Cut-in speed is the wind speed at which the turbine is rotating fast enough to produce useful electrical energy.

The cut-in speed is an important feature to weigh when considering a turbine design, purchase or installation site. Usually, this term is used in reference to wind turbines that are used to charge a battery or battery bank. In order to begin charging, the produced energy must be at a voltage that is higher than that of the battery. A turbine that spins steadily all day long producing 10V output would provide exactly zero charge to a 12V battery.

Note to self: might mention Bergey-style boost circuits and DC-DC converters with respect to lowering cut-in speed.