A Stirling engine, powered by the heat difference between the inside and outside of a house. In the winter the house is warmer than the outside, in the summer the outside is warmer when the A/C is operating inside, at all times the air under the roof is hotter...regardless there is always a difference. A Stirling engine would convert this temperature difference into an electrical charge. Like with solar panels this charge can be stored in batteries, or fed into the grid.
- Mark Abramovich