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

Graig Fatha generated 674MWh of electricity in October, the highest monthly generation to date. This month’s generation exceeded even the most optimistic P50 estimate. This is enough power for an EV to drive to the moon and back 4 times. 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. As we move into the winter months, generation from Graig Fatha is expected to rise significantly. 

Over the coming months we are likely to see even higher monthly generation levels.  December is estimated to be the month with the highest generation of the year.  Generation in December is more than double the generation in May. 

Total generation has now reached 3223MWh, enough electricity to power over 1100 homes for an entire year. This cumulative generation is just shy of the P90 estimate (again, with a mathematician’s eye roll). You can track Graig Fatha’s generation here.

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