I've heard that investors use the P90 of a wind resource when considering an investment. What kind of measure is it, what does it actually mean, and how is it calculated?

  • From this site: P90 denotes the level of annual wind-driven electricity generation that is forecasted to be exceeded 90% of the year.
    – THelper
    Commented Sep 28, 2016 at 12:18

1 Answer 1


From the Wind Measurement International site:

P90 is all about quantifying the uncertainty of annual energy yield predictions. P90 is the energy WMI predicts that a wind turbine is 90% likely to produce over an average year, given the uncertainties in the measurement, analysis and wind turbine operation. P50, on the other hand, is the average annual energy yield predicted for your site - the annual energy output you are most likely to achieve.

Investor decisions are commonly based on P90 (P95 or even P80 are sometimes used). Better instrumentation, long term data, analysis and wind turbine operation are all able to close the gap between P90 and P50, so increasing your project value.

  • What does that mean though? 90% likely to produce and exceed? Or 90% likely to be no greater than. (And it's unclear from that as to which average P50 is? Mean, median, mode, or something else?)
    – 410 gone
    Commented Sep 28, 2016 at 17:36

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