The bus line of an electric power system is connected to an inverter which can be placed in a forward operation mode or in a reverse operation mode, and batteries are connected to the inverter to store the dc energy thereof. When a commercial power source is normal, an ac switch is closed, so that the inverter is placed in the forward operation mode to perform an active filter operation for eliminating harmonic current and reactive current. When necessary, the batteries are charged, and the inverter is placed in the reverse operation mode to perform a peak cut power generation. At loss of the commercial power source or momentary drop of the supply voltage, the switch is opened so that the inverter is placed in the reverse operation mode to perform an UPS operation (emergency power generation). Hence, electric power is supplied without interruption to an important load and the emergency load.