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The net capacity factor is the unitless ratio of actual electrical energy output over a given period of time to the theoretical maximum electrical energy output over that period. The theoretical maximum energy output of a given installation is defined as that due to its continuous operation at full nameplate capacity over the relevant period. The capacity factor can be calculated for any electricity producing installation, such as a fuel-consuming power plant or one using renewable energy, such as wind, the sun or hydro-electric installations. The average capacity factor can also be defined for any class of such installations and can be used to compare different types of electricity production.
The actual energy output during that period and the capacity factor vary greatly depending on a range of factors. The capacity factor can never exceed the availability factor, or uptime during the period. Uptime can be reduced due to, for example, reliability issues and maintenance, scheduled or unscheduled. Other factors include the design of the installation, its location, the type of electricity production and with it either the fuel being used or, for renewable energy, the local weather conditions. Additionally, the capacity factor can be subject to regulatory constraints and market forces, potentially affecting both its fuel purchase and its electricity sale.
The capacity factor is often computed over a timescale of a year, averaging out most temporal fluctuations. However, it can also be computed for a month to gain insight into seasonal fluctuations. Alternatively, it can be computed over the lifetime of the power source, both while operational and after decommissioning. A capacity factor can also be expressed and converted to full load hours.
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If consistent units are used for energy and power—such as megawatt-hours (MWh) for energy and megawatts (MW) for power—and time is expressed in hours, the units "cancel out". For example, if a 1 MW plant produces 0.5 MWh in one hour, its capacity factor is:
Each of Palo Verde’s three reactors is refueled every 18 months, with one refueling every spring and fall. In 2014, a refueling was completed in a record 28 days, compared to the 35 days of downtime that the 2010 capacity factor corresponds to.
In 2019, Prairie Island 1 was the US unit with the highest factor and actually reached 104.4%.
it has produced 6416 GWh since its commissioning 7 years ago, i.e. an average annual production of 875 GWh/year and a capacity factor of
Sites with lower capacity factors may be deemed feasible for wind farms, for example, the onshore 1 GW Fosen Vind, which is under construction in Norway, has a projected capacity factor of 39%. Feasibility calculations may be affected by seasonality. For example, in Finland, capacity factor during the cold winter months is more than double compared to July. While the annual average in Finland is 29.5%, the high demand for heating energy correlates with the higher capacity factor during the winter.
Certain onshore wind farms can reach capacity factors of over 60%, for example, the 44 MW Eolo plant in Nicaragua had a net generation of 232.132 GWh in 2015, equivalent to a capacity factor of 60.2%, while United States annual capacity factors from 2013 through 2016 range from 32.2% to 34.7%.
Since the capacity factor of a wind turbine measures actual production relative to possible production, it is unrelated to Betz's coefficient of , which limits production vs. energy available in the wind.
Three Gorges Dam in China is, with its nameplate capacity of 22,500 MW, the largest power generating station in the world by installed capacity.
In 2015 it generated 87 TWh, for a capacity factor of
Hoover Dam has a nameplate capacity of 2080 MW and an annual generation averaging 4.2 TW·h. (The annual generation has varied between a high of 10.348 TW·h in 1984, and a low of 2.648 TW·h in 1956.) Taking the average figure for annual generation gives a capacity factor of
For example, Agua Caliente Solar Project, located in Arizona near the 33rd parallel and awarded for its excellence in renewable energy has a nameplate capacity of 290 MW and an actual average annual production of 740 GWh/year. Its capacity factor is thus
A significantly lower capacity factor is achieved by Lauingen Energy Park located in Bavaria, near the 49th parallel. With a nameplate capacity of 25.7 MW and an actual average annual production of 26.98 GWh/year it has a capacity factor of 12.0%.
A plant can be out of service or operating at reduced output for part of the time due to equipment failures or routine maintenance. This accounts for most of the unused capacity of base load power plants. Base load plants usually have low costs per unit of electricity because they are designed for maximum efficiency and are operated continuously at high output. Geothermal power plants, nuclear power plants, and bioenergy that burn solid material are almost always operated as base load plants, as they can be difficult to adjust to suit demand.
A plant can also have its output curtailed or intentionally left idle because the electricity is not needed or because the price of electricity is too low to make production economical. This accounts for most of the unused capacity of peaking power plants and load following power plants. Peaking plants may operate for only a few hours per year or up to several hours per day. Many other power plants operate only at certain times of the day or year because of variation in loads and electricity prices. If a plant is only needed during the day, for example, even if it operates at full power output from 8 am to 8 pm every day (12 hours) all year long, it would only have a 50% capacity factor. Due to low capacity factors, electricity from peaking power plants is relatively expensive because the limited generation has to cover the plant fixed costs.
A third reason is that a plant may not have the fuel available to operate all of the time. This can apply to fossil generating stations with restricted fuels supplies, but most notably applies to intermittent renewable resources. Solar PV and wind turbines have a capacity factor limited by the availability of their "fuel", sunshine and wind respectively. A hydroelectricity plant may have a capacity factor lower than 100% due to restriction or scarcity of water, or its output may be regulated to match the current power need, conserving its stored water for later usage.
