The goal of this study is to find the optimal sizes of renewable energy systems (RES) based on photovoltaic (PV) and/or wind systems for three energy storage system (ESS) scenarios in a micro-grid; (1) with pumped hydro storage (PHS) as a long-term ESS, (2) with batteries as a short-term ESS, and (3) without ESS. The PV and wind sizes are optimally determined to accomplish the maximum annual RES fraction (F-RES) with electricity cost lower than or equal to the utility tariff. Furthermore, the effect of the use of battery and PHS on the electricity cost and F-RES are studied. A university campus on a Mediterranean island is selected as a case study. The results show that PV-wind hybrid system of 8 MW wind and 4.2 MW PV with 89.5 MWh PHS has the highest F-RES of 88.0%, and the highest demand supply fraction as 42.6%. Moreover, the results indicate that the economic and technical parameters of RESs are affected significantly by the use of ESSs depending on the type and the capacity of both the RES and the ESS.