Abstract: In this paper, the structure of off grid photovoltaic (PV)/battery micro grid system and the power model of photovoltaic array are introduced. The reliability index of this system is proposed. The effect of peak load shifting can be measured by loss of load probability (LOLP) index. To describe the peak load shifting duty of BESS, an operation model based on storage battery is established, and an optimal BESS configuration problem is discussed. In the end, a real time and dynamic control strategy of energy storage system for peak load shifting is put forward in the paper. The analysis on actual example testifies the reasonability of proposed strategy.