Wind Power
Power generation from wind.
Theoretically power in moving air  or wind  can be calculated
P = ρ A v^{3} / 2
= ρ π d^{2} v^{3} / 8 (1)
where
P = power (W)
ρ = density of air (kg/m^{3})
A = wind mill area perpendicular to the wind (m^{2})
v = wind speed (m/s)
π = 3.14....
d = wind mill diameter (m)
Be aware that the density of air decreases with temperature and altitude and that the major factor in wind power generation is wind speed.
 20% increase in wind velocity will increase the power generation with 73%
The theoretical and rated wind power generation from a typical windmill is indicated in the "wind speedpower curve" below. Cutin wind speed, rated wind speed, shutdown wind speed and rated power for windmills with 20% and 40% efficiency.
Actual Available Power
Actual available wind power can be calculated
P_{a} = ξ ρ A v^{3} / 2
ξ ρ π d^{2} v^{3} / 8 (2)
where
ξ = efficiency of the windmill (in general less than 0.4  or 40%)
Example  Wind Power
The actual available power from a wind mill with diameter 1 m, efficiency 0.2 (20%)  with wind velocity 10 m/s  can be calculated as
P_{a} = (0.2) (1.2 kg/m^{3}) π (1 m)^{2} (10 m/s)^{3} / 8
= 94.2 W
Related Mobile Apps from The Engineering ToolBox
 free apps for offline use on mobile devices.
Online Wind Power Calculator
The calculator below can be used to calculate the available power from a windmill:
Generated Wind Energy
The energy generated over time depends on the wind mill potential power generation (as indicated above)  and how often, or how many hours the wind blows  or more scientifically  the "wind speed frequency distribution" at the actual location.
The total energy generated over a year can be calculated by summarizing the power generation for all velocities (ranging from the actual windmill cutin speed to the shutdown speed) multiplied with the no. of hours the wind blows at the actual speeds.
A typical wind speed frequency distribution diagram with energy produced at the different velocities is indicated below.
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