Renewable energy was the biggest source of new power added to U.S. electrical grids last year as falling prices and government incentives made wind and solar increasingly viable alternatives to fossil fuels. Developers installed 16 gigawatts of clean energy in 2015, or 68 percent of all new capacity. Over half of the growth came from wind farms. This was the second straight year that clean power eclipsed fossil fuels. “This is a long-term trend,” says Colleen Regan, a New Energy Finance analyst who follows North American power markets. “System costs have really come down for renewables, which makes the case for installing them a lot stronger.”