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Electricity explained How electricity is generated

An electric generator is a device that converts a form of energy into electricity. There are many different types of electricity generators. Most of world electricity generation is from generators that are based on scientist Michael Faraday’s discovery in 1831 that moving a magnet inside a coil of wire makes (induces) an electric current to flow in the wire. He made the first electricity generator called a Faraday disk, which operates on this relationship between magnetism and electricity and which led to the design of the electromagnetic generators that we use today.

Electromagnetic generators use an electromagnet—a magnet produced by electricity—not a traditional magnet. A basic electromagnetic generator has a series of insulated coils of wire that form a stationary cylinder—called a stator—surrounding an electromagnetic shaft—called a rotor. Turning the rotor makes an electric current flow in each section of the wire coil, which becomes a separate electric conductor. The currents in the individual sections combine to form one large current. This current is the electricity that moves from generators through power lines to consumers. Electromagnetic generators driven by kinetic (mechanical) prime movers account for nearly all of U.S. electricity generation.

Turbine driven generators

Most of U.S. and world electricity generation is from electric power plants that use a turbine to drive electricity generators. In a turbine generator, a moving fluid—water, steam, combustion gases, or air—pushes a series of blades mounted on a rotor shaft. The force of the fluid on the blades spins/rotates the rotor shaft of a generator. The generator, in turn, converts the mechanical (kinetic) energy of the rotor to electrical energy. Different types of turbines include steam turbines, combustion (gas) turbines, hydroelectric turbines, and wind turbines.

Diagram of an electric turbine generator.

Steam turbines are used to generate the majority of the world’s electricity and they accounted for about 44% of U.S. electricity generation in 2020. Most steam turbines have a boiler in which a fuel is burned to produce hot water and steam in a heat exchanger, and the steam powers a turbine that drives a generator. Nuclear power reactors use nuclear fuel rods to produce steam. Solar thermal power plants and most geothermal power plants use steam turbines. Most of the largest U.S. electric power plants use steam turbines.

Combustion gas turbines, which are similar to jet engines, burn gaseous or liquid fuels to produce hot gases to turn the blades in the turbine.

Steam and combustion turbines can be operated as stand-alone generators in a single-cycle or combined in a sequential combined-cycle. Combined-cycle systems use combustion gases from one turbine to generate more electricity in another turbine. Most combined-cycle systems have separate generators for each turbine. In single-shaft combined cycle systems, both turbines may drive a single generator. Learn more about different types of combined-cycle power plants. In 2020, combined-cycle power plants supplied about 35% of U.S. net electricity generation.

Combined-heat-and-power (CHP) plants, which may be referred to as cogenerators, use the heat that is not directly converted to electricity in a steam turbine, combustion turbine, or an internal combustion engine generator for industrial process heat or for space and water heating. Most of the largest CHP plants in the United States are at industrial facilities such as pulp and paper mills, but they are also used at many colleges, universities, and government facilities. CHP and combined-cycle power plants are among the most efficient ways to convert a combustible fuel into useful energy.

Hydroelectric turbines use the force of moving water to spin turbine blades to power a generator. Most hydroelectric power plants use water stored in a reservoir or diverted from a river or stream. These conventional hydroelectric power plants accounted for about 7% of U.S. electricity generation in 2020. Pumped-storage hydropower plants use the same types of hydro turbines that conventional hydropower plants use, but they are considered electricity storage systems (see below). Other types of hydroelectric turbines called hydrokinetic turbines are used in tidal power and wave power systems. Learn more about different types of hydroelectric turbines.

Wind turbines use the power in wind to move the blades of a rotor to power a generator. There are two general types of wind turbines: horizontal axis (the most common) and vertical-axis turbines. Wind turbines were the source of about 8% of U.S. electricity generation in 2020.

Ocean thermal energy conversion (OTEC) systems use a temperature difference between ocean water at different depths to power a turbine to produce electricity.

Other types of generators

There are many different types of electricity generators that do not use turbines to generate electricity. The most common in use today are solar photovoltaic (PV) systems and internal combustion engines.

Solar photovoltaic cells convert sunlight directly into electricity. They are used to power devices as small as wrist watches and can be connected together in panels that are connected together in arrays to power individual homes or form large power plants. Photovoltaic (PV) power plants are now one of the fastest growing sources of electricity generation around the world. In the United State, PV power plants were the source of about 2% of total utility-scale electricity generation in 2020.

Internal combustion engines, such as diesel engines, are used all around the world for electricity generation including in many remote villages in Alaska. They are also widely used for mobile power supply at construction sites and for emergency or backup power supply for buildings and power plants. Diesel-engine generators can use a variety of fuels including petroleum diesel, biomass-based liquid fuels and biogas, natural gas, and propane. Small internal combustion engine generators fueled with gasoline, natural gas, or propane are commonly used by construction crews and tradespeople and for emergency power supply for homes.

Other types of electricity generators include fuel cells, Stirling engines (used in solar thermal parabolic-dish generators), and thermoelectric generators .

Energy storage systems for electricity generation include hydro-pumped storage, compressed-air storage, electrochemical batters, and flywheels. These energy storage systems use electricity to charge a storage facility or device, and the amount of electricity that they can supply is less than the amount they use for charging. Therefore, the net electricity generation from storage systems is counted as negative to avoid double counting electricity use for charging the storage system.

Electricity generators by major type and share of total annual U.S. utility-scale net electricity generation in 20201
Generator Plant type Main fuel/energy source Share of annual electricity generation
Steam turbine Single cycle All sources 43.9%
Coal 19.1%
Nuclear 19.7%
Natural gas   2.7%
Biomass (1.1%); Others (1.3%)   2.4%
Multiple Combined cycle Natural gas2 34.5%
Combustion turbine 22.0%
Steam turbine 11.1%
Dual/single shaft   1.5%
Combustion gas turbine Single cycle Natural gas2   3.4%
Wind turbine All types Wind   8.4%
Hydroelectric turbine Conventional Water   7.1%
Photovoltaic All types Solar   2.1%
Others3 Various   0.6%
Storage systems4 Various  -0.1%

1Includes generators at power plants with at least one megawatt electric generation capacity.
2Natural gas is the main energy source (99%) for combined-cycle power plants.
3Other sources include internal combustion engines, fuel cells, and binary-cycle turbines.
4Storage systems include hydro-pumped storage, electrochemical batteries, compressed-air storage, and flywheels. Generation from storage systems is netted out of total annual electricity generation to avoid double counting of energy storage charging sources.
Source: U.S. Energy Information Administration (EIA), Form EIA-923 Power Plant Operations Report, 2020

Last updated: November 1, 2021