Today, energy crises attracted many researchers’ attention to renewable energy technologies especially photovoltaic (PV) systems. The main challenge of PV systems is unpredictable nature of solar power generation. To overcome this challenge, a storage system is integrated which reduces demand reliance on electricity grid and uses excess energy that solar panels produce. As investment cost of the storage system is considerable, finding an optimal technology, size, and configuration are crucial. In this paper, the optimal battery system is excluded from existing PV plant installing in a commercial building located in Mashhad/Iran. Here, the sizing procedure is based on a financial evaluation which considers the damage costs due to outages that the commercial building is experiencing during the PV system life spam. To compare results, the battery size is also determined by stochastic methods, e.g. Monte-Carlo simulation method. The simulation results confirm the advantages of the proposed approach compare with classic ones.