An energy storage system and a method of controlling the energy storage system, the energy storage system including a power converting unit for converting a voltage output from the power generating system into a direct current (DC) link voltage, a bidirectional converter enabled to perform mutual conversion between an output voltage of the battery and the DC link voltage, a DC link unit for constantly maintaining a level of the DC link voltage, a bidirectional inverter for converting the DC link voltage into an alternating current (AC) voltage appropriate for the grid, and for converting an AC voltage of the grid into the DC link voltage, and an integrated controller for controlling the power converting unit, the bidirectional converter, and the bidirectional inverter, and for controlling operation modes of the energy storage system. In particular, the integrated controller may control operation modes in accordance with the DC link voltage.