Our business customers expect electricity to be available whenever they turn on a switch. Satisfying these instantaneous demands requires an uninterrupted flow of electricity. To meet this requirement, utilities and non-utility electricity power producers operate several types of electric generating units, powered by a wide range of fuel sources. These include fossil fuels (coal, natural gas and petroleum), uranium and renewable fuels (water, geothermal, wind and other renewable energy sources).

The combination of energy sources used is referred to as the fuel or generation mix.

Current Generation Mix*

U.S. Primary Energy Consumption by Source and Sector, 2004
(Quadrillion Btu)

¹Excludes 0.3 quadrillion Btu of ethanol, which is included in "Renewable Energy.”
²Includes coal coke net imports.
³Conventional hydroelectric power, wood, waste, alcohol, geothermal, solar, and wind.
⁴Includes industrial combined-heat-and-power (CHP) and industrial electricity-only plants.
⁵Includes commercial combined-heat-and-power (CHP) and commercial electricity-only plants.
⁶Electricity-only and combined-heat-and-power (CHP) plants whose primary business is to sell electricity, or electricity and heat, to the public.

Note:  Sum of components may not equal 100 percent due to independent rounding.
Source:  Energy Information Administration, Annual Energy Review 2004, Tables 1.3 and 2.1b-2.1f.

Types of Generating Units
Steam-electric generating units burn fossil fuels such as coal, natural gas and petroleum. The steam turns a turbine that produces electricity through an electrical generator. Natural gas and petroleum are also burned in gas turbine generators where the hot gases produced from combustion are used to turn the turbine, which spins the generator to produce electricity. Additionally, petroleum is burned in generating units with internal-combustion engines. The combustion occurs inside cylinders of the engine, which is connected to the shaft of the generator. The mechanical energy provided from the engine drives the generator to produce energy.

In nuclear-powered generating units, the boiler is replaced by a reactor in which the fission of uranium is used to make steam to drive the turbine.

Hydroelectric power units use flowing water to spin a turbine connected to a generator. In a falling water system, water is accumulated in reservoirs created by dams, then released through conduits to apply pressure against the turbine blades to drive the generator. In a run-of-the-river system, the force of the river current applies the pressure to the turbine blades to produce electricity.

Non-water renewable sources of electricity generation contribute only small amounts to total power production. These sources include geothermal, refuse, waste heat, waste steam, solar, wind and wood.

Source: www.eia.doe.gov