Householders installing wind turbines and solar panels must wait many years for a payback on their investment. In Germany they’ve found a way to cut down that payback time. It’s called a feed-in tariff and it is giving a big boost to the renewables industry. Under German law, householders with operating solar or wind power can sell their spare capacity back to power companies at a premium. Demand for renewable energy systems is up, and as the numbers increase the installation are steadily falling. Forty countries now follow the German lead.