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Frequently Asked Questions (FAQs)

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What is the difference between electricity generation capacity and electricity generation?

Electricity generation capacity is the maximum electric output an electricity generator can produce under specific conditions. Nameplate generator capacity is determined by the generator's manufacturer and indicates the maximum output of electricity a generator can produce without exceeding design thermal limits.

Net summer electricity generation capacity and net winter electricity generation capacity are typically determined by a performance test and indicate the maximum electricity load a generator can support at the point of interconnection with the electricity transmission and distribution system during the respective season. Two primary factors affect or determine the difference in capacity between summer and winter months:

  • The temperature of cooling water for thermal power plants or the temperature of the ambient air for combustion turbines
  • The water flow and reservoir storage characteristics for hydropower plants

Electricity generation is the amount of electricity a generator produces during a specific period of time. For example, a generator with 1 megawatt (MW) capacity that operates at that capacity consistently for one hour will produce 1 megawatthour (MWh) of electricity. If the generator operates at only half that capacity for one hour, it will produce 0.5 MWh of electricity. Many generators do not operate at their full capacity all the time. A generator's output may vary according to conditions at the power plant, fuel costs, and/or as instructed by the electric power grid operator.

Electricity net generation is the amount of gross electricity generation a generator produces minus the electricity used to operate the power plant. These electricity uses include fuel handling equipment, water pumps, combustion and cooling air fans, pollution control equipment, and other electricity needs. Some electric generators that are used to supply power for only short periods of time, either by design or economics, such as peaking power plants and energy storage facilities for electricity generation, may be idle for relatively long periods of time. However, they require power from the power plant (in which they are part of) and/or from the electric power grid to be in operable condition when called upon by grid operators to supply power for a short period of time. Over an entire month, their electric generation may be less than the power they used while they were waiting to be dispatched.

Capacity factor of electricity generation is a measure (expressed as a percentage) of how often an electricity generator operates during a specific period of time using a ratio of the actual output to the maximum possible output during that time period.

The U.S. Energy Information Administration (EIA) publishes average monthly and annual capacity factors for different types of electric generators in Table 6.07.A and Table 6.07.B of the Electric Power Monthly. The capacity factors are based on a time-adjusted capacity. Monthly time adjusted capacity is the summer capacity of generators in operation for an entire month. Electric generators that began operation during a month or that retired during a month are excluded. The annual time adjusted capacity is a time-weighted average of the monthly capacity factors. Details on how EIA calculates capacity factor are included on page 21 of the Electric Power Monthly Technical notes.

Learn more:
Energy Explained: Electricity generation, capacity, and sales in the United States
Data on electric power plants generating capacity
Data on electricity generation and thermal output
Existing nameplate and net summer capacity by state, type of producer, and energy source (historical data file from 1990 to most recent year available)
Net generation by state, type of producer, and energy source (historical data file from 1990 to most recent year available)
Electric Power Monthly: includes monthly and annual data on electricity generation, generation capacity, and capacity factors.
Electricity Monthly Update
Today in Energy articles on electricity

Last updated: April 12, 2022

Other FAQs about Electricity