This article presents research on modelling the operation of an independent electricity generation system consisting of a photovoltaic installation and energy storage in the form of electrochemical batteries (PV/BAT). The generation system was considered primarily in the context of its sizing process, i.e., the selection of the installed power of the photovoltaic installation (PV) and the rated capacity of the battery (BAT). Traditionally, the model includes a one-year analysis of the generation system based on initial (nominal) parameters without considering component performance degradation. The novelty of this research lies in the long-term simulation of the system operation, considering the degradation of its components. The sizing process was based on the numerical method. The best solution is selected on the basis of the economic criterion, while satisfying the reliability condition. The simulations were conducted using Matlab software. Using a comparative analysis, the scale of technical and economic oversizing of the system was determined by considering long-term reliability. For the assumed customer load profile, insolation profile, and battery operation in the range of 25–100% of the available capacity, providing the assumed level of reliability after accounting for degradation in the sizing process resulted in a 33.33% increase in the rated battery capacity, an 18.75% increase in the installed photovoltaic plant capacity, and a 19.5% increase in the system cost of electricity (LCOE) relative to the results of the sizing process without accounting for component performance degradation over the years of operation.