Publisher Summary Optimum sizing of a photovoltaic system, particularly in stand-alone applications is an important part of a system design. The optimum sizing of a photovoltaic system requires knowledge of the solar radiation data for the site, the load profile, and the importance of supply continuity. This chapter present the sizing procedure of photovoltaic battery energy systems mainly based on energy balance of a particular application. A system has been chosen to provide electricity for a small and remote located clinic. A methodology is developed for calculating the correct size of this system and for optimizing the management of this system. The power for the system comes from PV panels. A battery bank is used as a backup unit. The combined system is as a stand-alone system, in the sense that no national grid electricity is brought in to meet the load.