Utilization of energy storage system (ESS) in microgrids has turned to be necessary in recent years and now with the improvement of storage technologies, system operators are looking for an exact modeling and calculation for optimal sizing of ESS. In the proposed paper, optimal size of ESS is determined in a microgrid considering demand response program (DRP) and reliability criterion. Both larger and small-scale ESSs have their own problems. A largescale ESS reduces microgrid operating cost but it includes higher investment costs while a small-scale ESS has less investment cost. The main goal of the proposed paper is find optimal size of ESS in which microgrid investment cost as well as operating cost are minimized. Since the renewable units may not have stable production and also because of the outages that conventional units may have, ESS is utilized and then a reliability index called reliability criterion is obtained. Furthermore, effects of reliability criterion and DRP on optimal sizing of ESS are evaluated. A mixedinteger programing (MIP) is used to model the proposed stochastic ESS optimal sizing problem in a microgrid and GAMS optimization software is used to solve it. Five study cases are studied and the results are presented for comparison.
