The availability factor of a power plant is the duration it achieves production of electricity divided by the duration that it was planned to produce electricity. In the field of reliability engineering, availability factor is known as operational availability, . The capacity factor of a plant includes numerous other factors which determine the durations the plant is planned to produce electricity. 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.