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Capacity is the maximum power that a plant is designed to output. A 1000 MW plant can output 1000 megawatts of power (or 1000 megajoules per second), but sometimes it outputs less than that. For instance, it might operate at a lower power or not at all while undergoing maintenance. A capacity factor is defined as the actual output of a electricity generating device over it’s maximum possible generation during the same time period.

For example, a power plant that is rated at 1000 MW capacity and generates 7500 GWh over a full year:

  • actual generation = 7500 GWh/year
  • possible generation = 1000 MW * 8760 hrs/yr * (1 GW / 1000 MW) = 8760 GWh/yr
  • capacity factor = 7500 GWh/yr / 8760 GWh/yr = 0.856

The plant operated 85.6% of time at full power. This could mean that all year it operated at 85.6% or that for 312 days it was at full power and the other 53 days it was shut off producing no power. More likely it was somewhere in between.

The EIA has plotted monthly capacity factor data for the US over several years:

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The monthly breakdown gives a better idea of the mix of sources as the year progresses. Nuclear power plants are refueled only once every 18 or 24 months. They try to time these outages for fall and spring because demand is lower in those months (less air conditioning and heating is used). The capacity factor of wind on the other hand reliably tends to be lowest in the late summer. Natural gas combustion turbines are usually used as peak power which means they only turn on when no other source on the grid can increase output (the hottest summer days).

Since the capacity factor changes throughout the year, the way to compare between sources is to use the annual average.

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A coal plant operates on average 65% of the time. A geothermal plant 70% of the time. The capacity factor shows how well the plant is used, but it doesn’t tell us which fuels Americans use more:

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Or combined into one plot with

cf_genshare

We need to rapidly transition our system away from using so much fossil fuels for our electricity. If wind has only a 30% capacity factor, it will need to radically expand its capacity in order to replace significant portions of other fuels. Transitioning from fossil fuels in other sectors like heating and transportation will be even more difficult.

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