This research article is concerned with reliability based long-term efficiency and production of photo-voltaic solar plant. In particular, an analytical and theoretically expression is evaluated by taking into account the reliability values of devices as panels, cables LV, MV, HV, transformers, DC/AC converters, power transformers and overhead lines. The results is the energy not supplied (ENS) equations and energy index. It involves reliability values with a real case study and a numerical case study.