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Definition in PVsyst.

The PNom ratio is defined as the ratio between  the PV array nominal power (PNom STC [kWp]) and the inverter's nominal power PNom [kWac].

This ratio is often named DC:AC ratio.

PNom and PMax

Many inverters now define a PMax value, i.e.  an excess Power which may be attained in some temperature conditions  (when the inverter is not too hot). This "bonus" is not taken into account in the PNom ratio definition of PVsyst. PVsyst always refers to the PNom as the nominal power of the inverter, i.e. the power usually mentioned in the model's name.

As the PNom ratio is used as an indicative parameter when sizing a system, this definition may lead to some confusion. The common value of the PNom ratio admitted for avoiding significant overload losses is 1.25 to 1.3  (see the distributions study). If the inverter operates at higher powers this hypothesis in no more true.

When checking the sizing of the PV array, PVsyst tries to evaluate the overload loss by a simplified way (histogram of array available power) involving the PNom. This evaluation cannot use the PMax, as the inverter temperature is not known at this stage. So that the overload result may be overestimated. The only way of getting a reliable overload loss is the perform the complete hourly simulation.