Playing off of a UPS battery backup??

[Soul Rebel]

Does anyone have any experience with some of the really big battery backup units? I live in South Florida, and it looks like we may have a hurricane this weekend, and if I have to sit around doing nothing, I'd obviously like to be productive and get some hands in. Since we could have power outtages that last a while, I'm looking to see how long I could potentially play with a battery unit. I have a Sony 17" laptop and a DSL modem, and I'm trying to find a backup system that will last for several hours (if that's possible). I looked through a bunch of web sites, but couldn't really tell how long each unit will power my setup. Anyway, any suggestions would be greatly appreciated. I'm willing to spend a few hundred bucks on it, but it has to be something I can go buy at CompUSA or whatever. Thanks for the help.