If you have a UPS laying around, that's fine, but you need a pretty big one (maybe 1500VA or better) to give you good runtime on a main pump. Powerheads you might get more timer.
If you don't have one around and are facing buying one, then a cheaper, better solution in my opinion: Get a 350 to 500 watt power inverter. They run about 60-90 bucks (less than a big UPS) and keep that around. If the power goes out, hook it up to your car, run an extension cord, and you're good to go as long as your gas tank lasts. A car will idle on a full tank of gas for a good 10+ hours. You could use the UPS method and use this as a secondary, long term solution too.
Me, I just use a generator, but I am a tool-freak, I guess that's not for everyone.