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.