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PVModule model: graphs

In the PV module definition dialog, the page "Graphs" shows the general behavior of the model. You can display the following sets of curves:

  • Current versus voltage (often called "I/V curves")
  • Power versus voltage (often called "P/V curves"),
  • Efficiencies as a function of the irradiance.
  • Efficiencies as a function of the temperature.

These plots show sets of parametric curves—that is, for different irradiances, temperatures, series resistance, or shunt resistance—

Efficiencies as a function of the irradiance

This plot is used mainly to determine Rseries and secondarily for Rshunt.

The relative efficiency as function of the irradiance is highly dependent on the Rserie.

For crystalline modules, the efficiency at 25°C is used as a reference for determining Rseries:

  • When measured (according to IEC 61853), the shape of efficiency as a function of irradiance is well represented by the one-diode model. We choose the Rseries that best represents this curve.
  • when low-light measurements are unavailable, PVsyst uses the relative efficiency for the determination of the default Rserie value, by imposing a relative efficiency (with respect to STC) of -3% under 200 W/m².

I/V curves in realistic conditions

The option "Module temperature according to irradiance" aims to show module behavior in realistic conditions.

Typically, people display this I/V graph at different irradiances but for the same temperature.

This educational tool attempts to show real conditions under sunlight. When irradiance increases, module temperature also increases: here each curve shows a temperature corresponding to the PVsyst thermal model:

Tmod = Tamb + (1/U) * Ginc * Alpha * (1-effic).

The main objective of this graph is to show that under real conditions, the Vmpp voltage remains nearly constant regardless of irradiance.

For Voc, the temperature-induced decrease often outweighs the logarithmic increase due to irradiance.