From the Danish Wind Energy Association website, this graph shows the power curve of a wind turbine which shows how large the electrical power output will be for the turbine at the varying wind speeds.

An anemometer is used to find the power curve. It is placed near but not too close to the turbine itself, because the wind created by the spinning wheel will cause an unaccurate wind speed.

The second image is of an anemometer. These are also used at weather station to measure wind speed.