How long will my UPS last on my pellet stove ?

UPDATE: Halloween storm 2011.. my little 1000W UPS with "modified" power went 15 hours (Sunday at 6AM I turned it off as it only had 2 lights out of 7).  Saturday night, after being on for 6 hours, I unplugged the DVR that eats some 90w due to the internal HD (as stated on most news shows), and a couple of AC adaptors.  So it could have run even longer !

DO NOT RUN a modified Sine Wave DC adaptor on a pellet stove..   These are the type that clamp on a car battery.  They could damage the electronics.   You need a "pure sine wave" DC converter..   for a few bucks more but won't damage your pellet stove electronics.  The stove is a "timer" and needs the correct power to work right.  Also.. don't charge batteries (it's in the manual) with these modified sine wave converters.  ALSO, if you have a generator.... again, many are modified sine wave.  Some are made for electronics (pure sine wave) so be warned.

My USP, as the page states, is a standard 1500 AH (amp hour) UPS that was being thrown out.  There are tons of them on E-bay with dead batteries.  But make sure the USP works, lots of reasons a UPS can die.  Don't pay for shipping dead batteries.

ONLY if you are good with electricity, can you do what I did.  Extend the UPS life.  You can get a new UPS from Staples, Sam's club or Best Buy.  A $200 one should keep your stove going for an hour or more.  The smaller ones if you have small 10 min blackouts.   I would always keep a stove on a UPS, the surge protection is the best against the $350 control panel. 
There's an easy way to tell how long it should last, the USP should have 2 LED systems (better UPS systems).

One LED set shows the charge of the battery.  
One LED set shows the current drain. 

If the only thing plugged into your UPS is the stove, it will show the drain.  Unplug the UPS and have the stove run off it.   Watch the power drain.  If it takes 1/2 hour to discharge the battery half way.  Then the stove should last an hour.  If after a half hour on the USP the battery is still full.  Then the USP will let the stove run for a few hours.  Just remember if the UPS dies (they will automatically shut off when they "beep fast") the stove will shut off and the stove WILL SMOKE until started again. (depending on your chimney setup)
A UPS is just a battery (bigger battery, longer up time) that converts to 120V.
I know enough about electricity that I made the 24V needed for my USP as car batteries.  It's still running since I made that web page (1996 ?)
I have replaced the batteries every 5 -7 years.  So there is an expense, even with the store bought self contained units.