How to account for climate variability when determining PR under P90 conditions?

[Fabian.HOQ]

How should the PR P90 be determined in PV energy yield assessments? Specifically, what considerations should be taken into account regarding climate variability? Should the variability of the solar resource affect both the numerator (delivered energy) and the denominator (theoretical energy), as it would in real measured PR calculations, or should it only affect the numerator, as some simulation methodologies assume?

[Michele Oliosi]

Assuming that the statistics of the yield of a PVsyst are only influenced by the climate, as a first approximation, the PR will not change. This is because as a first approximation, the yield is proportional to the average irradiance (from which the climate variability is taken).

However, at second order there will be other effects, that are too complicated to track down. Indeed, there is no definite definition of a P90 year useable for a simulation, only the statistical meaning that the yield should be better 90% of the times. I.e. you cannot exactly pinpoint whether this or that combination of entries in the time series has led to the reduction of the yield.

However, since the very definition of P90 production also assumes a linear relation between irradiance and yield, it should be okay to consider the PR as constant.