Max panel string voltage - STC or NOCT values?

jjdh

Wholesale Solar has a guide for string sizing here.

In it, they say something that surprised me. They say, in order to calculate the max string output, you should:

1) Take the STC Voc (eg. the Voc measured at 25C), 48.08V in their example above
2) Take the temperature at NOCT, 44C for their example panel
3) Find the delta from the NOCT temperature and your minimum design temperature, 77.3C in their example
4) Use the panels Voc temp coefficient to derate the STC Voc by that delta

I guess my question is obvious:Why are they using the NOCT temperature (44C) but the STC voltage? I would have thought if you were derating the STC voltage you'd "start" at 25C, the STC temp, rather than 44C?

BB.

JJDH,

I agree with you... They did not start with Vmp-noct and Voc-noct... They only had Vmp-std and Voc-std where STD=25C.

Looking at the current (365 Watt panel) Mission Solar Data Sheet, they only list the NOCT temperature, not any of the other electrical values.

https://www.missionsolar.com/resources/#productsheet
https://secureservercdn.net/198.12.145.239/l7u.263.myftpupload.com/wp-content/uploads/2020/06/C-SA2-MKTG-0015-Data-Sheet-for-SQ9S.pdf

I do not understand why they did what they did (I may be wrong--But don't really see how).

-Bill