Engadget has a device that looks like the kind of thing I invent. A clock...that eats things.
The clock catches flies and other insects, and digests them for energy. Presumably it has an initial charge so that you don't have to feed it the first fly yourself. It continuously recharges its battery by throwing all the flies it catches into a pit of bacteria, which produce electricity from the dead flies.
This is convenient because all houses have a few flies and they are incredibly annoying. In the absence of flies, moths, wasps, and other pests also work, but may not be attracted to the built-in flypaper.
Other carnivorous devices are also possible in theory, but most homes don't have the volume of flies required to support, say, a refrigerator or a computer, unless they are some kind of garbage dump. (Perhaps not intentionally, some people are really really slovenly.)