Wind Turbine Power Calculator

How it is calculated

P = ½ × ρ × A × v³ × Cp, with A = π × (D ÷ 2)². D is the rotor diameter in metres, v the wind speed in m/s, ρ the air density in kg/m³ (default 1.225, sea level at 15 °C) and Cp the efficiency entered as a percent (default 40). The result is shown in watts, then kW (÷ 1000), and the daily figure is kW × 24, which assumes the wind blows at that exact speed all day. Real sites have variable wind, so the daily energy is an upper-style estimate, not a forecast.

Diameter and wind speed must be above zero. Efficiency is limited by the Betz limit: a value of 0 or above 59.3% (16/27) gives no result, because no turbine can extract more of the wind's power than that. Air density must also be above zero. Leaving efficiency or air density empty uses the defaults.

Common mistakes

• Entering the radius instead of the diameter. The swept area then comes out four times too small.
• Entering efficiency as a decimal (0.35) instead of a percent (35). The tool reads 0.35 as 0.35%, and the same turbine drops to 8 W.
• Using the daily kWh figure multiplied by 365 as a yearly forecast. It assumes a constant wind speed for 24 hours, while real wind varies a lot and, because of the cubic law, gusts and calms do not average out.
• Using wind speed in km/h or mph. The input is m/s, and since power goes with the cube, a 3.6-fold error in speed is a very large error in watts.
• Entering a turbine's rated peak efficiency as Cp without allowing for generator and inverter losses, or trying a value above 59.3%, which gives no result.