The formula
Because of the cube, doubling the wind speed gives eight times the power. A turbine that makes 400 W at 12 m/s may make only 50 W at 6 m/s.
Rough annual energy
Wind speed varies all the time, and the fast gusts carry most of the energy. For a typical site where wind follows a Rayleigh distribution, the average power is about 1.91 times the power at the average wind speed. This calculator uses that factor when you enter your site's average wind speed. It's an estimate only. Real turbines cut in at 2.5–3.5 m/s and limit output in high winds.
Worked example
A 1.5 m rotor (1.77 m²) at 5 m/s with Cp 0.3: 0.5 × 1.225 × 1.77 × 125 × 0.3 = 41 W. At an average of 5 m/s that suggests roughly 680 kWh a year, about 1.9 kWh a day, on an exposed site.
Siting matters most
- Mount the turbine well clear of turbulence: at least 9 m (30 ft) above anything within about 150 m (500 ft) is a common guideline.
- Rooftop and low mast installations in towns often produce a fraction of the brochure figure.
- Use the wind speed at height calculator to see how a taller mast helps.
Frequently asked questions
How much power does a 400W wind turbine produce?
400 W is usually its output at 12–13 m/s (about 27–29 mph). At a typical average of 4–5 m/s it may average only 20–60 W.
Is a small wind turbine worth it for off-grid?
On exposed coastal, hill or boat sites with steady wind, yes, especially in winter when solar is weak. In sheltered gardens, solar is almost always better value.