Computer UPS's are only designed to run a computer for no more than 20 min or so. Just long enough for you to finish what you are doing, save your work and shut down. A computer draws 300-400 watts and the monitor draws about 100 watts. As stated by hawk, keeping the tank warm is the major consideration, especially during the winter when it seems most power outages occur. SO your tank heater is likely 300watts or so plus figure about 100W for one circulation pump. So you're right up there with the draw of a computer. 20 min of backup power, even 40 min, is not gonna make much of a difference either way. Your tank can survive just fine all on its own for that little time. Where you need to be concerned is with the outages that last over an hour and up to several hours.
There was a thread on reefcentral awhile ago on a DIY battery backup unit using a deep cycle marine battery and a power inverter and a trickle charger.
http://www.reefcentral.com/forums/sh...battery+backup
One of the beauties of this set up is you can recharge it with your car if it runs down or even hook it up to your car with jumper cables to keep it going indefinitely.