Provided are a system and a method which allow an improved operating time of an energy storage system in a power system by reflecting a state of charge of a battery and a state of the power system on a control of an amount of charging power or discharging power of the energy storage system. The system includes: a battery configured to charge or discharge power; a battery monitor configured to monitor a state of the battery; a power system monitor configured to monitor a state of a power system; and a charge/discharge controller configured to control an amount of charging power or an amount of discharging power of the battery using a state of charge of the battery detected by the battery monitor and the state of the power system detected by the power system monitor.