At 2200 square feet, including the bedroom basement, I wouldn't call my cabin a mansion. I designed, built in 2005 with energy efficiency being a top priority. It is well insulated, with a Burnham high efficiency oil boiler for heat and hot water. No energy-wasting chimney. The boiler is located in my insulated basement garage and the stainless steel vent passes 15 feet horizontally over the conditioned portion of the basement, increasing the efficiency above its stated rating. At its stated rating, current fuel cost, I estimate a net output of 21,600 net BTU per dollar. With my setup, I may get a bit more, say 22,800.
I also have a co-axial vented propane stove, Vermont Castings so highest quality and efficiency. Net BTU/$ is 20,900, so that costs a little more to run than fuel oil.
Since we have cold winters here at high elevation heat pumps are wishful thinking. They go into full electric mode, and the cost of a KWH here in the mountains (TVA sourced) is twice what it it in the Piedmont (with Duke Energy). Most homes here are part-time use "vacation" homes and were built all-electric, without forward thinking of heating costs, and the owners pay dearly during the heating season. In fact on cold weekends they use so much electricity, that the voltage drop is a big issue to commercial users here.