Will Brinton, the founder of Woods End Laboratories, a bioenergy consultancy, predicts a future without landfills. Instead we'll use table scraps and sewage to power our homes. Just dump the waste into a household digester, and bacteria will break it down and release the natural gas methane. Farms could sell their copious poop-based energy supplies back to the grid. But how much energy do animals yield? We ran the numbers and found that you might want to consider a pet elephant.