Electricity Production and Distribution

Photo of electricity transmission lines with wind turbines in the background.

Sources of U.S. Electricity Generation, 2012

Pie chart shows U.S. sources of electricity generation in 2011: Coal 37%; Natural Gas 30%; Nuclear 19%; Renewables 12%; Petroleum, less than 1%

—U.S. Energy Information Administration

Plug-in hybrid electric vehicles (PHEVs) and all-electric vehicles (EVs) store electricity in batteries to power one or more electric motors. The batteries are charged primarily by plugging into off-board sources of electricity, produced from fossil fuels, nuclear energy, and renewable energy sources.

EVs and PHEVs in all-electric mode do not produce tailpipe emissions. However, there are emissions associated with the majority of electricity production in the United States. See the emissions section for more information.

Production

Most of the electricity in the United States is produced by steam turbine generators at power plants from primary resources such as coal, natural gas, and nuclear energy. According to the U.S. Energy Information Administration, in 2012, 37% of the nation's electricity was generated by coal, 30% by natural gas, 19% by nuclear energy, and 1% by petroleum.

Electricity is also produced from renewable sources of energy, including hydropower, biomass, wind, geothermal, and solar power. Together, renewable energy sources generated about 12% of the country's electricity in 2012. Production capacity from renewable sources (excluding hydropower) has been increasing steadily over the last decade.

With the exception of photovoltaic (PV) generation, most of the primary sources of energy are used directly or indirectly to move the blades of a turbine connected to an electric generator. The turbine generator set converts mechanical energy to electrical energy. In the cases of coal, oil, natural gas, nuclear fission, biomass, geothermal and solar thermal, the heat produced by these primary resources is used to create steam, which moves the blades of the turbine. In the cases of hydro and wind power, turbine blades are acted upon directly by flowing water and wind, respectively. PV panels convert sunlight directly to electricity using semiconductors that exhibit the photovoltaic effect.

The sources of energy used to produce electricity vary from one geographic region to the next. To find out about the mix of fuels and other energy sources used in your area, see the emissions section. Learn more about electricity production from the U.S. Department of Energy's Energy Information Administration.

Electricity Transmission and Distribution

Electricity in the United States travels long distances from generating facilities to local distribution substations through a transmission grid of nearly 160,000 miles of high-voltage transmission lines. Generating facilities provide power to the grid at low voltage, from 480V in small generating facilities to 22 kV in larger power plants. Once electricity leaves a generating facility, the voltage is increased, or "stepped up," by a transformer to minimize the power losses over long distances. As electricity is transmitted through the grid and arrives in the load areas, voltage is stepped down by transformers at distribution substations (ranges of 69 kV to 4.16 kV), and finally further lowered for use by customers (residential customers use 120V and 240V; commercial and industrial customers typically use 120V, 208V, and 480V).

Plug-In Vehicles and Electricity Infrastructure Capacity

EVs and PHEVs represent a new source of demand for electricity. However, they are unlikely to strain much of our existing electricity infrastructure in the near term. Large increases in the number of these vehicles in the United States will not necessarily require the addition of new electricity-generation capacity or substantial upgrades to transmission and distribution infrastructure.

Demand for electricity rises and falls, depending on time of day and time of year. Electricity production, transmission, and distribution capacity must be able to meet demand during times of peak use; but most of the time, the electricity infrastructure is not operating at its full capacity. In the United States, roughly 50% of the generation capacity is used 100% of the time, while only 5% of the time (about 400 hours per year) generation greater than 90% of the capacity is used. Usually the most costly and inefficient generation is used during these peak periods. As a result, EVs and PHEVs have the potential to create little or no need for additional capacity, as long as they charge predominantly during off-peak times, such as late at night, when the electric load on the system is at a minimum.

According to a study by Pacific Northwest National Laboratory, existing U.S. electricity infrastructure has sufficient capacity to meet about 73% of the energy needs of the country's light-duty vehicles. According to deployment models developed by researchers at the National Renewable Energy Laboratory (NREL), the diversity of household electricity loads and electric vehicle loads should allow introduction and growth of the plug-in vehicle market while "smart grid" networks expand. These networks will provide the capability to monitor and protect residential distribution transformers from future vehicle impacts, ensure that charging occurs during off-peak periods, and reduce costs to utilities, grid operators, and consumers. The NREL analysis also demonstrated the potential for synergies between plug-in vehicles and distributed sources of renewable energy. For example, small-scale renewables, like solar panels on a rooftop, can both provide clean energy for vehicles and reduce demand on distribution infrastructure by generating electricity near the point of use.

Utilities, vehicle manufacturers, charging-equipment manufacturers, and researchers are working to ensure that EVs and PHEVs are smoothly integrated into the U.S. electricity infrastructure. Some utilities offer lower rates at off-peak times in order to encourage vehicle charging when electricity demand is lowest. Vehicles and many models of electric vehicle supply equipment (EVSE, or charging stations) can be programmed to restrict charging to off-peak times. "Smart" models are even capable of communicating with the grid, enabling them to charge automatically when electricity demand and prices are lowest.