POWER SYSTEM

The power system includes a fuel cell stack, a system accessory, a battery, and a control device. The control device executes, based on a state of a vehicle and the battery, one of the following processes: a normal power generation process during which the control device makes a net output greater than 0; a first idling stop process during which the control device makes the net output equal to or less than 0 while continuing operation of the system accessory and power generation by the stack; a second idling stop process during which the control device makes the net output less than 0 by stopping the power generation while continuing the operation of the system accessory; and a third idling atop process during which the control device makes the net output equal to 0, by stopping both the operation of the system accessory and the power generation.