A method for controlling an energy storage system (ESS) and devices performing the method are disclosed. A method for controlling an ESS according to example embodiments, the method includes an operation of analyzing energy information of an area for which a peak power load is to be calculated received from a plurality of clients included in the area, an operation of generating, based on an analysis result, scheduling control information for controlling the ESS connected to the plurality of clients on a daily basis, an operation of determining, based on the scheduling control information, whether to generate real-time control information for controlling the ESS on a basis of a predetermined control time period, and an operation of correcting, based on the real-time control information, the scheduling control information.