An energy storage device including at least one battery pack; a communication module configured to transmit power-on information or energy storage amount information to a power management device and to receive a charge command or discharge command from the power management device; a connector configured to receive alternating current (AC) power, supplied to an internal power network through a photovoltaic module, from the internal power network based on the charge command or to output AC power to the internal power network based on the discharge command; and a power converter configured to, when the charge command is received from the power management device, convert the AC power from the internal power network into direct current (DC) power based on the charge command, or, when the discharge command is received from the power management device, convert DC power stored in the at least one battery pack into AC power based on the discharge command.