UPS

From Gentoo Linux Wiki

UPS: Uninterruptible Power Supply

UPS is a device that is used to maintain a constant power supply to computer hardware (or other electronic device) by providing battery backup power (at normal mains rating) in the event of main power supply failure or power spikes. Most common uses are for computer servers (or other devices) that are business critical, hospitals, airports, and any other institutions that require 100% power stability.

UPS's are normally accompanied by a generator. As a UPS runs off battery power, converting this to mains power, it is limited in the amount of time it can supply constant power.