They found the cost of wind power to be 15.1 cents per kilowatt hour if natural gas is used to back-up the wind energy or 19.2 cents per kilowatt hour if coal is used as the back-up fuel. These costs are 1.5 to 2 times the 9.6 cents per kilowatt hour estimate the EIA is using for generating electricity from wind in its models.[iii]
http://www.instituteforenergyresearch.org/analysis/the-hidden-costs-of-wind-power/