Did you know?
A standard unit for measuring electricity is the kilowatt (kW), which is equal to 1,000 watts. A watt is a measure of energy named after the Scottish engineer James Watt. One kW of electricity generated or used over the course of one hour is a kilowatthour (kWh). Other units for measuring electricity capacity and electricity generation and consumption are
- Megawatt (MW) = 1,000 kW; megawatthour (MWh) = 1,000 kWh
- Gigawatt (GW) = 1,000 MW; gigawatthour (GWH) = 1,000 MWh
Did you know?
Electricity generating capacity has three general categories. Nameplate capacity is determined by the generator's manufacturer and indicates the maximum output of electricity a generator can theoretically produce at a certain point in time without exceeding specified thermal limits.Net summer capacity and net winter capacity indicate the maximum instantaneous electricity load a generator can support during the respective season. These values may differ because of seasonal variations in the temperature of the cooling water or ambient air used by the generating unit.
Did you know?
The number of small-scale distributed solar photovoltaic (PV) systems, such as those found on the roofs of buildings, has grown significantly in the United States over the past several years. Estimates of small-scale solar PV capacity and generation by state and sector are included in the Electric Power Monthly. As of the end of 2017, almost 41% of total U.S. small-scale solar PV electricity generating capacity was in California.
Three terms are important to understand when learning about electricity production and consumption:
- Generation is a measure of the amount of electricity produced over a period of time. Most electric power plants use some of the electricity that they produce to operate the power plant itself.
- Capacity is the maximum level at which electric power (electricity) can be supplied at a specific point in time under certain conditions.
- Sales are the amount of electricity sold to customers over a period of time. They represent a proxy for most electricity consumption.
The amount of electricity generated is greater than the amount of electricity sold. The main reason for this difference is that some energy is lost (as heat) in the transmission and distribution of electricity. In addition, some electricity consumers generate electricity and use most or all of it. This use is called direct use. These consumers include industrial, manufacturing, commercial, and institutional facilities, as well as homeowners who have their own electricity generators. The United States also exports and imports some electricity to and from Canada and Mexico. Total U.S. electricity consumption by end-use consumers is equal to U.S. retail sales of electricity plus direct use of electricity.
The U.S. Energy Information Administration (EIA) publishes data on two general types of electricity generation and electricity generating capacity:
- Utility scale includes electricity generation and capacity of generating units (generators) located at power plants with at least one megawatt (MW) of total electricity generating capacity.
- Small scale includes generators with less than 1 MW of generating capacity that are usually at or near the location where the electricity is consumed. The most common type of small-scale systems are solar photovoltaic systems installed on building rooftops.
In 2017, net generation of electricity from utility-scale generators in the United States was about 4.01 trillion kilowatthours (kWh). EIA estimates that an additional 24 billion kWh (or 0.02 trillion kWh) were from small-scale solar photovoltaic (PV) systems, most of which was direct use. EIA estimates that total direct use of net electricity generation in 2017 equaled about 138 billion kWh.
In 2017, about 63% of U.S. utility-scale electricity generation was produced from fossil fuels (coal, natural gas, and petroleum), about 20% was from nuclear energy, and about 17% was from renewable energy sources. The shares of U.S. electricity generation by major energy sources in 2017 were
- Natural gas—32%
- Renewables (total)—17%
- Nonhydroelectric renewables—10%
- Petroleum and other—1%
Electricity generating capacity
Having sufficient capacity to generate electric power is important. The physical properties of electricity require that enough electricity be produced and placed on the electric power system, or grid, at every moment to instantaneously balance electricity demand.
Most of the time, many power plants are not generating electricity at their full capacities. Three major types of generating units vary by intended usage:
- A base load generating unit normally satisfies all or part of the minimum, or base, demand (load) on the system. A base load generating unit runs continuously, producing electricity at, essentially, a constant rate. Base load generating units generally have the largest capacity of the three types of units.
- A peak load generating unit helps to meet requirements during periods of greatest, or peak, load on the system such as when hot weather increases electricity demand for air conditioning.
- An intermediate load generating unit meets system requirements that are more than base load but less than peak load. Intermediate load generating units are used during the transition between base load and peak load requirements.
Generators powered by wind and solar energy supply electricity only when these resources are available (i.e., when it's windy or sunny). When these renewable generators are operating, they may reduce the amount of electricity required from other generators to supply the grid.
Distributed generators are connected to the electricity grid, but they are mainly used to supply some or all of the electricity demand of individual buildings or facilities. Sometimes, these systems may generate more electricity than the facility consumes, in which case the surplus electricity is sent to the grid. Most small-scale solar photovoltaic systems are distributed generators.
