Renewable energy resources (RERs), mainly wind and photovoltaic (PV), can be integrated into the power system to reap the gains of clean energy. However, the stochastic output of large-scale RERs introduces many integration challenges on the power system, especially when replacing the conventional generation by RERs. To avoid the power-gap between the generation and demand because of variable sources existence, the energy storage devices (ESDs) are introduced. Batteries that have high energy density and reasonable time response are the appropriate technology for steady state operation. Their sizing is essential for assuring the intact operation of the power system. In this work, the load flow algorithm that emulates a proper battery power profile is used. Then, a developed algorithm to adjust the battery size and its initial value is proposed. Different case studies are presented to comprehensively analyze the impact of RERs integration with the ESDs on the power system operation. The proposed algorithm with the aid of power system analysis toolbox (PSAT) is applied to the medium voltage (MV), IEEE-14 bus test system to validate and emphasize its applicability.