
The industrial sector accounts for 26 percent of U.S. electricity sales, with the residential and commercial sector evenly sharing the remainder. (see Figure 7).
Figure 7: Retail Sales of Electricity to Ultimate Customers, Total by End Use Sector (2011)
Source: EIA, Electric Power Monthly, Table 5.1, May 29, 2012. http://www.eia.doe.gov/cneaf/electricity/epm/table5_1.html.
The primary end uses of electricity vary by sector. In the residential sector, space heating, water heating, space cooling and lighting together account for more than half of household electricity use (see Figure 8). In the commercial sector, lighting is the largest electricity end use (see Figure 9). In the manufacturing sector, half of all electricity use is for powering electric motors (see Figure 10).
Figure 8: Residential Electricity Consumption by End Use (2010)
Source: DOE, Buildings Energy Data Book, Table 2.1.5, March 2012. http://buildingsdatabook.eren.doe.gov/ChapterIntro2.aspx.
Figure 9: Commercial Electricity Consumption by End Use (2010)
Source: DOE, Buildings Energy Data Book, Table 3.1.5, March 2012. http://buildingsdatabook.eren.doe.gov/ChapterIntro3.aspx.
Figure 10: Manufacturing Electricity Consumption by End Use (2006)
Source: EIA, Manufacturing Energy Consumption Survey (MECS), Table 5.2, 2006. http://www.eia.doe.gov/emeu/mecs/.
Since 1949, U.S. electricity generation has grown dramatically, with an average annual growth rate of 4.3 percent (see Figure 11). Since 2000, however, U.S. electricity generation has grown at an average rate of less than 1 percent. During this time, generation from natural gas has increased at an average annual rate of 4.9 percent, and non-hydro renewable generation has increased at an average annual growth rate of 8.3 percent. Coal generation has decreased at an average annual rate of 1.1 percent, and in 2011 fell below the level generated in 1996.
Figure 11: U.S. Net Electricity Generation by Source (1949-2011)
Source: EIA, Total Energy, Electricity Net Generation, 2012. http://www.eia.gov/totalenergy/data/monthly/index.cfm#electricity.
From 1990 to 2010, U.S. electricity generation-related greenhouse gas emissions grew an average of 1.1 percent per year, while a decrease in annual emissions was seen in several years (see Figure 12). During this time, the proportion of electricity generation-related greenhouse gas emissions from coal combustion, which peaked in 1996 at around 85 percent, has fallen to 79 percent in 2010. The share of emissions from natural gas combustion has grown from around 9 percent in 1990 to just over 17 percent in 2010, and the share of emissions from petroleum has fallen from around 5 percent to a little more than 1 percent over the same period.
From 1990 to 2010, CO2 emissions from electricity generation, electricity generation, and real gross domestic product (GDP) grew at annual average rates of 1.1, 1.5, and 2.5 percent, respectively (see Figure 13). This illustrates that the U.S. economy grew less electricity-intensive per value of output while the electricity generation also became less carbon intensive over this period.
Figure 12: U.S. Electricity Generation-Related Greenhouse Gas Emissions (1990-2010)
Source: EPA, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2010, Table 2-13, 2012. http://www.epa.gov/climatechange/emissions/usinventoryreport.html.
Figure 13: Relative Growth of Electricity Generation, CO2 Emissions from Electricity Generation, and GDP
Source: EPA, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2010, Table 2-13, 2012; EIA, Total Energy, Table 8.2a, 2012. http://www.eia.gov/totalenergy/data/annual/index.cfm#electricity; Bureau of Economic Analysis, Gross Domestic Product, http://www.bea.gov/national/index.htm.
Globally, CO2 is the most abundant anthropogenic greenhouse gas, accounting for 76 percent of total anthropogenic greenhouse gas emissions in 2008; the CO2 emissions from fossil fuel use alone account for 62 percent of total greenhouse gas emissions. ,  Electricity generation is by far the largest single source of CO2 emissions (see Figure 14).
Figure 14: Sources of Global CO2 Emissions (1970-2004, Direct Emissions by Sector Only)
Source: Intergovernmental Panel on Climate Change (IPCC), "Introduction." In Mitigation of Climate Change. Contribution of Working Group III to the Fourth Assessment Report. Cambridge: Cambridge University Press, 2007. Figure 1.2. http://www.ipcc.ch/ipccreports/ar4-wg3.htm
Notes: (1) Including fuel wood at 10% net contribution, large-scale biomass burning averaged data for 1997–2002, including decomposition and peat fires, excluding fossil fuel fires; (2) other domestic surface transport, non-energetic use of fuels, cement production, and venting/flaring of gas from oil production; (3) including aviation and marine transport.
