A power system 10 is provided with a power conversion device 110 connected to an input power supply 200, a battery 120 and a load 130 connected in parallel with each other to the output side of the power conversion device 110, and a control device 140 controlling charging/discharging of the battery 120, wherein the control device 140 receives output power of the power conversion device 110, determines, based on the received output power, charge or discharge power of the battery 120 such that the output power becomes close to a first value, and charges or discharges the battery 120 based on the determined charge or discharge power.