Electricity explained in the United States

Most of the electricity in the United States is produced using steam turbines.

A turbine converts the kinetic energy of a moving fluid (liquid or gas) to mechanical energy. In a steam turbine, steam is forced against a series of blades mounted on a shaft, thus rotating the shaft connected to the generator. The generator, in turn, converts its mechanical energy to electrical energy based on the relationship between magnetism and electricity.

In steam turbines powered by fossil fuels, such as coal, petroleum (oil), and natural gas, the fuel is burned in a furnace to heat water in a boiler to produce steam.
Fossil Fuels Generate Most U.S. Power

Coal is the most common fuel for generating electricity in the United States. In 2008, nearly half (48%) of the Country’s 4.1 trillion kilowatthours of electricity used coal as its source of energy.
Share of electricity generated from each energy source, 2007. Coal 48.5%, Natural gas 21.6%, nuclear power 19.4%, hydroelectric 5.8%, Other renewables 2.5%, petroleum 1.6%, other gases 0.3%, other 0.3%.
Natural gas, in addition to being burned to heat water for steam, can also be burned to produce hot combustion gases that pass directly through a turbine, spinning the turbine’s blades to generate electricity. Gas turbines are commonly used when electricity utility usage is in high demand. In 2008, about 21% of the Nation’s electricity was fueled by natural gas.

Petroleum can be burned to produce hot combustion gases to turn a turbine or to make steam to turn a turbine. Residual fuel oil, a product refined from crude oil, is often the petroleum product used in electric plants that use petroleum to make steam. Petroleum was used to generate just over 1% of all electricity in the United States in 2008.
Nuclear Power Provides About One-Fifth of U.S. Electricity

Nuclear power is a method in which steam is produced by heating water through a process called nuclear fission. In a nuclear power plant, a reactor contains a core of nuclear fuel, primarily uranium. When atoms of uranium fuel are hit by neutrons, they fission (split) releasing heat and more neutrons. Under controlled conditions, these other neutrons can strike more uranium atoms, splitting more atoms, and so on. Thereby, continuous fission can take place, creating a chain reaction releasing heat. The heat is used to turn water into steam, that, in turn, spins a turbine that generates electricity. Nuclear power was used to generate about 21% of all the Country’s electricity in 2008.
Renewable Energy Sources Make Up the Rest

Hydropower, the source for 6% of U.S. electricity generation in 2008, is a process in which flowing water is used to spin a turbine connected to a generator. There are two basic types of hydroelectric systems that produce electricity. In the first system, flowing water accumulates in reservoirs created by dams. The water falls through a pipe called a penstock and applies pressure against the turbine blades to drive the generator to produce electricity.

In the second system, called run-of-river, water is diverted from a river using a relatively low dam or weir into penstocks and turbines. The dam does not store a large volume of water in a reservoir. Run-of-river power plants are more dependent on river flows than hydro plants with reservoirs for storing water which can produce electricity even when natural river flows are low.

Biomass is material derived from plants or animals (i.e. biogenic) and includes lumber and paper mill wastes; food scraps, grass, leaves, paper, and wood in municipal solid waste (garbage); and forestry and agricultural residues such as wood chips, corn cobs, and wheat straw. These materials can be burned directly in steam-electric power plants, or converted to gas that can be burned in steam generators, gas turbines, or internal combustion engine-generators. Biomass accounts for about 1% of the electricity generated in the United States.

Wind power is produced by converting wind energy into electricity. Electricity generation from wind has increased significantly in the United States since 1970, but wind power remains a small fraction of U.S. electricity generation, about 1%.

Geothermal power comes from heat energy buried beneath the surface of the earth. In some areas of the United States, enough heat rises close to the surface of the earth to heat underground water into steam, which can be tapped for use at steam-turbine plants. This energy source generated less than 1% of the electricity in the Country in 2008.

Solar power is derived from energy from the sun. There are two main types of technologies for converting solar energy to electricity: photovoltaic (PV) and solar-thermal electric. PV conversion produces electricity directly from sunlight in a photovoltaic (solar) cell. Solar-thermal electric generators concentrate solar energy to heat a fluid and produce steam to drive turbines. In 2008, less than 1% of the Nation’s electricity was from solar power.

