GT String sizing calculator temperature compensation
The GT string sizing tool uses a compensating value of 35ºC above the user input temperature range.
Cell temp is always higher than ambient temp. If you've ever placed your hand on a solar panel on a sunny day, that's very evident. It's the cell temp that determines the change in Vmp, thus the need for the temperature compensation. It's our way of determining the lowest array voltage during the hottest part of the day.
The temperature difference that our calculator is using is fixed at 35C above ambient. This value was chosen based on several research reports stating that the average cell temp (above ambient temp) is 35C above ambient temp in a range of ambient temps and module mounting configurations.