A power supply system having a cogeneration unit equipped with an internal combustion engine and a generator driven by the engine to generate power to be supplied to a power destination and hot water to be supplied to a hot water destination by heat exhausted from the engine, comprising: a natural energy generation unit that generates power with natural energy; a power supply unit that receives the power generated by the generator and the natural energy generation unit to supply the received power to the power destination; a connecting bus that electrically connects the cogeneration unit, the natural energy generation unit and the power supply unit; a voltage detector that detects voltage of the power flowing through the connecting bus; an electric heater that heats the hot water by the power supplied through the connecting bus; a storage unit that is connected to the connecting bus and stores the power not supplied to the power destination; and a detector that detects voltage of the power stored in the storage unit; characterized in that the power supply unit controls the operation of the electric heater to on: when the detected connecting bus power voltage exceeds a predetermined first voltage (α) and the detected stored power voltage exceeds a predetermined second voltage (δ) (S10 - S24); or when the detected connecting bus power voltage exceeds a predetermined first voltage (α), and is equal to or greater than a predetermined third voltage (γ), and the detected stored power voltage exceeds a predetermined second voltage (δ) (S10 - S24). a natural energy generation unit that generates power with natural energy; a power supply unit that receives the power generated by the generator and the natural energy generation unit to supply the received power to the power destination; a connecting bus that electrically connects the cogeneration unit, the natural energy generation unit and the power supply unit; a voltage detector that detects voltage of the power flowing through the connecting bus; an electric heater that heats the hot water by the power supplied through the connecting bus; a storage unit that is connected to the connecting bus and stores the power not supplied to the power destination; and a detector that detects voltage of the power stored in the storage unit; characterized in that the power supply unit controls the operation of the electric heater to on: when the detected connecting bus power voltage exceeds a predetermined first voltage (α) and the detected stored power voltage exceeds a predetermined second voltage (δ) (S10 - S24); or when the detected connecting bus power voltage exceeds a predetermined first voltage (α), and is equal to or greater than a predetermined third voltage (γ), and the detected stored power voltage exceeds a predetermined second voltage (δ) (S10 - S24). when the detected connecting bus power voltage exceeds a predetermined first voltage (α) and the detected stored power voltage exceeds a predetermined second voltage (δ) (S10 - S24); or when the detected connecting bus power voltage exceeds a predetermined first voltage (α), and is equal to or greater than a predetermined third voltage (γ), and the detected stored power voltage exceeds a predetermined second voltage (δ) (S10 - S24).