As a rule of thumb, a building site should have an average wind speed of 10 mph to make the installation of a turbine worthwhile. At wind speeds much below this the turbine cuts out and no power is generated.
The U.S. Department of Energy has published a map showing average wind speeds that can be useful in calculating the potential of a site.
Wind conditions are highly localized. Especially hilly areas may require site-specific data collection to make sure it’s not in a wind shadow even if the general area looks promising.
The two most important variables determining a turbine’s power production are the rotors’ swept area and the wind speed. When it comes to wind power production, sites with low wind speeds are useless. Available wind power is proportional to the cube of wind speed. That’s why a site with an average wind speed of 18 mph can produce eight times more power than a site with an average wind speed of 9 mph.
Assuming a turbine’s efficiency is constant, a turbine that produces 300 watts at 9 mph will produce 8 times more power — that is, 2,400 watts — when the wind speed is doubled to 18 mph.
Before you purchase a wind turbine, it would be great to know your site’s average wind speed. Unfortunately, wind site assessment is expensive. Most small wind turbines are installed without any prior wind monitoring; instead, installers look for clues to high wind speeds like flagging on trees. “To get an anemometer up high enough, you need to spend about $7,000 for a tower,” says Anne Bijur, a customer communications representative for Earth Turbines in Hinesburg, Vermont. “So you might as well put the money into a turbine.”
One rule of thumb: a site isn’t windy enough to justify a turbine unless the wind…