Think about how much money you'd save if you didn't have to pay for gas or electricity every month. This concept house from the University of Stuttgart manages to generate enough power for itself, plus two electric cars, with enough left over to sell back to the power company for a profit.
The Plus-Energy house is completely covered in a shell of photovoltaic panels and solar thermal systems, which generate electricity and hot water. The rest of the house is designed to make the most of the energy available, by maximizing natural light and using extremely efficient insulation. The house also comes with a pair of electric cars which can subsist off of spare electricity from the solar panels.
The whole thing is completely modular, without a foundation even, and since it doesn't need grid access, you just have to plop it down and hook up to the nearest fire hydrant and you're more or less good to go.
This seems like a pretty sweet deal, but in my (admittedly limited) experience with solar power, it's miserably inefficient, even in the best of conditions. Solar is a great way to supplement other sources, but I just don't see how this house can keep all of my computers humming and the lights on and the heat on and the water hot and two electric cars charged all the time. Especially in Germany. In winter, in a blizzard, at night, when there's a solar eclipse. Yeah, I'm thinking worst-case scenario, but still, you get the point.
That said, unlike most concepts, this house is actually going to be built in Germany and will apparently be busily powering itself and its cars with juice to spare by the middle of next year. If it does, I'll be impressed.