An energy storage system for charging/discharging power of a plurality of battery packs connected between a grid and a variable power source, the energy storage system including an inverter connected between the grid, the plurality of battery packs, and the variable power source, the inverter inverting an AC power into a DC power and inverting a DC power into an AC power according to charge/discharge, a plurality of converters connected between the battery pack, the variable power source, and the grid, the converters being connected to the battery packs in a one-to-one relationship to convert a charge/discharge power of the battery packs, respectively, a plurality of battery switching devices, the battery switching devices being respectively connected between the converters and the battery packs, and a controller connected to the inverter and the converter to control driving, the controller controlling a turn-on/off operation of the battery switching devices.