The availability factor of a power plant is the percentage of the time that it is available to provide energy to the grid.
The availability of a plant is mostly a factor of its reliability and of the periodic maintenance it requires.
Because a plant is available, it does not mean that it can generate all of its nameplate capacity. For instance, a hydroelectric plant may be available, but it may not be possible to release all the water needed for it to reach its rated output. In the same way, in the middle of the night, an available solar photovoltaic plant will not generate any output. The same solar PV plant will not generate as much power in December as it generates in July, even if it is available.
Availability factors are not widely collected for given technologies or plants. Instead, the commonly tracked metric is the Capacity Factor, which can be measured externally.
Gas, coal, and nuclear plants carry availability factors over 80%, often around 90% or higher. Most modern wind farm availability factors top 95%, while solar PV plants reach over 98%.