P50 and P90

An energy yield assessment does not produce a single number, because wind resource is variable and the estimate itself carries uncertainty. It produces a distribution, described by exceedance probabilities. P50 is the central estimate, the level a site should exceed in half of all years. P90 is the conservative case, the level it should exceed in nine years out of ten.

The gap between them is a statement about confidence rather than about the wind. It widens with short measurement campaigns, sparse met mast coverage, complex terrain, and uncertain wake modeling, and it narrows as a site accumulates its own operating record. Lenders size debt against P90 and owners plan against P50, which is why the two numbers appear together in every project financial model.

Once a site is operating, the estimate can be tested. Several years of measured annual energy production either sit inside the predicted band or do not, and a persistent shortfall against P50 is either a resource assessment that was optimistic or a fleet that is not delivering what it should. Those are very different problems with very different remedies.

Separating them is exactly the question an owner needs answered before a refinancing or a sale. A shortfall explained by recoverable underperformance is an operational opportunity; a shortfall explained by the wind is a permanent revision to the asset's value.