Income from turbines
Energy output from a wind turbine
If you have read the page on Annual energy output from a wind turbine, this graph will already be familiar to you. The graph shows how annual energy production in million kWh varies with the windiness of the site. With a mean wind speed of, say 6.75 m/s at hub height you get about 1.5 million kWh of energy per year.
As you can see, annual energy output varies roughly with the cube of the wind speed at turbine hub height. Just how sensitive energy production is to wind speed varies with the probability distribution for the wind, as explained in the page for Variability. In this graph we have three examples with different k-values (shape factors). We will be working with the red curve (k=2) in our example.
The availability factor
The figures for annual energy output assume that wind turbines are operational and ready to run all the time. In practice, however, wind turbines need servicing and inspection once every six months to ensure that they remain safe. In addition, component failures and accidents (such as lightning strikes) may disable wind turbines.
Very extensive statistics show that the best turbine manufacturers consistently achieve availability factors above 98%, i.e. the machines are ready to run more than 98% of the time. Total energy output is generally affected less than 2%, since wind turbines are never serviced during high winds.
Such a high degree of reliability is remarkable, compared to other types of machinery, including other electricity generating technologies. The availability factor is therefore usually ignored when doing economic calculations, since other uncertainties (e.g. wind variability) are far larger.
Not all wind turbine manufacturers around the world have a good, long reliability record, however, so it is always a good idea to check the manufacturers' track record and servicing ability before you go out and buy a new wind turbine.