"..ISO New England, which oversees the six-state power market, concluded after months of research that the region’s power grid has been strengthened by cold-weather sunlight, an unanticipated finding that adds fuel to a growing debate over the impact that renewable energy has on the nation’s labyrinth of power plants and transmission lines. The findings stand to alter regulators’ view of rooftop solar, which many had seen as a small and unpredictable source of energy.