One of the most important risks when investing in the wind energy sector is the variability of the wind resource, compared to the forecast. To minimise this risk, energy yield assessment studies are performed by wind experts to provide the Net Yield and exceedance probability figures.

The aim of these next 2 Databytes is to understand the difference between all these terms in order to help you select the most appropriate figure when creating your business model.


The P50 figure is the level of production that is forecasted to exceed 50% of the project’s life. The P90 figure is the level of production that is forecasted to exceed 90% of the project’s life.

From wind measurements to P50 

When developing a wind farm project, it is important to collect on-site wind measurements over a minimum of a one year period. As no one can predict the wind over the next 20 years, the data measured at the meteorological mast over a short-term period are extrapolated to the future, using historical data. This data is provided by land-based stations or mesoscale models. The wind distribution issued from the long-term correction process is then extrapolated to the future wind turbine locations using numerical models. Finally, the wind resource at each turbine location is converted into production via the theoretical power curve of the selected turbine model.

The result from this whole process is called Gross Yield.

The Net Yield figure is derived from the Gross Yield by including the losses all over the project lifetime. According to the latest and best practices, these losses are; turbine availability and maintenance losses, grid and BOP availability, electrical losses, siting conditions, blade degradation and curtailment plans.

By Carla Vico, Technical Asset Manager