Difference between PVSyst PR and PVWatts derate factor

[lucied]

Hello,

I use PVWatts as a preliminary production estimator and when we get into system design, we use PVsyst.

PVWatts and PVsyst both have a performance index - PVWatts uses the Derate Facor, PVsyst uses the Performance Ratio.

The PR includes the array losses (Shadings, PV conversion, mismatch, wiring resistance, etc) and the system losses (inverter efficiency in grid-connected, or storage/battery/unused losses in stand-alone, etc).

Unlike the "Specific energy production" indicator, expressed in [kWh/kWp/year], this is not directly dependent on the meteo input or plane orientation. It is related to the system quality, and allows the comparison between installations in different locations and orientations."

I thought PVWatts derate factor had the same definition, but the derate factor and the PR are usually different on a same project.

Can someone explain what the difference is between those performance indexes?

Thanks in advance!

[André Mermoud]

I don't know the exact definition of the Derate factor of PVWatts.

But finding a different value between both values doesn't mean that they are differently defined.

The PR of PVsyst involves the result of the full simulation.

If PVWatts performs a different simulation and gets a different result, its indicator will be different of course...