In this study, we calculated regional optimal energy storage system (ESS) capacity to maximize customer profits for the grid-connected PV-ESS system. We used the results of calculating the photovoltaic (PV) power generation in 24 hours, representing a year in each region, with the data from the Renewable Energy Cloud Platform operated by the Korea Energy Agency. In addition, to increase the utilization of the Renewable Energy Cloud Platform, we proposed to provide information to users on ESS installation capacity by applying the method of calculating the regional optimal ESS capacity developed in this study to the platform. As a result, when PV power generation companies install the grid-connected PV-ESS systems in each region, they select and install the most economical ESS capacity. The results of this study is expected to expand the supply of renewable energy and ESS.