An energy storage system includes: a new renewable energy unit generating a new renewable energy power; a DC link connected to the new renewable energy unit and generating a DC link power; a power system connected to the DC link and generating a power system power; a battery connected to the DC link and generating a battery power; an auxiliary power generator connected to the power system, the new renewable energy unit, the DC link, and the battery, respectively, and generating an auxiliary power; and an integrated controller connected to the auxiliary power generator and receiving the auxiliary power. The auxiliary power is selected from one of the power system power, the new renewable energy power, the DC link power, and the battery power.