An energy storage system including a plurality of loads each converting direct current (DC) power stored in a battery thereof into alternating current (AC) power and outputting the AC power, a plurality of slave power controllers detecting zero crossing points of AC voltage signals output from one of the plurality of loads and controlling the plurality of loads in accordance with a control signal received from a master power controller, and the master power controller controlling the plurality of slave power controllers so as to control the plurality of loads in accordance with the control signal received from the master power controller after a preset time has lapsed since the detected zero crossing point.