Any research I did suggested that a square wave UPS won't work for aquarium equipment. Even an approximation to a sine wave UPS is iffy. I found a true sine wave UPS, but they were $650 each. I wanted something that would kick in while not home, but figured I should have at least something, so I went to C-Tire to get one of those Eliminator back-ups. While there, I happened upon this...
...the beauty of which is that it's automatic. This one will power a MaxiJet 1200 for 13 hours. There are bigger ones available. Up to 1500W, I think.
HTH,