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Wind speed requirements: what your site actually needs
The number that matters is the annual mean wind speed at hub height. Here is the threshold, how to estimate it, and the three mistakes that make people overestimate their site by 40 %.
The threshold: 4.5 m/s, ideally 5.5
Below 4.5 m/s at hub height, a grid-tied small turbine generally will not recover its cost within its design life. Between 4.5 and 5.5 m/s it is marginal and depends on your electricity price. Above 5.5 m/s it becomes a solid investment, and above 7 m/s small wind is the cheapest power available off-grid.
Note that this is the annual mean, not the gust speed and not the speed on a windy afternoon. A site that hits 12 m/s twice a week and averages 3.5 m/s is a bad wind site, not a good one.
Height: the cheapest wind resource you can buy
Wind speed increases with height following a power law: in open terrain, doubling the height adds roughly 10 % to the mean speed, which adds about 33 % to the energy. Over a village or a forest, the exponent is larger and the gain is bigger — 40–50 % is common going from 10 m to 20 m.
This is why a 12 m tower is the minimum we recommend and 15–18 m is normal for farms. The tower is 20 % of the cost and often 40 % of the output difference.
Obstacles, and the 10-metre rule
Any obstacle within 10 times its own height upwind creates a wake that cuts speed and adds turbulence. A 6 m house therefore affects the wind for 60 m downwind. The practical rule: the hub should be at least 10 m above anything within 100 m.
Turbulence matters as much as speed. A turbine in a turbulent 6 m/s flow produces less than one in a smooth 5 m/s flow, and it fatigues its blades far faster. This is the real reason rooftop installations disappoint.
How to measure properly
For anything above 1 kW: a calibrated cup or sonic anemometer at hub height, logging 10-minute averages for at least three months, ideally covering the windiest season. Add a wind vane to identify the prevailing direction and to check that the tower site is clear in that sector.
For smaller systems, a two-week measurement plus a wind atlas is a reasonable compromise. What is not reasonable is using the weather app on your phone: it reports a 10 m wind over open ground, often at a site 20 km away, and it is routinely 30–40 % off in complex terrain.
Reading a wind atlas without being fooled
Most atlases give the mean at 50 m or 100 m over smooth terrain. To get your 12 m hub height, apply the shear exponent downward — which typically cuts the figure by 15–25 %. Then apply an obstacle correction if the site is not open. If the corrected number is still above 5 m/s, you are in business.
