I was looking at this awhile ago. There's lots of online mods to computer ups's. The big thing to watch for is that most computer ups's are made to provide power for only a few min to shut down. So they do not offer any cooling for the inverter or transformer. There story's of house fires from the transformers melting through the case. I've read that you should only run them at 25% loading if you are going to run continuous. It's doable, look at gel battery's since they are safer indoors, just pricey.
I'm sure lots of the prince Rupert area reefers have some decent idea's after the past few months. Some multi day outages there...
http://www.instructables.com/id/Rewo...sive-Capacity/