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Performance ratio and efficiency per cells area

Nicolas A

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Good morning M. Mermoud,

I have done some simulations on PVsyst 6.35 to calculate PR and I am surprised of a result: Sunpower E20 327 gets a better PR than Sunpower X21 345 for the same experimental protocol. Anyway, X21 should be better than the E20, but here X21 has got more irradiance losses than the E20.

One explanation might be the influence of "efficiency per cells area" for PR calculation. Indeed, it values 82% for Panasonic HIT-N245 and only 23.44% for sunpower 327 and 22.3% for sunpower 345. Is it the reason of the PR difference I have explained before ? By the way, this characteristic is "N/A" for SolarWorld 255 and 280, so is it mechanistically considered equal to 100% in this case ?

Thanks a lot for this software and for your support.

Yours sincerely,


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The PR doesn't depend on the STC efficiency of the modules in any way, as it is related to the nominal power PNom.

See our FAQ How is calculated the PR?

However it may be affected by the low-light relative efficiency, which depends on the parameters chosen for the PV module (namely the Rserie, see the FAQ How should the Rserie be specified?).

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Hello, M. Mermoud,

First of all, thanks for you quick and accurate answer.

Indeed PR calculation is specific and I have used EarrRef and Earray values for it, not EarrNorm.

I am still surprised of the PR value, because the Sunpower X21 345 is supposed to be great for low-light efficiency.

Moreover, the Rserie resistance itself is lower for the X21 345 (0.39) than for the E20 327 (0.435).

I might have not understood some parameters and I still not understand this PR issue (-0.5% for X21, for two simulations, in Cadarache and Freiburd).

Yours faithfully,


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The values of the parameters (Rserie and Rshunt) in the database have been set by Sunpower people.

Now the SPR-X21-345 module has a very low Rserie = 0.39 ohm, leading to bad low-light performances (relative efficiency -1.1% at 600 W/m2, and -5.4% at 200 W/m2).

When the SPR-E20-327 has a higher Rserie = 0.54 ohm, leading to better low-light performances (relative efficiency -0.3% at 600 W/m2, and -3.1% at 200 W/m2).

Therefore it is quite normal that the Irradiance losses of the X21 module are higher than the E20 module.

By the way, we can observe that these low-light performances (as stated by Sunpower for the PVsyst database) are much lower than the performances of all equivalent modern modules.

The new standard of PVsyst for the default value of Rserie is to set -3% at 200 W/m2, and this is a cautious estimation.

NB: The temperature coefficient stated for the X21 (-0.30%/°C) is significantly lower than for the E20 (-0.38%/°C). This compensates a part of the Irradiance loss effect.

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