How to plan a photolvotaic system

In practice, the planning of a photovoltaic system is based on software tools that allow to calculate predictions of the produced energy taking into account quantities such as module specifications, weather predictions and expected irradiance. Such caluclations allow investors and operators predict and assess the profitablity (return on investment).

In those calculations, the nominal power output according to the specifications of the manufacturer is used. However, the nominal power output may substantially differ from the true power output. Since some PV modules of a shipment are usually underperforming, this requires overperforming modules to be delivered, in order to reach, on average, the planned power production. Some manufacturers therefore sell positively selected modules. To summarize, the quality of the delivered modules in terms of the real power output at the time when the PV system is installed has a hight impact on energy produced by the photovoltaic system as well as the return on investment.

The real power output can only be measured in a photovoltaic laboratory and cannot be calculated from the data sheets. Of course, such a laboratory inspection cannot be based on all modules but has to be based on a small sample.

PVStatLab proposes the following 5-step approach:

1. Plan your PV system using software tools such as PVSYS or PVSOL.
2. Agree with the manufacturer on the quality parameters as well as parameters defining the statistical properties of the decision procedure.
3. Apply APOS to calculate the number n of modules one should check in a lab.
4. Measure the performance of modules from the sample.
5. Use APOS to decide whether the shipment should be accepted or rejected.