To assess the solar resource or energy yield potential of a site we model the solar resource/energy yield using best available information and methods. The resulting estimate is the P50 estimate or in other words the “best estimate”.
P50 is essentially a statistical level of confidence suggesting that we expect to exceed the predicted solar resource/energy yield 50% of the time. However, this may represent too much risk for some investors. Therefore other probabilities of exceedance values such as P90 (exceeded 90% of the time) or P75 (exceeded 75% of the time) are considered.
Lenders and investors typically use P90 estimates to be confident that sufficient energy will be generated, allowing to safely repay their project debt.
Why is TMY energy production higher with P50 compared to P90?
When selecting months for TMY P50 generation, we try to choose months from multi-year time series that are close to the monthly averages.
When selecting months for TMY P90, the yearly sum will constitute a more conservative, lower value. However, all months in the TMY P90 dataset need not have lower values than the long-term average monthly value.
Indeed in real situations, if we look at real data with low yearly irradiation conditions, it is almost impossible to find a year when all months are lower than the long-term average (the only exception can be year following huge volcano eruptions such as the Pinatubo eruption).