The Availability Factor
When looking at figures for estimated annual energy output, it is typical to assume that they have been based on the wind turbine being operational all of the time. Unfortunately, in practice, a wind turbine will need to be serviced and inspected every six months so as to be sure that it is still in safe working order. Although servicing and inspection does not reduce total output by an appreciable amount, component failures or accidents (lightning strikes for instance) can disable wind turbines and leave them inactive for some time until repairs can be completed.
A very extensive meta-analysis of the statistics from a wide selection of wind turbines worldwide, shows that turbines from the best manufacturers will consistently meet 98% operational time. This means that the machines are ready to run for more than 98% of the time, but makes no judgment on whether they are not producing electricity due to lack of wind. The effect on total energy output would be less than 2% of the total, as wind turbines would never be serviced during peak operating conditions (high winds).
Such an amazing level of reliability is really quite remarkable. Especially when compared to many other types of machinery, even other technologies used for generating electricity. However the availability factor is usually ignored for economic calculations, since other variables, such as the unpredictable reliability of the wind, have a much greater impact on the output.