PHOTOVOLTAICS ARE STILL, ON AVERAGE, A PRICEY, subsidy-dependent source of electricity. However, rooftop panels are beginning to beat the grid in a number of jurisdictions with high retail power rates—and their ranks are projected to swell over this decade. A growing number of economists say that rapidly shrinking costs have turned distributed solar generation into a disruptive technology that's set for runaway growth. In fact, they say, it could ultimately upend the power distribution market.