We might quietly scoff at the Toyota (NYSE: TM) Prius drivers - after all, the car only gets slightly better mileage than the average car in its class, so it's not all that special as far as environmentalism goes.

But don't scoff too hard, because it just might be that we'll all be driving hybrid cars in the not-so distant future.

You might be thinking that we simply don't have a model of fuel-source change for automobiles - so we really don't know what the future will hold - and whether our cars will be powered by natural gas, lithium-ion, or even solar power - or perhaps some combination.

And you're right - there's basically no model for automobile fuel conversion.

But there is a very robust model for home heating conversion.

Today there are at least as many heating technologies as there are fuel types, but 100 years ago, most people used coal and wood.