To provide an electrical power system capable of supplying power to a load without delay even when external power supply is interrupted or its voltage drops.

The electrical power system includes a detecting means for detecting the voltage of a power supply joint 50 as direct-current voltage portion; a controlling means for controlling an adder 72, a regulator 73, and the like that is for controlling the electricity generated by a generator 20 through a converter 40 so that the detected voltage value of the power supply joint 50 is kept at a target value when the external power source 30 is normal; a limiter 74 that limits the electricity generated to a predetermined value; and a diode in the external power source 30 that keeps the target value (detected voltage value) at a value equal to or higher than the output voltage of the external power source 30 when normal. The electrical power system is so constructed that, when the electricity generated limited by the limiter 74 is lower than the power requirement of the load 10, the detected voltage value is lowered to a value lower than the target value, and deficient power is supplied from the external power source 30 to the load 10. When the detected voltage value falls below the predetermined value, power interruption or voltage drop in the external power source 30 is detected.