I came across some yearsold discussion about calculating the temperature of the panels from the string voltage, based on TC of Vmp. Panel specs list Voc and Vmp, and the temperature coefficient of Voc, but not the temperature coefficient of Vmp. Is the temperature coefficient of Vmp something that can be obtained from the manufacturer, or is it being estimated based on the coefficients for Isc, Voc and Pmax?
temperature coefficient of Vmp?
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I’ve never through to use anything but the VOC since that is the highest voltage that would happen and that could possibly fry a MPPT.
I guess under hot circumstances when a panel is producing power at VMP where the voltage is lower, if not sized correctly, not enough voltage may be present to meet the minimum production requirements for an MPPT.Comment

J.P.M. You were party to these old discussions. Is there any chance you still remember?
It's been a while since I stopped playing with my array but I think I got a lot of good and consistent data.
Perhaps the best short answer to your question from what I found is that the average temperature coefficient of voltage for my operating array was always pretty close to 0.184V/deg K. The published open circuit voltage listed on the spec sheet for the panels (Sunpower E20/327's) is 0.176V/deg. K.
A sort of a precis of what I did:
Early on during my 6 year saga of measuring a bunch of array and weather data, I began by measuring array and weather variables on 34 very clear winter and 26 very clear summer days over 6 or so minutes on either side of the daily minimum solar incidence angle for my array. The results of these measurements was then used to find(estimate) a working coefficient of voltage for the similar measurements that followed over the next 6 years.
I cleaned the array with soap and water and rinsed with distilled H2O each morning.
A very important parameter I initially measured (but did not repeat for later maesurements over the years) was the panel temp. measured with a pretty good (or at least pretty precise) IR thermometer at 4 random places over the back of each of the 16 panels. Then, I went to the (string) inverter in the garage and wrote down voltages and currents.
BTW, I designed the roof mounted array with ~ 12+" of clearance between the roof deck and the bottom edge of the panel frames, partly to be able to get under the array to measure panel temps.
For these initial (60 days) and all subsequent measurement days on the array (~ 500 or so measuring days over the 6 yrs.) I then ran back up to the roof and repeated the measurement process with as much consistency as possible.
For these initial 60 measuring days, I mapped each average panel temp. for that day (that is, of the average of the 4 temps for each of 16 panels taken that day, and averaged all 16 panel average temps. (64 readings for both before and after the inverter readings, 128 total readings for that day) and averaged the whole bunch as being representative of the back of panel temp.
I adjusted each panel's average CELL temp. upward by use of a correlation from NREL (SAM) that accounts for irradiance and panel type but not wind heat transfer film coefficients.
Then, using the data, I compared average panel temp. to what voltage the inverter display showed at the middle point in the measurement process.
Along with that data I also measure and record weather data including irradiance at 1 minute intervals (1440 intervals/day) using a Davis Pro II+ located about 4 ft. north of the array centerline and about 12 " above the array highpoint.
The total summer and winter data described above each gave a mean coefficient of temp. for the array voltage as ~ 0.184 V/degree K. with std. dev. of 0.120 (for N=60) or, 0.148 for the winter data (N=34), and 0.068 (N=26) for the summer data.
Therefore, the difference between summer and winter average coeff. of temp as f(inverter shown voltage) was Zero.
I then used that coeff. of temp. for all subsequent measurements (the 500 or so additional data sets) as 0.184 V/deg. K.
there's a lot more to all this stuff I got tied up in than just the temp. coeff. of voltage. One of the other aeras I looked into was array fouling and its rates and its effect(s) on performance.
Lastly, for a concise and to the point discussion of PV operating and theoretical cell temp., see chap. 23 of Duffie and Beckman (ISBN # 0471510564, John Wiley & Sons, 1991), particularly, sec. 23.3 (p. 779 of the 2 ed.). That chap., even though the book is mainly about solar thermal, is probably about the best, brief but still detailed technical introduction to PV I've seen. The book is online as a PDF.Last edited by J.P.M.; 10242024, 08:45 AM.Comment

J.P.M. Thanks for the detailed explanation. I'm not quite so ambitious to go to the same lengths as you to calculate the TC as you did.
The 184/176 ratio for Vmp/Voc is helpful as a comparison point to my attempts to interpolate a value based on the available specs I have.Comment

J.P.M. Thanks for the detailed explanation. I'm not quite so ambitious to go to the same lengths as you to calculate the TC as you did.
The 184/176 ratio for Vmp/Voc is helpful as a comparison point to my attempts to interpolate a value based on the available specs I have.
Regards,Comment
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