Maybe, but that other 70-80% is waste - we could turn it into useful heat, but instead we vent it to the environment. I've seen plans to run an engine in your

Maybe, but that other 70-80% is waste - we could turn it into useful heat, but instead we vent it to the environment. I've seen plans to run an engine in your furnace, thus creating both electric (at 20-30%, and capturing the rest as heat to the house. I've never seen this in the real world and experts who have looked as those systems said they won't enter a building that has one using that design. (it is possible to make a design they would trust, but they haven't seen any)