The generation profile of global electricity production is similar to that of the United States, with coal being the largest energy source for electricity production (see Figure 15). Globally, 5.1 percent of electricity is generated by oil, whereas in the United States oil makes up less than 1 percent. Also, hydropower makes up a larger share of global electricity generation, while the United States gets a greater proportion of its electric power from nuclear. The United States contributes more than one-fifth of global carbon dioxide emissions from electricity and heat production; China and the United States are the largest single emitters (see Figure 16).
Figure 15: World Electricity Generation by Fuel (2009)
Source: International Energy Agency (IEA), Key World Energy Statistics. Paris: IEA, 2011. http://www.iea.org/Textbase/publications/free_new_Desc.asp?PUBS_ID=1199
Notes: Other includes geothermal, solar, wind, combustible renewables & waste, and heat.
Figure 16: CO2 Emissions from Fossil Fuel Combustion for Electricity and Heat (2009)
Source: IEA, CO2 from Fossil Fuel Combustion 2011. Paris: IEA, 2011. http://www.iea.org/Textbase/publications/free_new_Desc.asp?PUBS_ID=1825.
Electricity Sector Mitigation Opportunities
In general terms, greenhouse gas emission reductions from the electric power sector can be achieved through efficiency (i.e., eliminating waste), conservation (i.e., reducing the amount of electricity generated), switching fuel sources (i.e., from coal to lower-emitting natural gas), and by incorporating low- and zero-carbon electricity generation technologies (i.e., reducing the emissions associated with electricity generation), such as renewable energy, carbon capture and storage, and nuclear power.
Many studies have analyzed the most cost-effective mix of emission-reduction options. Some of the most widely cited studies include:
C2ES Work on Electricity
Over the past 20 years U.S. electricity generation has become less carbon intensive, and the U.S. economy has grown less electricity-intensive. Further mitigating greenhouse gas emissions from electricity generation will require a comprehensive approach, including lower-, low- and zero-carbon electricity generation technologies, incorporating renewable energy, switching to lower-emitting fuels, coal or gas with carbon capture and storage, and nuclear power, as well as energy efficiency and conservation. Several types of policies can be employed to promote these mitigation techniques, including emissions pricing (e.g. carbon tax or cap and trade), electricity portfolio standards (also known as clean energy standards), emission performance standards, financial incentives for clean energy deployment and energy efficiency, and research and development to support innovative technologies.
Our work at C2ES covers all types of electricity-related topics, including policy and regulation, low-carbon technology status and outlook, and technology innovation. We track and inform policymaking at the state, federal, and regional levels, collaborate on research for papers and briefs, blog about current energy issues, and educate policymakers and others with up-to-date online resources about important low-carbon technologies.
Tracking policy - We track policy progress at the state, federal, and international level. Our state maps provide information about which states have implemented policies that promote low-carbon electricity technologies and energy efficiency. We also track and analyze policy at the national level, including what is happening in Congress and the Executive Branch.
Research - We produce research, including reports, white papers, and briefs, on climate and clean energy issues. For example, our 2005 report titled The U.S. Electric Power Sector and Climate Change Mitigation is a comprehensive look at reducing greenhouse gas emissions from the electricity sector.
Climate Compass Blog - Our blog includes posts about current issues related to electricity, and you can view relevant posts here.
Climate Techbook - The Climate Techbook provides briefs describing technologies related to electricity generation and energy efficiency. Below is a list of the Techbook factsheets that pertain to electricity.
International Energy Agency (IEA)
· World Energy Outlook
· Energy Technology Perspectives 2010: Scenarios & Strategies to 2050
U.S. Department of Energy (DOE)
· Electric Power
U.S. Energy Information Administration (EIA)
· Electricity Overview
· Electricity Explained
U.S Environmental Protection Agency (EPA)
· US Greenhouse Gas Emissions Inventory
· Clean Energy
· GHG Performance Standards for Power Plants
Bipartisan Policy Center
Electric Power Research Institute (EPRI)
Environmental Defense Fund (EDF)
Natural Resources Defense Council (NRDC)
Resources for the Future (RFF)
World Resources Institute (WRI)
Related Business Environmental Leadership Council (BELC) Companies
 Baseload generation describes electric power plants that typically run all day and night, seven days a week.
 Peak generation describes electric power plants that run only during times of the highest demand. For example, high demand can occur in the morning when many consumers are waking up and switching on electric appliances or on hot summer afternoons when many air conditioners are running simultaneously.
 Intermediate generation describes electric power plants that typically run only during daytime hours to support higher use of electrical appliances, computers, lighting and so on.
 International Energy Agency (IEA), CO2 Emissions From Fuel Combustion (2011). Section III, Figure 1, p III.4.
 Intergovernmental Panel on Climate Change (IPCC). Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report. Cambridge: Cambridge University Press, 2007. See Figure 1.1b, available at http://www.ipcc.ch/ipccreports/ar4-wg3.htm.