PNom Ratio - DC:AC

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—that is, excess power that may be achieved under certain temperature conditions (when the inverter is not too hot). This "bonus" is not included in the PNom ratio definition used by PVsyst. During sizing and in reports, PVsyst refers to PNom as the nominal power of the inverter—that is, the power usually mentioned in the model name.

Because the PNom ratio is used as an indicative parameter when sizing a system, this definition may cause confusion. The common PNom ratio for avoiding significant overload losses is 1.25 to 1.3 (see the distributions study). If the inverter operates at higher powers, this assumption is no longer valid.

When checking PV array sizing, PVsyst evaluates overload loss in a simplified way (histogram of available array power) using PNom. This evaluation cannot use PMax because inverter temperature is not known at this stage, so overload results may be overestimated. The only way to obtain reliable overload loss is to perform the complete detailed simulation.