Wind power density is a useful way to evaluate the wind resource available at a potential site (see wind resource evaluation). The wind power density, measured in watts per square meter, indicates how much energy is available at the site for conversion by a wind turbine. Classes of wind power density for two standard wind measurement heights are listed in the table below. Wind speed generally increases with height above ground.

 

Classes of Wind power density at 10 m and 50 m(a)
.	10 m (33 ft)		50 m (164 ft)
Wind power class	Wind power density (W/m2)	Speed(b) m/s (mph)	Wind power density (W/m2)	Speed(b) m/s (mph)
1	< 100	< 4.4 (9.8)	< 200	< 5.6 (12.5)
2	100 - 150	4.4 (9.8) / 5.1 (11.5)	200 - 300	5.6 (12.5) / 6.4 (14.3)
3	150 - 200	5.1 (11.5) / 5.6 (12.5)	300 - 400	6.4 (14.3) / 7.0 (15.7)
4	200 - 250	5.6 (12.5) / 6.0 (13.4)	400 - 500	7.0 (15.7) / 7.5 (16.8)
5	250 - 300	6.0 (13.4) / 6.4 (14.3)	500 - 600	7.5 (16.8) / 8.0 (17.9)
6	300 - 400	6.4 (14.3) / 7.0 (15.7)	600 - 800	8.0 (17.9) / 8.8 (19.7)
7	> 400	> 7.0 (15.7)	>800	> 8.8 (19.7)
(a) Vertical extrapolation of wind speed based on the 1/7 power law
(b) Mean wind speed is based on the Rayleigh speed distribution of equivalent wind power density. Wind speed is for standard sea-level conditions. To maintain the same power density, speed increases 3%/1000 m (5%/5000 ft) of elevation. (from the Battelle Wind Energy Resource Atlas)

 

Table of data compiled by the American Wind Energy Association