Having messed around with a computer-style UPS unit (1200 watt) in my basement where most of my tanks are, I sprung for a beefier unit, this Duracell powerpack thing... it's intended for longer runtime than the UPSes are, and doesn't beep constantly when the power's off. Duracell Powerpack 600 | Backup Power Source | Duracell Power
As for what to run on it, frankly I'd just run the filter or aerator. Unless my house is going to be cold for several days, the fish can handle a few degrees drop in temps and a little extra darkness. They can't live without the O2
though. Most filters I've tested don't take up too much juice either. If you have gas water heating, you can do partials with warm water out of the tap to maintain tank temps.