To a wet finger lifted to the wind, not much. But those three miles per hour could be the difference between a sensible investment and an expensive mistake. Wind turbines harness the power of the wind with spinning rotors that drive electrical generators. These rotors act somewhat like airplane propellers. Wind flows over one side of the blade faster than the other, creating a pressure difference called lift that pulls the blade around the hub.
Faster winds create more lift and thus more power.
But there's a little surprise lurking in the physics. It turns out the wind's power increases with the cube of the speed. This means that a ten mile per hour wind has nearly three times the power of a seven mile per hour wind. Seeking out these extra three miles per hour is well worth the effort. Ideal sites are the tops of smooth hills, where the wind speeds up due to funneling.