One of the big plusses of using a UPS is the instant change over. Most of the time we have an outtage, the power only stays off until I start the generator.
I run my DVC 500 (130 measured watts) plugged into a APC-1500 - gives me 1-2hrs of run time - just enough time to get over 90% of the glitches we have here. I'm not really worried about the house loosing heat in that time period - or for that matter the fire going out, but more protecting the stove against the heat of improper shutdown or power surges (DVC-500 has a CPU)
If you can't fill in the run time gap with a generator - you could toss the on board UPS batteries and rig up either an external deep cycle marine battery or better yet four golf (6v) cart batteries. You can get *vast* run times with external batteries.
There's info on the web about doing this - Big VA used UPS's can be had for cheap on Ebay - the bigger the UPS - the heftier the charging circuit. (have them toss the batteries for you - save on shipping)
I run one of the lighting circuits in my house on one of these - the only way you know the power went out is when you hear all the UPS's beeping around the house. That's when the neighbors call and want to know why you still have lights....