During 2017, wind energy provided 2.4% of U.S. energy consumption. Because wind speeds fluctuate and are sometimes not strong enough to drive turbines for days at a time, most wind power capacity must be backed up by other energy sources to generate electricity when the wind doesn't blow. As the amount of wind capacity rises in a given region, so do the challenges and costs of backing up its unreliable energy output. This limits wind production in the U.S. and throughout the world.