I picked up one of these standby PC UPS units at a thrift store a few weeks ago for $5. Put a new battery in it and it seems to function fine. From fully charged it ran a 80 watt lamp for about 50 mins and then started to sound an alarm for I assume low battery voltage, but it was still holding the load.
I put a scope on the output and the wave form looks like a bipolar PWM signal. My Fluke true RMS meter showed 111VAC.
I'm not sure what kind of loads would not like the rather squarish waveform and not sure how fast the unit switches from normal to back-up mode. Also I noticed that the unit will not start-up on battery only mode even if fully charged, it first has to have an AC input active before it will turn on? So that seems to limit it's usefulness as a emergency source of AC power AFTER an AC power outage, it will have had to be already on prior to the outage.
Anyway I don't know if I will have a application for this unit, just too good a price to not purchase and play with. I noticed that there are tons of these on E-bay for cheap, but factor in the cost of a new lead acid gel cell battery.
Lefty