A decent-scale solar=thermal array which captures solar heat and transfers it to circulating fluid can significantly offset the cost of heating a home as well as heating its hot water. We had such an array on our first house and despite heating a 2000 sq foot home and its hot water it was quite common on sunny days to see the pressure-relief valve letting off steam into the atmosphere because more heat was being captured than was being used or being lost to circulation costs.

Of course, this was southern California which has an above average solar profile compared to Oregon or New Jersey, for example. But still.