Energy Storage System and Control Method Thereof

An energy storage system and a control method thereof are provided. The control method includes steps of: (a) providing an energy storage system including N energy storage modules; (b) obtaining a first sequence and a second sequence by sorting the N energy storage units based on the quantity of electricity in descending order and ascending order respectively; (c) determining a required power of the power grid according to a grid frequency; (d) when the required power is positive, controlling first X energy storage units in the first sequence to discharge for collectively providing an electrical energy, having same magnitude with the required power, to the power grid; and (e) when the required power is negative, controlling first Y energy storage units in the second sequence to collectively receive an electrical energy, having same magnitude with the required power, from the power grid for charging.