Also on Energy Explained

* Use of Electricity
* Factors Affecting Electricity Prices
* Use of Nuclear Power
* Energy Use in Homes
* Energy Use in Commercial Buildings

Learn More

* Electric Power Industry Overview 2007 — http://www.eia.doe.gov/cneaf/electricity/page/prim2/toc2.html#change
* Edison Electric Institute — http://www.eei.org/
* American Public Power Association — http://www.appanet.org/
* How clean is my electricity? — http://www.epa.gov/cleanenergy/energy-and-you/how-clean.html
* Renewable Energy Information — http://www.eia.doe.gov/cneaf/solar.renewables/page/rea_data/rea_sum.html

Last Updated: February 2, 2010

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There is more than one way to size up U.S. electricity. Looking at how much and from what sources electricity is generated (“generation”) is only one piece of the puzzle. Electricity sales and capacity also provide important perspectives on U.S. electricity:

* Generation — The amount of electricity produced over a period of time.
* Sales — The amount of electricity sold to customers over a period of time.
* Capacity — The maximum level at which electric power can be supplied at a point in time.

Electricity Generation
bar chart showing U.S. Electric Power Industry Net Generation, 2008. Coal 48.5%, Natural gas 21.6%, nuclear power 19.4%, hydroelectric 5.8%, Other renewables 2.5%, petroleum 1.6%, other gases 0.3%, other 0.3%.
Click to enlarge »

Data for this figure

About 90% of U.S. electricity is generated by three fuels: coal, natural gas, and nuclear:

* Coal — 48.2%
* Natural Gas — 21.4%
* Nuclear — 19.6%
* Hydroelectric — 6.0%
* Other Renewables — 3.1%
* Petroleum — 1.1%

Net generation of electricity fell to 4,119 billion kilowatthours (kWh) in 2008 from 4,157 billion kWh in 2007. A combination of weak economic activity and reduced summer electricity demand for cooling contributed to the 0.9% decrease in net generation.
Electricity Sales

U.S. electricity sales to customers totaled nearly 3,765 billion kilowatthours (kWh) in 2008, a 0.8% decline over 2007.

Sales to each customer class in 2007 totaled:

* Residential — 1,380 billion kWh (37% of electricity sold)
* Commercial — 1,336 billion kWh (36%)
* Industrial — 1,009 billion kWh (27%)
* Transportation — 8 billion kWh (0.2%)

Sales by each kind of provider in 2008 totaled:

* Investor-owned electric utilities — 60% of electricity sold
* Public utilities — 15%
* Energy-Only Providers — 14%
* Cooperatives — 11%
* Federal Power Marketing Authorities — 1%

Electricity Capacity
line graph showing U.S. Electric Power Industry Net Summer Capacity, 2007
Click to enlarge »

Data for this figure

Electricity capacity is essentially the maximum level at which electric power can be supplied at a point in time.

Sufficient capacity is important, because enough electricity must always be produced to meet demand at every moment.

At most times, many power plants are not generating electricity at their full capacity. There are three major types of generating units that vary by intended usage:

* A baseload generating unit is normally used to satisfy all or part of the minimum or base load of the system and, as a consequence, produces electricity at an essentially constant rate and runs continuously. Baseload units are generally the newest, largest, and most efficient of the three types of units.
* A peakload generating unit, normally the least efficient of the three unit types, is used to meet requirements during the periods of greatest or peak load on the system.
* An intermediate-load generating unit meets system requirements that are greater than base load but less than peak load. Intermediate-load units are used during the transition between baseload and peak load requirements.

Since the late 1990s, natural gas has been the fuel of choice for the majority of new generating units. Natural gas now accounts for about 40% of total generating capacity.