At the end of 2017, the United States had about 1,084,783 MW—or 1.1 billion kilowatts (kW)—of total utility-scale electricity generating capacity and about 16 million kW of small-scale distributed solar photovoltaic electricity generating capacity.
Generating units fueled primarily with natural gas account for the largest share of utility-scale electricity generating capacity in the United States. The shares of utility-scale electricity generating capacity by primary energy source were
- Natural gas—42%
- Nonhydroelectric renewables—12%
- Other sources—0.4%
Energy sources for U.S. electricity generation
The mix of energy sources for generating electricity in the United States has changed over time, especially in recent years. Natural gas and renewable energy sources account for an increasing share of U.S. electricity generation, while coal-fired electricity generation has declined. In 1990, coal-fired power plants accounted for about 42% of U.S. electricity generating capacity and about 52% of total electricity generation. By the end of 2017, coal's share of electricity generating capacity decreased to 24% and accounted for 30% of total electricity generation. Over the same period, the share of natural gas-fired electricity generating capacity more than doubled from 17% in 1990 to 42% in 2017, and its share of electricity generation nearly tripled from 12% in 1990 to 32% in 2017.
Most nuclear and hydropower plants were built before 1990. Nuclear energy's share of total U.S. electricity generation has held steady at about 20% since 1990. Electricity generation from hydropower, historically the largest source of renewable electricity generation, fluctuates from year to year because of precipitation patterns.
Total monthly U.S. electricity generation from nonhydro renewables now routinely exceeds hydroelectricity generation
Renewable electricity generation from sources other than hydropower has steadily increased in recent years, mainly because of additions to wind and solar generating capacity. In 2014, the total annual electricity generation from utility-scale nonhydro renewable sources surpassed hydropower generation for the first time.
Wind energy's share of total utility-scale electricity generating capacity in the United States grew from 0.2% in 1990 to about 8% in 2017, and its share of total annual utility-scale electricity generation grew to 6%.
While relatively small in terms of its share of total U.S. electricity capacity and generation, solar electricity generating capacity and generation has grown significantly in recent years. Utility-scale solar electricity generating capacity rose from about 314 MW—or 314,000 kW—in 1990 to about 26,665 megawatts (MW) at the end of 2017, of which about 93% was solar photovoltaic systems and 7% was solar thermal/electric systems. Solar energy's share of total U.S. utility-scale electricity generation in 2017 was about 1%, up from less than 0.1% in 1990. In addition, EIA estimates that by the end of 2017, the United States had 16,224 MW of small-scale solar photovoltaic generating capacity, and total electricity generation from small-scale photovoltaic systems was about 24 billion KWh during the year.
Various factors influence the mix of energy sources for electricity generation
The major factors that have contributed to changes in the U.S. electricity generation mix in recent years include
- A decline in natural gas prices
- State requirements to use more renewable energy sources
- Availability of government and other financial incentives for building new renewable capacity
- Federal air pollution emission regulations for power plants
- A reduction in the growth of electricity demand
The declining price of natural gas has been a major factor in the rise in natural gas-fired electricity generation and the decline in coal-fired electricity generation since 2008. When natural gas prices are relatively low, high-efficiency, natural gas-fired combined-cycle generators can supply electricity at a lower cost than coal-fired generators. Coal-fired power plants then operate less often and earn less revenue, which decreases their profitability and reduces the incentive to invest in new coal-fired generating capacity. Sustained low natural gas prices encourage development of new natural-gas fired capacity. In contrast to coal-fired generators, natural gas-fired generators
- Can be added in smaller increments to meet grid generating capacity requirements
- Can respond more quickly to changes in hourly electricity demand
- Generally have lower compliance costs with environmental regulations
Retail electricity sales
U.S. retail electricity sales to end-use customers totaled 3,682 billion kWh—or 3.7 trillion kWh—in 2017, about 80 billion kWh less than in 2016. Retail sales include net imports (imports minus exports) of electricity from Canada and Mexico. The sales of electricity to major types of U.S. retail customers and share of total sales in 2017 were
- Residential—1,379 billion kWh—37%
- Commercial—1,349 billion kWh—37%
- Industrial—946 billion kWh—26%
- Transportation—8 billion kWh—0.2%
Who sells electricity?
Six major types of electricity providers sell electricity to end-use consumers. The shares of electricity sales by type of provider in 2016 (the most recent annual data available at the time of this update) were
- Investor-owned utilities—51%
- Power marketers—22%
- Federal, state, and local utilities—14%
- Electric cooperatives—11%
In addition to sales to end-use customers, electricity is also often traded on wholesale markets or through bilateral contracts.