An energy storage system includes a power conversion system configured to produce a control signal for regulating a frequency of power flowing from a power generation system to an electric-power system, and a battery system including a first battery rack, a second battery rack, a charger/discharger configured to perform a charging/discharging operation of the second battery rack, and a rack BMS configured to control the charging/discharging operation of the first and second battery racks using the control signal, and to control the charger/discharger, thus controlling a state of charge (SOC) of the second battery rack.