The fraction of a power plant's maximum possible energy output actually delivered over a given period — a key measure of reliability and a major driver of electricity cost.
Capacity factor (CF) is the ratio of actual energy produced over a time period to the energy the plant would have produced if it operated continuously at its full nameplate rating:
CF = Eactual / (Prated × T)
A 1,000 MW plant running at full power for 7,008 of the 8,760 hours in a year achieves a capacity factor of 80 %. Downtime for scheduled maintenance, unplanned outages, plasma disruptions, component replacement, and fuel processing all reduce CF.
Modern nuclear fission plants routinely achieve capacity factors of 90–93 %, reflecting decades of operational learning.1 Combined-cycle natural gas plants typically reach 55–65 %, partly because they cycle to follow load. Wind and solar capacity factors range from 25–45 % and 15–30 % respectively, limited by resource intermittency rather than equipment reliability.2
Capacity factor enters the denominator of the levelized cost of energy. For a capital-intensive technology like fusion — where construction costs may dominate the lifetime cost — every percentage point of CF lost raises LCOE significantly.3 A plant designed for 1,000 MW net at 85 % CF produces 7,446 GWh per year; dropping to 70 % CF reduces annual output by 1,314 GWh and raises the per-megawatt-hour cost proportionally.
Achieving high CF in fusion requires solving several engineering challenges simultaneously: neutron-resistant first-wall and blanket materials, reliable tritium fuel cycle operation, divertor longevity, and — for tokamaks — steady-state plasma scenarios that avoid disruptions.4 Stellarators and certain mirror concepts offer inherently steady-state operation, potentially easing the path to high capacity factor, though they introduce different engineering complexities.