The Pmpp value calculated by the one-diode model in PVsyst may be different from the Nameplate value.
Indeed you have 3 values which may all be different:
- Pnom (nameplate),
- specified Vmpp*Impp at STC,
- Vmpp*Impp as computed by the model at STC.
- The Nameplate Pmpp is sometimes not equal to the Impp*Vmpp as specified by the manufacturer at STC.
Since recently, some manufacturers include the positive sorting in the STC values. That is, for a given nameplate module, they define STC Vmpp*Impp values superior by 1-3%. This of course distorts the interpretation of the PR in the simulation results (produces an artifical increase of the PR). In the database of PVsyst, we don't accept a discrepancy (PNom - Imp*Vmp(STC)) higher than 0.2%, and we correct the Imp value in order to get the exact Pnom (some old modules may have not been corrected).
In the versions >= 6.39, you have an information when a discrepancy occurs. And the modules imported from PHOTON are corrected in this way.
- The Impp*Vmpp calculated by the one-diode model at STC may be higher than the Imp*Vmp specified value.
Indeed, 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 specidied (Vmp,Imp) point is the true maximum of this P/V curve. If the true maximum is different, it will of course be higher.
This discrepancy may be sometimes up to 1% !
This last effect could 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 more likely this indicates that the manufacturer's specifications are not correct! For establishing their datasheets, manufacturere usually avail of the mesurements of one set or identical modules (of very close powers). We don't know how they extend the (Vmp, Imp) specifications to the other power classes.
According to the matching of most modules, we consider that the one-diode model is reliable in this respect.
NB: in the PV module definition dialog, when pointing the mouse on the PNom value, all 3 values will be displayed.
Effect on the PR and the Specific production
The PR (Performance ratio) is related to the Pmpp(STC) value. The "official" definition (as shown on the results page) uses the Nameplate value PNom.
But if you evaluate the PR from the simulation results (e.g. the loss diagram), it will involve the Pmpp of the module at STC, as computed by the one-diode model. The PR will be increased in the same proportion as the ratio Pmpp (Model)/Pmpp(Nameplate).