Capacity Factor Definition

A 1 GW nuclear plant and a 1 GW solar farm have identical nameplate capacity. But the nuclear plant produces about 8,000 GWh per year while the solar farm produces about 2,200 GWh. Same rated power, vastly different energy. Capacity factor is the number that explains the gap.

Capacity factor is the ratio of a plant's actual energy output to its maximum possible output over a given period: CF = Actual Output / (Nameplate Capacity x Time). A nuclear plant running 92% of the time has a 92% capacity factor. A solar farm averaging 25% of its peak output (due to night, clouds, and seasons) has a 25% capacity factor. The number captures everything that prevents a plant from running at full power all the time: fuel availability, maintenance, weather, and dispatch decisions.

Capacity factor converts capacity (power) into expected energy (power x time). Without it, comparing generation technologies by nameplate capacity alone is meaningless.

Worked Example

A 500 MW wind farm produced 1,314,000 MWh in a year. There are 8,760 hours in a year.

• Calculate maximum possible output. 500 MW x 8,760 h = 4,380,000 MWh.
• Calculate capacity factor. 1,314,000 / 4,380,000 = 0.30, or 30%.

A politician says "we added 10 GW of wind, equivalent to 10 nuclear plants." Is this accurate?

No. At 30% CF, 10 GW of wind produces the energy equivalent of about 3.3 GW of nuclear (at 92% CF). The politician compared capacity; the correct comparison uses energy: 10 x 0.30 / 0.92 = 3.3.

Nameplate capacity is what a press release announces. Capacity factor is what the meter actually records, and the gap between the two is where most misleading energy claims live.

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