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 ...

The Cox family is part of a growing trend in Phoenix and other sunny locales: homeowners using rooftop solar panels to generate their own power and sell the excess to the local electric utility grid. More than 127,000 homes in Arizona now have rooftop arrays. The Coxes bought their panels outright, after doing the math that proved that the system—with a life of 20 years or more—would pay for the $12,000 cost in roughly six years, through savings on their electricity bill and tax breaks. “Last year, in May, June and July, we had a zero utility bill, which is awesome to see,” Cox recalls. Other months the Cox household produces excess power, which gets sent to the local grid for credits on future bills. “Our meter goes backward,” he says ...