The nameplate capacity of a wind turbine is somewhat arbitrary. Therefore, so is the capacity factor. 
The power available in the wind is proportional to the cube of the windspeed. With an excellent wind turbine of 50% efficiency, the hypothetical power output can be caluclated by multiplying the square of the radius (in meters) by the cube of the windspeed (in meters per second). Most wind turbines have a cutoutspeed of about 55 to 60 miles per hour (25 meters/second), according to a correspondent who works in the field. Consider, for example, a wind turbine of 23meter radius. In a 25m/s wind, it could hypothetically produce 8.3 MW of electricity. The Enron Type Z750 wind turbine has a 23m radius. It is rated at 750 kW, not 8.3 MW. The machine is designed to produce 750 kW of electrical power for all speeds in excess of about 11 or 12 m/s. Its expected capacity factor in a wind farm in Minnesota is about 35%. If the wind turbine were rated at 14 m/s, the nameplate power would be about 1.5 MW instead, but the power delivered to the grid would still be about the same. Its capacity factor would be 17% (possibly a bit higher, depending upon windspeed distribution) instead of 35%. Early wind turbine manufacturers (and proponents)
tended to emphasize nameplate power. More recently, they have gone
for relatively constant power (at least in higher winds) and higher capacity
factors.
