Irradiance loss

In PVsyst, the evaluation of "Losses" for a PV array (as defined in the normalized performance ratio) starts with the energy that would be produced if the system always operated at STC conditions (1000 W/m², 25°C, AM1.5).

The loss due to operating temperature (rather than 25°C STC) is well-known and widely referenced.

It is surprising that the loss due to irradiance level—which is of the same type—is rarely discussed. Look at the graphs of a crystalline PV module's behavior (in the PV module dialog, select "Graphs" > "Efficiency vs Irradiance"), and you will see that efficiency decreases at lower irradiances. This causes "Irradiance loss" (relative to 1000 W/m²). Therefore, irradiance loss is a direct consequence of the intrinsic behavior of PV modules, as described by the "one-diode" model.

In the one-diode model, the efficiency at low levels depends on two parameters:

• The Rshunt exponential behavior: as irradiance decreases, Rshunt increases exponentially (and the corresponding loss decreases).

The lower Rshunt  at STC, the more losses to be retrieved by this process, and therefore the higher "low irradiance" efficiency.

• The Rserie (series resistance) increases with the square of the current, and thus with power. If Rserie is high (poor), losses are higher at STC (or conversely, efficiency improves at low irradiance levels).

Therefore, "poor quality" modules (low Rshunt, high Rserie) perform best at low irradiance levels (compared to STC specification).

This can be understood from the opposite perspective: if you start with a module's performance at low irradiance (say, 200 W/m²), efficiency at higher irradiances is penalized by these losses, resulting in poor STC performance. However, in practice, you purchase based on the STC value.

This explains (together with the temperature coefficient) why amorphous modules show better productivity [kWh/kWp] than crystalline modules in Central European climates.