emergency power supply; backup power source
Refers to a power source used when the main supply fails, such as a generator or battery backup.
The hospital is equipped with an emergency power supply.
The backup power source automatically started during the blackout.
予備電源 is a more general term for a spare or standby power source, while 非常用電源 specifically implies use in emergencies.
Compound of 非常 (emergency) + 用 (for use) + 電源 (power source).