There are wind energy converters with a gearbox and without. Cause you said windmill, I assume you mean the classical wodden windmill (so I assume without a gearbox, there is nevertheless a transmission, especially with the classical windmill on a huge scale).
Therefore the formula is:
P_el = 1/2 * A * rho * v³ * eta (* i )
with being:
P_el = electrical performance (Watts)
A = Area (m²)
rho = density of the medium used for propelling (around 1,2 kg/m³ for air at 68 degree Fahrenheit)
v = velocity ( m/s)
eta = energy conversion efficiency (percent, no unit)
i = transmission (percent, no unit)
There is something called the Betz Coefficient. It says that, if the kinetical Energy of the wind is used to propell the mill, there is a limit for its usage, cause the wind would theoretically have to stand completly still after hitting a blade if the Energy conversion efficiency was to be 100%.
So according to Betz law, the maximum of usable Energy from the Wind is: 59,3 %. A classical wodden windmill has a energy conversion efficiency of maybe 10 - 30 %, depending on its configuration. Best way to measure it is to measure the electrical output it generates and then enter the numbers in the equation above and compare for the energy efficiency.
In your case, I would assume (with 10 feet blades being = 3 m in length and assuming 0,5 m in width = 1,6 feet , and an energy conversion factor of 25%) :
P_el = 1/2 * (3 * 0,5) m² * 1,2 * 10³ * 0,25 = 225 Watts
Here you can read in detail about it (article is quite good):
Wind turbine - Wikipedia, the free encyclopedia