An energy storage system capable of improving the lifespan of a battery pack while supplying power in a secured manner is provided. In one embodiment, the energy storage system includes a plurality of battery packs connected between a grid and a solar cell to charge/discharge power. The energy storage system includes an inverter connected among the grid, the solar cell, and the battery pack and inverting an alternating current (AC) power to a direct current (DC) power and vice versa according to charging/discharging operations. The energy storage system includes a converter connected to the solar cell, the battery pack, and the grid and converting power supplied from the solar cell, and a plurality of bidirectional converters connected to the plurality of battery packs, the solar cell, and the grid and connected in one-to-one correspondence to the plurality of battery packs and converting the power for charging/discharging the battery pack. The energy storage system further includes a controller connected to the inverter, the converter, and the plurality of bidirectional converters and applying drive signals to sequentially drive the plurality of bidirectional converters.