capacity factor

Capacity factor is the bridge between what a power plant could produce and what it does produce. The United States Energy Information Administration defines it as the ratio of the electrical energy a generating unit produced over a given period to the energy it could have produced running at continuous full power for that same period. A plant that ran flat out every hour of the year would score 100 percent, which nothing does, because every machine needs maintenance and most depend on a fuel or a resource that is not always there.

The number varies far more by technology than by effort. Solar generates only in daylight and less under cloud, and wind only when the wind blows, so both sit well below a coal or nuclear plant that can run whenever it is fuelled and maintained. That gap is why a country can build a fleet that is more than half non-fossil and still take most of its electricity from fossil fuels. Raising it is a job for storage, transmission and flexible demand rather than for more generators.