# Temperature Coefficient

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I wanted to get a little clarity on muVoc mV/°C vs %/°C. We have often done string sizing calculations by hand by using the %/C value on the data sheet but when using the same temperature values we receive a close but slightly different maximum Voc value.  For example:

STC 25°C with an absolute low of -14°C.

VOC Coef = -.0026V/C

string size of 18

Starting Voc of 49.8

(-14 - 25)(-.00265)(49.8)+49.8=54.95VocMax

54.95*18=989V

a -14°C system with a Voc Coef would indicate a max voltage of 983.33 for a string of 18 with an VOC of 49.8 and a VOC Coef of -.265% for a 25°C STC system. The pan file indicate a muVoc of -150mV/°C. When I use the Pan defined MuVoc value It indicates the max Voc at 994V, if I use the one diode model we see a max Voc of 999V. This isn't a problem with this solution as both methods are in are inverter range, but we have run into being a large enough discrepancy. Essentially, PVsyst indicates we should lower our string size, but the datasheet calc indicates we are in an acceptable range. I have tried to read the Module parameters section of the help section and it indicated the one diode can no longer be edited because its an R shunt and R series calc. Values which I wouldn't know where to start with. Any advice on this subject would be greatly appreciated on why these values aren't inline and if its a problem to proceed with the one calculation over the other would be greatly appreciated.

Pan defined MuVoc

Pan File

One Diode model

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The muVoc as result of the one-diode model is indeed not equal to the muVoc (beta parameter) of the datasheets.

When establishing the one-diode model, it is indeed impossible to specify simulatneously the  muPmpp and  the muVoc temperature coefficient.

By default, PVsyst uses the  muVoc result of the one-diode model for the sizing voltage limit.

However in the PV module definitions, you can specify the muVoc of the datasheets  (page  "Additional data => Secondary parameters", item "Specified mVoc).

And for using this value during the sizing process, you have to choose this in the project's settings, page "Design conditions => muVoc values"