All power sources need back-up and no energy technology can be relied upon 100%. The electricity grid copes with big fluctuations in energy demand on a daily basis. Although wind speed varies, this can be forecast fairly accurately. In Denmark, nearly 40 % of electricity demand is already supplied by the wind, and is managed successfully by the Transmission System Operator.
The International Energy Agency's (IEA) report on Gird Integration of Renewables from 2014 showcases how "There is no technical limit on how much variable generation a power system can absorb. But system transformation with increased flexibility is required for higher shares. Technically much higher shares are feasible.
Posted in: Wind power myths