The majority of Photovoltaic (PV) plants do not include components with moving parts and often they are wrongly considered maintenance-free. The present work consists of a Reliability Analysis (RA) of five PV systems with different size from 50 kW to 1 MW. The exponential distribution is used to calculate the reliability of electrical components, and failure rates are derived from literature. The results are compared to experimental data, obtained by the analysis of maintenance activity in two years of life of the PV plants. In addition, the calculation of the related energy losses of the PV plants is presented. From these results, it can be argued that, in some cases, energy losses are not negligible.