the like power of a predetermined voltage e g single phase 200 v is supplied from a power company substation to the primary side of the transformer 318d at the transformer 318d the voltage at the primary side is stepped down to the commercial voltage e g 200 v secondary side voltage accordingly power is usually supplied from the secondary side of the transformer 318d to an ordinary household as illustrated by arrows a in fig2 on the other hand power generated by the solar cell 310d is supplied to the secondary side of the transformer 318d as illustrated by arrows b in fig2 so as to increase the power capacity a microcomputer 320d for controlling the inverter circuit 312d monitors the voltage inputted at the a d converter and the current from the current transformer ct and controls the switching operation of the inverter circuit 312d so that the solar cell 310d operates optimally further control of the opening and closing of the parallel off conductor 316d provided between the transformer 314 and the transformer 318d is also carried out at the microcomputer 320d the parallel off conductor 316d has a switching function for either supplying power generated at the solar cell 310d to the transformer 318d or cutting off the supply the microcomputer 320d controls this on and off operation the microcomputer 320d computes the power passing through the isolation transformer 314d and transmits this value to the exterior unit of the air conditioner via the signal line s in the.