You can't flick a switch and turn backup power on as if you were turning on a light bulb. You need something called "spinning reserve" which is a huge inertia of spinning rotors in synchronous generators that will run at all times in the no-load position. Thus they still use coal or nuclear or hydro for the synchronous gens. It takes between 7-8 HOURS to start a thermal unit (coal) because it has to heat a series of pipes that contain water to create steam to move the turbine to turn the rotor and bring it up to synchronous speed. If you want to make any changes to a nuclear output it takes upwards of 1 WEEK.So when the wind dies down for 2 hours how will you provide power to the essential service buildings, industry, and residential areas unless you have the spinning reserve running? The answer is that you can't with today's technology. This is why nuclear (best in my opinion) or hydro or coal are good for providing the base load and wind can provide some variable load with the support of extra spinning reserve.