Quick Answer — How Much Will a Small Wind Turbine Generate?
There are two ways to estimate annual output, and a domestic wind quote is only credible when they agree.
The nameplate route is what suppliers quote: Annual kWh = rated kW × 8,760 hours × capacity factor. A 5 kW turbine at a 30% capacity factor produces 5 × 8,760 × 0.30 = 13,140 kWh a year.
The physical route is what the rotor can actually extract: P = ½ × air density × swept area × power coefficient × wind speed³. A 5.5 m rotor sweeps 23.76 m², and at 6.5 m/s with a power coefficient of 0.35 that is 1.40 kW at the mean wind speed. Multiplied out across the year with a Rayleigh energy pattern factor of 1.91 — the standard correction for the fact that energy scales with the cube of speed, so windy hours count far more than the average — it gives 23,403 kWh.
Here the physics comfortably exceeds the nameplate figure, so 13,140 kWh stands. When it does not, the nameplate number was never achievable and the calculator says so.
The cubed term is the single most important fact about wind. Double the wind speed and the power goes up eightfold. A site at 5.0 m/s and a site at 6.5 m/s are not 30% apart in output; they are roughly twice as good.