A monitoring system for an energy storage system is provided. The monitoring system for an energy storage system includes a power conversion system controlling charging and discharging of a battery; a charging control unit receiving data from the power conversion system and controlling the power conversion system; and a system control unit receiving data from the charging control unit and transmitting control data to the charging control unit, wherein the system control unit and the charging control unit transmit and receive data according to a priority if data traffic is equal to or higher than a certain level, and when the data traffic is the equal to or higher than the certain level, the charging control unit transmits data to the system control unit only if control data is not transmitted from the system control unit.