Pnom and Pmpp(STC) definitions
NB: The Maximum Power under STC (or the nominal efficiency) may be slightly different between the 3 concurrent definitions:
| The Nameplate value | Pnom is the nominal (commercial denomination) of the module, which is the basis of the definition of the installed "Nominal Power" of the system. The nameplate value is used for the estimation of the PR (performance ratio). |
| The Impp*Vmpp | as specified by the manufacturer at STC may be different of PNom. This is a problem in the Manufacturer's specifications. Recently, some manufacturers begun to include the positive sorting in this STC specifications. That is, for a given nameplate module, they define STC Vmpp*Impp values superior by 2-3%. This of course distorts the interpretation of the PR: it produces an artificial increase of the PR, which is not acceptable. In the PVsyst database, we now check that the Imp*Vmp product matches the PNom value within 0.2%. If not we modify either the Vmpp or the Impp or both for getting PNom. This may be a reason why the PVsyst data don't exactly correspond to the datasheets. |
| The Modelled Pmpp | calculated by the one-diode model (see for example "internal model results tool") may be higher than the specified Impp*Vmpp. The reason is that the model is established for passing the I/V (or P/V) curve through the 3 specified points (0,Isc), (Vmp, Imp), (Voc,0). Now, nothing ensures that the specified (Vmp, Imp) point is the true maximum of this P/V modelled curve. If the true maximum is different, it will of course be higher ! The modelled Pmpp is used as starting point for the evaluation of the array losses in the PVsyst loss diagram. Therefore the PR calculated from the array loss diagram may be slightly different from the PR shown on the results. This last effect may be considered as a weakness of the one-diode model: the model doesn't reproduce the exact I/V curve shape of the manufacturer. But this can also indicate that the manufacturer's specifications are not quite correct. |