During 2019, wind provided 2.7% of all U.S. energy and 7.3% of all U.S. electricity. 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 for 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. It can also cause blackouts during dangerous weather in areas where wind provides a significant portion of electricity.