Here is a real world example. It is quite bit different than book numbers.
Wright-Hennepin Electric put up a Jacobs Wind Energy Systems 31-20 wind turbine which is on a 120′ tower and has 31′ diameter blades.
This HAWT has produced 5981kWh year to date as of today which works out to 18kWh per day this year.
The average wind speed in Rockford MN is around 5.3m/s.
So according to the book calculation with a cP of 25% this unit should put out 38kWh per day.
HWP = 0.5 x 1.23 x SA x WV^3 x 0.25
(HWP) Harvestable Wind Power (w/hr) = 0.5 x AD (air density kg/cu m) x SA (sweep area m^2) x WV^3 (wind velocity m/s) x cP (coefficient of performance Betz limit 59.26%, R^1 or Copper loss, linear power curve to cubic curve matching — 0.25 for most systems)
0.5 x 1.23 x 3.14 x 15.5 x 15.5 x 0.09260304 x 5.3 x 5.3 x 5.3 x 0.25 = 1599 Wh
1.599Wh x 24 = 38.4kWh per day.
But according to the meter the year to date out put is 5981 kWh works out to 5981kWh / (365 days – 35 days) = 18kWh per day.
At a cost of $100,000 it would only take 15 years to pay back if there was no maintenance.
$100,000 / 18.5 x 365 x $0.40 = 14.89 years
And that is at the inflated rate of $0.40 per kWh that Ontario is willing to pay.