##### P90/P95 or Average?

In a normal distribution dataset, 50% of the points will be below the average and 50% above. Using the average to assess the performance and return of a solar project is risky. The results have 50% chance to be below the average.

Solar developers and investors need greater certainty. For example, what is the minimum return that could be achieved 95% of the time over the project lifetime? Or what is the minimum annual cash flow that could be generated 90% of the time. This requires to calculate the *“exceedance probability”* (Pxx) i.e. the level of solar energy that would be exceeded x % of the time.

The following graph plot the cumulative probability of GHI solar data from the Cranbrook Airport Weather Station. The dotted blue curve plots the cumulative probability of annual data variations, the red curve, the combined variability of annual variations and monitoring errors, the yellow curve the 25-yr average variance. The purple dot represents the P90 level at one year, and the green dot P95 at 25-yr. It indicates a 90% chance that the GHI will exceed 1,090 kWh/kW/m2/yr in any articular year and 95% chance that the GHI 25-yr average will exceed 1,181.

These numbers can be used to make a solar project *“investment-grade”.*

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