How do we know when the wind is going to blow?

No one can accurately predict the weather or wind levels in the long term. So we use estimations when calculating how much a wind farm may generate in the future. The generation from a wind farm is estimated using ‘P values’ – typically P90, P75, and P50. The P value indicates the confidence of exceeding a given generation level on average over a 10 year period.  

There’s a 90% chance the average generation over 10 years would be greater than the P90 level.  So there’s a 10% chance it would be lower over 10 years.  Of all the P values the P90  is the lowest, most conservative estimate.  

The P50 indicates the generation expected on average to be exceeded over 10 years with a 50% probability.  It is the highest of the P values.  The P75 is in between.  

It’s important to remember that the P values show averages over 10 years.  Within those 10 years there will be some years that are higher and some that are lower than the level.  

Based on wind data, typical monthly generation estimates can be extrapolated from these 10 year values, as shown below. Extrapolating annual or monthly data from the P values would no doubt make a mathematician’s eyes roll, but it does give a rough  indication of likely levels. 

Graig Fatha’s generation

October’s generation exceeded even the most optimistic P50 estimate. It generated enough power that month to power an EV to drive to the moon and back 4 times. January 2023 was Graig Fatha’s highest generation to date, generating 793 MWh of electricity. The most recent generation in May was also a fantastic month, exceeding the P50 estimation. As you can see from the chart, some months have been above the P50, some have been below the P90.  Such variation is to be expected in individual months across the 10 year period the P values are based on. 

Wind speeds in winter tend to be higher compared to those in summer. So as you can see, generation between October 22 and March 23 was much higher. As expected it’s now starting to decrease as we come into the summer months.

Total generation has now reached 6760 MWh. This cumulative generation is just above the P90 estimate (again, with a mathematician’s eye roll). You can track Graig Fatha’s generation here.

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