The
boon to all you
network administrators out there. We luv 'em.
Basically, a UPS is just a large computer battery. These large-scale power devices are run off mains power in to the battery, and then have standard computer power outlets to connect devices in to. The idea is, you run your computers, routers, switches etc off the UPS battery constantly, while it gets its recharging supply from the mains.
The ratings of these batteries are quite large and varied - from small-scale 200VA to the mother of all power supplies, 7500VA. A 200VA UPS will run a small computer for an hour to a couple of hours, based on demand, if power was totally removed from the UPS. The big'uns can run a rack of computer networking equipment for a good 5 hours in total darkness. It always pays to have a torch available for emergency network works.
Lately, the modern UPS has also had remote shutdown facilities included in them. This is usually in the form of a serial cable (or networking variants) and some software installed on the device that you are powering. If there is a blackout, and the batteries are running low, the UPS instructs the equipment to shut down safely. A handy little feature.
Unfortunately, no UPS is smart or mechanically gifted to turn the computers back on when power is restored. *Sigh*