Governments differ in their wiliness to accept risks of power outages and lack of resilience against natural disasters and military attack on electricity grids. Examples of historical events impacting grid resilience are the 1991 Gulf War air campaign against civilian infrastructure, 2015 Ukraine power grid hack, 2021 Texas power crisis and Russian strikes against Ukrainian infrastructure (2022–present). Low risk tolerance may require electricity grids to be more significantly overbuilt to mitigate the potential costs of electricity grid interruptions and outages, impacting on a technology-by-technology basis the amount of generation curtailment necessary under normal grid conditions.
Other reasons that a power plant may not have a capacity factor of 100% include restrictions or limitations on air permits and limitations on transmission that force the plant to curtail output.
When hydroelectric plants have water available, they are also useful for load following, because of their high dispatchability. A typical hydroelectric plant's operators can bring it from a stopped condition to full power in just a few minutes.
are variable, due to the natural variability of the wind. For a wind farm, the capacity factor is determined by the availability of wind, the swept area of the turbine and the size of the generator. Transmission line capacity and electricity demand also affect the capacity factor. Typical capacity factors of current wind farms are between 25 and 45%. In the United Kingdom during the five year period from 2011 to 2019 the annual capacity factor for wind was over 30%.
Solar energy is variable because of the daily rotation of the earth, seasonal changes, and because of cloud cover.
For example, the Sacramento Municipal Utility District observed a 15% capacity factor in 2005.
However, according to the SolarPACES programme of the International Energy Agency (IEA), solar power plants designed for solar-only generation are well matched to summer noon peak loads in areas with significant cooling demands, such as Spain or the south-western United States, although in some locations solar PV does not reduce the need for generation of network upgrades given that air conditioner peak demand often occurs in the late afternoon or early evening when solar output is reduced.Muriel Watt Value of PV in summer peaks Government of South Australia (2007), p.13,14 South Australia’s Feed-In Mechanism for Residential Small-Scale Solar Photovoltaic Installations
SolarPACES states that by using thermal energy storage systems the operating periods of solar thermal power (CSP) stations can be extended to become dispatchable (load following).Thomas R. Mancini and Michael Geyer (2006). Spain Pioneers Grid-Connected Solar-Tower Thermal Power SolarPACES, OECD/ IEA, p. 3.
Geothermal power has a higher capacity factor than many other power sources, and geothermal resources are generally available all the time.
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!colspan="8"| Non-fossil fuels !! Coal !!colspan="4"| Natural gas !!colspan="3"| Petroleum liquids
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!Nuclear !! Hydro !! Wind !! Solar PV !! Solar CSP !! Landfill gas
However, these values often vary significantly by month.
Capacity factors by energy source
Worldwide
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United States
According to the US Energy Information Administration (EIA), from 2013 to 2017 the capacity factors of utility-scale generators were as follows:
and !! Other biomass,
including wood !! Geothermal !! !! !! !! !! !! !! !!
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|89.9% ||38.9% ||32.4% ||NA ||NA ||68.9% ||56.7% ||73.6% ||59.8% ||48.2% ||4.9% ||10.6% ||6.1% ||12.1% ||0.8% ||2.2%
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|91.7% ||37.3% ||34.0% ||25.9% ||19.8% ||68.9% ||58.9% ||74.0% ||61.1% ||48.3% ||5.2% ||10.4% ||8.5% ||12.5% ||1.1% ||1.4%
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|92.3% ||35.8% ||32.2% ||25.8% ||22.1% ||68.7% ||55.3% ||74.3% ||54.7% ||55.9% ||6.9% ||11.5% ||8.9% ||13.3% ||1.1% ||2.2%
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|92.3% ||38.2% ||34.5% ||25.1% ||22.2% ||69.7% ||55.6% ||73.9% ||53.3% ||55.5% ||8.3% ||12.4% ||9.6% ||11.5% ||1.1% ||2.6%
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|92.2% ||43.1% ||34.6% ||25.7% ||21.8% ||68.0% ||57.8% ||74.0% ||53.7% ||51.3% ||6.7% ||10.5% || 9.9% ||13.5% ||0.9% || 2.3%
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|92.6% ||42.8% ||37.4% ||26.1% ||23.6% ||73.3% ||49.3% ||77.3% ||54.0% ||57.6% ||11.8% ||13.7% ||NA ||13.9% ||2.5% ||NA
United Kingdom
The following figures were collected by the Department of Energy and Climate Change on the capacity factors for various types of plants in UK grid:
Nuclear power stations 58.5% Combined cycle gas turbine stations 38.9% Coal-fired power stations 12.7% Hydroelectricity 33.1% Wind power 29.3% Offshore wind power 37.4% Photovoltaic power stations 10.0% Marine (Wave power and tidal power stations) 2.8% Bioenergy 56.6%
See also
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