Every six months or so Doug Cox washes his roof so he can make more electricity. The 16 solar panels on the southern face can produce nearly four kilowatts of electricity in the strong Phoenix sunshine—enough to offset much of the power required to cool his home in this hot climate. But the two-year-old system gets dusty, which slows the current flow. “My wife is up there, hosing the soap off as I'm scrubbing,” says Cox, a 37-year-old high school math teacher. “We've even had our five-year-old daughter help us. She's on the ground, and we tell her when to turn the hose on and off.”