I guess if you put a turbine or solar array in your backyard, you can use the electricity to electrolise water to power a car, but every amp joule and calorie you use to do this is one less available for other uses.

I know I'm being flippant about the methane thing, but to me there seems to be much more point to it. We already have processes that imitate the intestine on a larger scale. Basically a big tank full of bacteria and enzymes and what-have-you that turns biomass into methane. Since that waste would biodegrade into methane anyway, it does seem to offer a chance of reducing greenhouse emissions as well as providing energy.