This paper presents electrical model of a photovoltaic-battery system. This model helps to understand the behaviour of a solar-battery system under various load and irradiance conditions. And also it assists with investigation of the performance of the system, Hybrid energy systems use different energy sources such as solar and wind with a backup unit such as a battery or a diesel generator. They are an economical option in areas remote from national grid. In this context the performance of the system to supply electric power in an efficient way of operation is important. The problem comes from uncertain renewable energy supply and load and also non linear characteristics of components in the system. The purpose of study we are involved in this relation is to see the behaviour of a solar-battery system under various load and irradiance conditions and to investigate the performance of the system. As a result an optimum system configuration and a correct and cost effective size of Balance of System (BOS) can be achieved. In this paper the complete electrical circuit of the entire hybrid system, the mathematical model and computational technique are presented.