During the early morning hours of April 15, with a steady breeze blowing down Colorado's Front Range, the state's biggest utility set a U.S. record -- nearly 57% of the electricity being generated was coming from wind power.

As dawn came and the 1.4 million customers in Xcel Energy's service district began turning on the lights, toasters and other appliances, the utility's coal and natural gas-fired power plants ramped up production and brought wind's contribution back closer to its 2012 average of 17%.

lities have long been wary of placing too much finicky renewable power on the grid.

"A lot of utilities don't want to contract large amounts of wind because it's volatile," said Amy Grace, a wind analyst at Bloomberg New Energy Finance. "Anything over 25%, and utilities get nervous."

Colorado's overnight high-water mark demonstrated that utilities can indeed incorporate cleaner power sources into the mix.