Wind Power Calculator can be used to calculate available power from a windmill
Available power in the wind can be expressed as
Pa = 1/2 ξ ρ A v3
where
P = power (W)
ρ = density of air (kg/m3)
ξ = efficiency of the windmill (in general less than 0.4, or 40%)
A = area wind passing through perpendicular to the wind (m2)
v = wind velocity (m/s)

Be aware that the density of air decreases with temperature and altitude and that the major factor in power generation is wind velocity.
a 20% increase in the wind velocity - increases the power generated with 73%

The energy generated by a windmill depends on the 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 cut-in speed to the shut-down speed) multiplied with the no. of hours the wind blows at the actual speeds.

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*This is a universal app that work for both iPhone and iPad.

ScreenShots:

Get Wind Power Calculator for iOS from Itunes App Store