For a wind
turbine to produce much energy, you need an annual average wind speed of close to 5 m/s (11 mph, or 9.7 knots), blowing at turbine hub height. It may not sound like much, but that is a reasonably windy place. Much of North America does not have that much wind, at 100 ft. or below.
In fact, the relationship between wind speed, and the energy it contains, is cubic, so power
decreases exponentially, with wind speed.
The ‘Silentwind’ and ‘Rutland 1200' claim
lower cut-in wind speeds [4.2 & 4.0 kts respectively], than most others [generally range from 6 to 7 kts]. I can’t speak to the accuracy of their claimed specifications.