capacity factor is the ratio of the electricity a power plant actually produces over a period to what it could have produced running continuously at full rated power — the standard measure of how hard a plant works.
Capacity factor is distinct from availability factor, its most commonly confused neighbor: availability counts the fraction of time a plant is capable of running, while capacity factor counts what it actually delivered, so a plant can be fully available and still post a low capacity factor if it is dispatched rarely or runs below rated power. The spread across sources is wide — the United States nuclear fission fleet runs above 90 percent, combined-cycle gas near 60, wind near a third, and utility-scale solar near a quarter — differences driven less by machinery than by fuel, weather, and dispatch.
For 📝fusion, capacity factor is where physics achievement meets the balance sheet. A fusion plant competes as firm, always-on clean power, which demands the high capacity factors of the fission fleet; reaching them requires long component lifetimes under 📝neutron damage, fast planned maintenance, and — in a pulsed design — a duty cycle high enough that the dwell between pulses barely dents output, the operational case for 📝steady-state operation. Because a plant's fixed costs are spread over every megawatt-hour it sells, capacity factor sits in the denominator of levelized cost of energy, and small changes in it move the economics disproportionately.