Householders installing wind turbines and solar panels must wait many years for a payback on their investment. So it has largely been an ethical choice, rather than an economic choice. Now Germany is buying back spare clean energy from customers at four times the price it charges them for conventionally generated electricity. Demand for renewable energy systems is up, installation costs down. Forty countries now follow the German lead. But some countries, like the U.K., use a quota system, which critics argue is inadequate. Earth Report investigates.