Trick question: They basically all pay for themselves in the long run! But a new infographic from One Block Off the Grid helps you choose home energy improvement projects based on up-front cost and how long they'll take to start paying you back. The quickest payback is a smart thermostat, with a payback time of seven months — but that one will only save you $3,600 over 20 years. By contrast, you could save $60,000 by switching to geothermal power, but it'll cost you $3,000 up front and take 10 years to pay off.