Just wanted to add a personal experience with that Xantrex 400 unit linked to on the first page fo the thread. I have one on my 46g that is plugged in to everything but lights, including 2x 100W heaters. Total draw with everything running at once is about 300W.
Depending on how warm the house is, and whether or not I unplug my skimmer, it will run between 4-6 hours before going dead. I've done it on a couple occassions, so I'm not BSing. Smaller heaters aren't that big of a deal because they aren't on all the time. No... it doesn't put out a true sine wave, but I've experienced no failures because of it. (Skimmer uses a Mag3 pump).
Anyway... I've since gotten a nice Honda generator, but I still keep the Xantrex unit hooked up for those "hour-long" outages that seem to happen in the middle of the night. Good piece of mind, in my opinion.
One thing to note - if you have a GFCI outlet that all your equipment is hooked up to, a battery backup will pretty much eliminate that safety protection since it kicks on as soon as it senses the power is out. So if your GFCI does it's job and cuts power just as you stick your hand in a shorted tank, your battery backup will then nicely turn the power back on for you! Oops. Running a portable GFCI between the load and the output of the battery backup is probably a really good thing to do if you go with any type of battery backup.
