In the IEC 61400-12 standard “Wind turbine power performance testing”, the availability of a wind turbine is defined as: “the ratio of the total number of hours during a certain period, excluding the number of hours that the wind turbine could not be operated due to maintenance or fault situations, to the total number of hours in the period, expressed as a percentage” [1]. It seems to be a clear definition, however in practice all parties involved, and often commercially bounded by guarantees, cannot interpret it unambiguously. This has all to do with the fact that the actual measuring and tracking is not done in a structured manner, mainly because it is not clear what the various (sub-)states of a wind turbine can be, and which sub-states have to be considered as available or not, and also for discerning who is liable. Moreover, due to heavy operation conditions and potential unintended built-in weaknesses, the ultimate availability of a wind turbine measured over the lifetime of 20 years, can easily tail off by 10% to 25%. Directly related to availability, the return on investment (ROI) will even decrease with a higher proportional part. Consequently, availability is a serious issue for manufacturers and investors, and may have extensive commercial implications. To better understand and improve the availability, from the beginning of the design process, this article presents the ‘State Model for Wind Turbines’. It underpins the strategy to optimize the availability versus the total costs. Furthermore the article provides some practical tools for specifying, design-in and measuring of the final availability.

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