In this study, a distribution system with very high penetration of Photo Voltaic (PV) generation installed is considered. A method is proposed for battery and PV capacity selection in two different scenarios. The first scenario considers an isolated distribution system. The second scenario assumes a 30% injection from the power system. The optimization problem of battery sizing is solved considering the power flow of the system (including the system losses) and the battery constraints. The effect of battery capacity and its initial State of Charge (SoC) on Loss of Load (LoL) is studied. We also consider the change in LOL as result of different contribution levels from the power system and find the minimum power injection needed from the system to avoid LOL.