The output from a computer UPS isn't the best for motors either. It's usually a square wave which doesn't work very well with AC motors. They are OK for electronics becuase the power supplies in pretty much all electronics change the 120v AC into 5 & 12 DC right off the bat. AC motors are designed to so work with a true sine wave. Using a PC UPS to run a motor will at best result in way less than peak performace for a shorter time, and at worse ruin the motor and the UPS.

You can get by this problem if you are using a DC powerhead like a stream(not the nanos), vortech, or the DC version of the Koralia for the reason I stated above.