[twindman]

If you start with a fully charge battery and turn on the furnace around 8 pm, it should last until 6 or 8 am. That is in 30 degree temps. I know because I used the lights from 4 to 8 and the battery was down a bit. My inverter started beeping at 5 am because the voltage was getting low. So had to go plug in the truck and run it in my pj's for 30 minutes or so to make it until light.
Oh, and I think the comment about 8amp per hour when running is about right. You can kind of calculate it out by figuring it will run about 50-75% of the time all night and multiplying things out to see how many amp hours you would use.