I've got an APC BX1500G UPS powering my modem, router, and iPod Touch charger. According to APC, it's a 1500va and 865w system. Total draw on it is 44w, which gives me 92 minutes before it shuts down.

I don't know how exactly one would calculate power draw from one of these things, but if you're only drawing ~18 watts with your phone/laptop and the UPC your using is 1000w vs. my 865w unit, than, at the very least, you should be getting a bit better than the hour-and-a-half I'm getting.