Hot Weather Voltage Degradation

So, I understand the increase in voltage with temps below STC, but (and the answer is probably self explanatory) is there a similar “degradation” of voltage when temps go above the 25c mark?

Yes. The rating is also for cell temps, not ambient. With 80% of the solar energy being turned into heat, your cells are going to be north of 40°C in most cases.

Look at the panel NOCT or NMOT rating - more of a real world typical performance.

Panels actually producing their rated output is pretty rare, but these effects sorta normalize performance.

In summer, you get lots of sun, but the cells are hot so you don't get rated.
In winter, you get less sun, but the cells run cooler, so it lessens the impact of getting less sun (but still not getting rated).

For me, it's typically in cool spring or fall days where I get my best performance.

sunshine_eggo said:

Look at the panel NOCT or NMOT rating - more of a real world typical performance.

Click to expand...

I was trying to find out about STC rating vs NMOT rating, to determine my panel setup. To see Isc and Voc.
And found this post.
For my panels, Voc is 40.6 (NMOT) vs 43.1 (STC).
I suppose to determine how many panels can go on a string, I need to use the higher number, even if in the real world, I probably will just see 36V.

Isc is also quite different -- 8.4A vs 10.4A!

The linked article at thegreenwatt.com was very useful, thank you.

CaliSunHarvester said:

I was trying to find out about STC rating vs NMOT rating, to determine my panel setup. To see Isc and Voc.
And found this post.
For my panels, Voc is 40.6 (NMOT) vs 43.1 (STC).
I suppose to determine how many panels can go on a string, I need to use the higher number, even if in the real world, I probably will just see 36V.

Isc is also quite different -- 8.4A vs 10.4A!

The linked article at thegreenwatt.com was very useful, thank you.

Click to expand...

NOCT/NMOT is approximately 80% irradiance (thus the 20% reduction in current), and cell/module temperature is assumed to be in the 40-50°C range (varies by panel), so the 40.6Voc is likely what you'll see, not 36V, and yes, you definitely use STC for string Voc calcs because STC TEMPERATURE conditions can be seen in the real-world (sunrise where ambient temp is at a daily minimum). Remember that irradiance has little impact on panel voltage... 200W/m^2 Voc will only be about 1-2% below STC 1000W/m^2 irradiance.

sunshine_eggo said:

Remember that irradiance has little impact on panel voltage

Click to expand...

No, I cannot remember that. I did not know it. And that's very critical information to be aware of!!! Appreciate it.

CaliSunHarvester said:

No, I cannot remember that. I did not know it. And that's very critical information to be aware of!!! Appreciate it.

Click to expand...

Heh... I'll work on my wording for that one in the future. Here's an example of a typical solar panel voltage/current/power curve plt where I've sketched on the peak power point. This corresponds to Vmp.




As you can see, it only changes a very small amount.

An easy way to think of it is:

Light gives voltage (you have light, you have your voltage).
Intensity of light gives current (low light = low current, intense light = high current).

sunshine_eggo said:

Light gives voltage (you have light, you have your voltage).
Intensity of light gives current (low light = low current, intense light = high current).

Click to expand...

"any sunshine = panel zappy"

sort of captures the idea for me.

the concept is kind of unintuitive. usually expect zappy only when super sunny, but voltage and amperage are different..

human danger is morning or dusk, solar panel still seeing dim sky, human still think safe to touch

still enough voltage and current to hurt skin (mostly heart)

equipment usually does not mind..