An energy storage system (ESS) is a promising tool for improving the power system's technical and economical functionalities. This device possesses diverse applications in terms of system operation and planning. One of the main applications of the ESS in the power system operation, specially a large scale unit, is defined as load leveling. To this end, in the literature, various ESS models are integrated into two main power system operation frameworks, i.e., optimal power flow (OPF) and unit commitment (UC). But the question is that in which framework the obtained benefit from the load leveling by ESS is maximum and why. Inother words, should ESS units schedule before generating unit commitment (UC) or after OPF and how. In this context, this paper performs three works. First, it proposes OPF and UC frameworks integrated with large scale ESS units. Second, the proposed models are implemented with identical parameters. Third. the obtained results are considered with respect to various outcomes, especially operation cost. This study is implemented on an IEEE 24 bus RTS system. The simulation results are analyzed and compared with respect to various parameters, and relevant conclusions are drawn based on the results.
