Long term solar yield reassessment



In order to validate and further improve its own methodology, and in answer to an increasing demand for the reassessment of the yield for operational PV plants, 3E has been working on an extensive programme reassessing long term yield for more than 70 projects in operation (in FR and BE) representing more than 50MW.

The aim of such reassessment is to use the operational data to correct the mean expected value of the initial long term energy yield assessment. Additionally, and often more important, the additional data will allow for a reduced uncertainty on the mean expected value, thus improving the P90 assumption of the project.

Energy meter data as well as recalculated performance indicators are used to better characterise the real behaviour of the system(s). This extensive work is done using detailed operation and maintenance reporting of the plant(s) under consideration. Only plants with more than 12 months of representative data are eligible.

The long term yield trend is then readjusted using a reference yield period for each plant as well as a common source of past and present irradiation. The uncertainties affecting the calculations are updated to benefit from the accuracy of actual data.


The comparison of the reanalysis results versus the initial expected yield is highly variable in terms of mean expected yield, as this very much depends on the meteo sources, simulation parameters and assumptions taken in the initial study. The updated result can thus be lower or higher than the initial yield calculation. However, the uncertainties used to generate the probabilities of exceedance (P90 for example) can generally be significantly reduced, and thus will have a positive impact on the financial plan of the project.