Right now I'm debating whether to buy or build. One of the great advantages of building is being able to control the way in which your house uses energy (to some extent, anyway). I would build the shit out of the most efficient house possible, and I think it would be a ton of fun doing it. Unfortunately, solar panels aren't as great in MI as HI, but I still think they'd be well worth installing.
David JC MacKay described his struggle to improve the energy efficiency of his 1940's era semi-detached house, with a history of his gas usage. He did quite well, but concluded that you get better results when you build with energy efficiency in mind, as did energy consultant Alan Foster. MacKay's book on sustainable energy is highly recommended, and available on the web site and as a free PDF.
Wow! Thanks! Guess I got some reading to do. Although, of course the finance of it runs deeper than energy efficiency, as it seems quite a bit cheaper per square foot to buy a preexisting home. But on the other hand, there's probably some non-quantifiable benefit to designing a home from scratch. Also, finding a suitable piece of property that doesn't already have a home isn't so easy, either.
I suppose in that case, you could install electric heating and appliances, and then you'd really be onto something. Right now, my combined energy bill is about $200/mo in the winter and summer, and about half that in the spring and fall. Are solar and geo typically amortized over the life of the mortgage? If so, I could definitely see it being at least price neutral.