Efficiency is closely related to insulation levels. A house that is insulated better will be more efficient. Since I've already talked about insulation, I will look at the other part of efficiency which is the mechanical systems of a house.
All homes in Northern Wisconsin are heated with something. There is electric heat, gas heat, and wood heat. If the house won't be a year round home, wood heat is not an option unless you plan to winterize it every year. Electric heat is simply inefficient in most cases, although a new home that is well insulated can do very well with a super-efficient mini-split electric heating system. The efficiency of gas heat depends on the efficiency of the boiler or furnace. An efficient heating system will save you money.
The final heat-related efficiency factor to look at is how the heat is delivered. Hydronic radiant heat costs less money to heat the house than forced air systems because they are more efficient in the way they deliver heat. Forced air systems lose a high percentage of air through duct losses. Radiant heat is also considered to be more comfortable.