Also on Energy Explained

* Use of Electricity
* Factors Affecting Electricity Prices
* Use of Nuclear Power
* Energy Use in Homes
* Energy Use in Commercial Buildings

Learn More

* Electric Power Industry Overview 2007 — http://www.eia.doe.gov/cneaf/electricity/page/prim2/toc2.html#change
* Edison Electric Institute — http://www.eei.org/
* American Public Power Association — http://www.appanet.org/
* How clean is my electricity? — http://www.epa.gov/cleanenergy/energy-and-you/how-clean.html
* Renewable Energy Information — http://www.eia.doe.gov/cneaf/solar.renewables/page/rea_data/rea_sum.html

Last Updated: February 2, 2010
Largest Utility Plants by Net Generation

(2007 data except where noted)
Rank Operator Name Facility Name State Net Generation
(megawatthours)
1 Arizona Public Service Co Palo Verde AZ 26,782,391
2 Georgia Power Co Scherer GA 25,053,667
3 Duke Energy Indiana Inc Gibson IN 23,342,347
4 Georgia Power Co Bowen GA 22,972,903
5 STP Nuclear Operating Co South Texas Project TX 22,178,508
6 Progress Energy Florida Inc Crystal River FL 22,008,891
7 U S Bureau of Reclamation Grand Coulee WA 21,632,495
8 Tennessee Valley Authority Browns Ferry AL 21,227,260
9 Duke Energy Carolinas, LLC Oconee SC 20,564,650
10 NRG Texas LLC W A Parish TX 20,068,090
Largest Utility Plants by Capacity
Rank Operator Name Facility Name State Summer Capacity
(Megawatts)
1 U S Bureau of Reclamation Grand Coulee WA 7,079
2 Arizona Public Service Co Palo Verde AZ 3,872
3 Florida Power & Light Co Martin FL 3,814
4 NRG Texas LLC W A Parish TX 3,651
5 Georgia Power Co Scherer GA 3,405
6 Florida Power & Light Co Turkey Point FL 3,330
7 Tennessee Valley Authority Browns Ferry AL 3,274
8 Georgia Power Co Bowen GA 3,254
9 Progress Energy Florida Inc Crystal River FL 3,148
10 Duke Energy Indiana Inc Gibson IN 3,131
Largest Utilities by Number of Customers
Rank Electric Company Class of Ownership Number of Customers
1 Pacific Gas & Electric Co Investor Owned 5,179,256
2 Southern California Edison Co Investor Owned 4,812,332
3 Florida Power & Light Company Investor Owned 4,496,593
4 Commonwealth Edison Co Investor Owned 3,747,358
5 Consolidated Edison Co-NY Inc Investor Owned 2,794,592
6 Virginia Electric & Power Co Investor Owned 2,362,317
7 Duke Energy Corporation Investor Owned 2,330,252
8 Georgia Power Co Investor Owned 2,324,874
9 Detroit Edison Co Investor Owned 2,161,388
10 TXU Energy Retail Co LP Power Marketer 2,127,931
Largest Utilities by Retail Sales
Rank Electric Company Class of Ownership Sales
(megawatthours)
1 Florida Power & Light Company Investor Owned 105,274,631
2 Georgia Power Co Investor Owned 86,084,305
3 Virginia Electric & Power Co Investor Owned 79,892,176
4 Southern California Edison Co Investor Owned 79,505,231
5 Pacific Gas & Electric Co Investor Owned 79,450,903
6 Duke Energy Corporation Investor Owned 79,000,682
7 Reliant Energy Retail Services Power Marketer 56,660,552
8 Alabama Power Co Investor Owned 56,641,831
9 PacifiCorp Investor Owned 53,390,478
10 Detroit Edison Co Investor Owned 48,816,410
Largest Utilities by Revenue
Rank Electric Company State Class of Ownership Revenue
(thousand dollars)
1 Florida Power & Light Company FL Investor Owned 11,263,206
2 Southern California Edison Co CA Investor Owned 11,217,201
3 Pacific Gas & Electric Co CA Investor Owned 10,902,816
4 Reliant Energy Retail Services TX Power Marketer 6,539,067
5 Georgia Power Co GA Investor Owned 6,499,579
6 TXU Energy Retail Co LP TX Power Marketer 6,083,540
7 Virginia Electric & Power Co VA Investor Owned 5,276,298
8 Consolidated Edison Co-NY Inc NY Investor Owned 5,158,192
9 Commonwealth Edison Co IL Investor Owned 5,000,736
10 PECO Energy Co PA Investor Owned 4,465,626 /tonto.eia.doe.gov

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