Nowadays, the decarbonization of the global and national economies by shifting from using fossil energy sources to using renewable energy sources represents an upward trend. The greatest potential has wind and photovoltaic sources, which are characterized by intermittency and unpredictability due to the intermittent nature of wind speed and solar irradiance. Thus, the main challenge for the integration of variable renewable energy sources (VRESs) into existing power systems is the gap between the electricity load of final users and the electricity generation of VRES. The main solutions for this challenge include using the reserve generation capacity and/or the energy storage systems (ESSs). Energy storage can be realized at different levels of the power systems: the end-users, the power plants, or the electricity grid. In this paper, we present the feasibility evaluation of the different types of ESS (battery and fuel cells) for the smoothing of the peak generation curve of the power plants using VRESs and the economically optimal capacity of ESSs.