Abstract The incorporation of energy storing units into hybrid systems reallocates the excess electricity to meet demand requirements in the deficiency periods. This study seeks to determine the optimal size of a Photovoltaic (PV)/wind/biomass hybrid system with and without energy storage built on the base of boosting the demand–supply fraction ( DSF ) and the renewable energy fraction (FR) with a net present value larger than or equals to zero. The Generalized Reduced Gradient algorithm has been utilized in this study to evaluate the optimal components’ capacities of the proposed system. Middle East Technical University Northern Cyprus Campus was used as a case study. The proposed system consists of 1.79 MW PV, 2 MW wind and 0.92 MW biomass systems with 24.39 MWh pumped hydro storage system and 148.64 kWh batteries achieving FR of 99.59%, DSF of 98.86%, and a cost of electricity equals to 0.1626 $/kWh. The simulations results proved that the integration of a hybrid energy storage system with the PV/wind/biomass system ensures very high autonomy approaching almost 99%. Finally, considering the significant excess energy produced by the tri-hybrid system, this excess could also be allocated towards meeting the campus’s thermal and domestic hot water demands creating a polygeneration power system.