Sorry - but this isn't an easy one to answer. It really depends on the temp outside, how warm you set your furnace, AND how old your battery is. I've had mine last as much as 8 hours and as few as 3-4 in really cold (18-20 degrees). We were out this weekend and I just gave up on the battery and ran the generator all night for 3 nights. Since we had the generator running, I also used some space heaters to limit the use of the propane down. I figured if I had to pay for the fuel one way (gas), I might as well limit the other (propane).