It’s worse than that. Companies rate windmills based on power output at a given wind speed. I have checked several manufacturers and typical rated wind speed is 28 to 31 mph. Energy produced is a function of the square of wind speed. So at a wind speed of 15 mph the windmills will produce one quarter of rated power, or at most 7.5 gW.
1. During storms and other high wind times the windmills have to shut down because of the high speeds involved.
2. In winter during low wind speed, the freezing cold causes the gear box oil to freeze, so the windmills have to be fed with electricity to keep them warm.
The working guts of a wind generator.....see the white hard hat on the top of the cabinet..(upper right)...gives some idea of the size of the beast.