I'm not 100%, but I believe the furnace ignitor and fan run off 12v. So the generator is not necessary for the Furnace. Now if you have a fireplace/electric heater, ours runs off a 15A breaker, so it's gotta be pulling less than 15A. And we also have a heat strip in one of our 15k BTU A/C's, which runs off 20A breaker.