PURPOSE: To improve system operating efficiency and reliability on a power supply system for a co-generation system installation people.

CONSTITUTION: A Na-S battery 2 used as a heat supply unit, for a co-generation system is kept in a consistently operating condition by parting hot gas 11 exhausted from an internal combustion engine 6 and guiding it to a constant- temperature tank 8 in power storage facilities which require heating. At this time, the flow rate of the hot gas is determined by controlling a flow control valve 12 depending on the temperature of the constant-temperature tank 8. Water absorbed by an exhaust heat boiler 1 which is operated by using exhaust heat from the internal combustion engine 1 is heated by a water-water heat exchanger 7 which uses return water of bearing cooling water for the internal combustion engine 1 for heat source and a gas-water heat exchanger 9 which is provided at the gas outlet of the constant-temperature tank 8, and the access of electric power to the Na-S battery 2 as the power supply unit is made through a DC-AC bidirectional convertor 18. In this way, new functions such as extra power adjustment in a power available time zone, reservation in the power supply unit and emergency power supply are added.