An air conditioning system comprises a converter for converting a commercial AC into DC, an inverter for reconverting the DC into an AC having a predetermined frequency corresponding to an air conditioner load, a compressor driven with the AC from the inverter under a compressor capacity control operation matching the load, a battery used for dealing with a voltage drop of the supply power from the commercial AC power source and for supplying a DC different from the DC converted by the converter to the inverter via a switching element, and a discharge control unit for controlling the switching element in the manner that the battery supplies a current having a predetermined value proportional to a difference between the AC supplied from the power source to the converter and a preset current value.