Solar power has been declared a winner before, only to flounder. It’s easy to remain skeptical today, given that solar power accounts for less than one percent of the global energy supply. But it is also expanding faster than any other power source, with an average growth rate of 50 percent a year for the past six years. Annual installations of photovoltaic panels increased from a capacity of less than 0.3 gigawatts in 2000 to 45 gigawatts in 2014—enough to power more than 7.4 million American homes. This time really is different: solar power is ready to compete on its own terms.