Power supply/demand device, power generation system, and control method

According to one embodiment, a power supply/demand device includes a virtual synchronous inverter and a storage battery. The virtual synchronous inverter is connected to a power system. Charging and discharging of the storage battery are controlled by the virtual synchronous inverter. The virtual synchronous inverter is connected to the power system in parallel with other inverters. The virtual synchronous inverter is capable of calculating a first standby time having any length and performing control of the storage battery corresponding to an operation instruction after a lapse of the first standby time, when receiving the operation instruction accompanied by output of power to the power system or input of power from the power system.