Hybrid energy storage systems have gained interest in recent years due to their potential to outperform single energy storage systems in certain situations. However, optimally designing and sizing HESS to effectively replace a single energy storage technology remains a complex challenge. In this study, a HESS combin-ing gravity energy storage (GES) with high-power electrochemical energy storage is integrated into a hybrid energy system (PV/WIND) to balance supply and demand in a renewable energy power system. This chapter investigates the design and optimization of this HESS. To achieve the best possible integration between the hybrid Gravity/Battery storage system and the WT/PV system, an energy management system linked to an optimization process has been developed. Prediction models have been used to forecast solar and wind generation. The obtained results have shown that the energy cost is significantly affected by the reliability percentage of the plant. When the reliability level is reduced to 20%, the optimal energy cost is 0.09 €/kWh, compared to 0.22 €/kWh at 100%, reliability. Reliability and efficiency in the utilization of renewable energy are enhanced by the integration of forecast models into the study and the hybridization of energy storage, which enhance the renewable power system’s optimal operation and design accuracy.