A solar photovoltaic plant is not planned to operate in the dark of a night, hence unplanned maintenance occurring whilst the sun is set does not impact the availability factor.
Newer plants tend to have significantly higher availability factors, but preventive maintenance is as important as improvements in design and technology.
Originally the term availability factor was used only for power plants that depended on an active, controlled supply of fuel, typically fossil or later also nuclear.
The emergence of renewable energy such as hydro, wind and solar power, which operate without an active, controlled supply of fuel and which come to a standstill when their natural supply of energy ceases, requires a more careful distinction between the availability factor and the capacity factor.
Photovoltaic power stations which have few or no moving parts and which can undergo planned inspections and maintenance during night have an availability factor approaching or equal to 100% when the sun is shining.