I heat my 1800 sq. ft. bilevel home in SE Pennsylvania for less than $200 per year ( 15,000 BTU propane space heater ). Temperature is a constant 70 degrees. The cost could be further reduced if I was more attentive to the solar situation (But "passive solar requires active people") Key elements : R-40 ceiling, R-30 side walls, square design to minimize the perimeter, and as the builder put it, there is more "mass" in the lower level than there is in the Luray caverns.