I have a 1000VA APC Smart-UPS. I read this thread a couple of days ago and thought I would give it a try. Here's my tank specs (for power):
1 AquaClear 200/50 HOB
1 Penguin 100 HOB
1 100W Heater
1 30" Flourescent light (not sure on the wattage. Came with the hood)
I ran everything constant, except the light. The light was on for 2 hours every 12 hours. The heater was set for 80F and I really couldn't monitor how long it was on/off (you know how those things go).
Remember that the Wattage of the UPS doesn't really determine how long something can run. It's really used for the load factor.
So my UPS gives me 1000VA. I figured I was throwing about 30W average (more with the heating/lighting). So, I was throwing roughly 43VA at my 1000VA UPS. Using the standard UPS sizing calculator, I take my 1000VA divided by the 43VA thrown at it. This gives me 23.25. Since it's over 3, I use a multiplyer of 1.5. This UPS can handle full load for 6.1 minutes. The formula is this:
(1000/43) x 1.5 x 6.1
This give mes 213 minutes, or about 3.5 hours.
Throwing everything on this to see if it works, I came close. I must have been off by my estimated Wattage being thrown because I lasted roughly 4.25 hours.
Just for kicks, a 350W UPS (common and cheap at under $100) will only give you about an hour. (71 minutes by the best full loaded 350W UPS)
Not very practical if you ask me. $1400 for 4 hours? No way!!!!!!! You can, however, make your own for very cheap using a battery an inverter (as mentioned).
. It's just my personal experience.