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Old 11-19-2018, 04:12 PM   #1
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Solar Panels and Temperature

I have noticed that many people equate solar panel and battery performance when it gets cold. While lead acid battery performance degrades as the temperature drops, solar panels do the opposite.

For this discussion, I chose the Renogy 100W, Monocrystalline, 12V panel as it seems to be very popular.

All solar panel manufacturers rate their panels based on STC (Standard Testing Conditions). The only spec we are concerned with is the STC Temperature; which is 25C (77F) panel temperature. This is NOT the ambient air temperature. The wattage (Pmax) of the panel, in this case 100W, is measured at 25C.

Panel manufacturers provide temperature coefficients which indicate how their panels perform as the panel temperature rises above or falls below the STC temperature. For the Renogy panel they are:

Temperature Coefficient of Pmax -0.44%/ēC
Temperature Coefficient of Voc -0.30%/ēC
Temperature Coefficient of Isc 0.04%/ēC

Pmax is the wattage of the panel.
Voc is the Open Circuit Voltage of the panel.
Isc is the short circuit Amperage of the panel.

In the below graph I have changed the temperatures from Centigrade to Fahrenheit for better understanding. All the lines cross at 100% and 77F (The STC rating of the panel). But, depending on how hot or cold the panels are the Pmax can either increase to 120% or decrease to 80% of the rated value. So your 100W panel could produce as much as 120W or as little as 80W depending on the temperature. The voltage and amperage perform the same way.

Pat
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Old 11-22-2018, 10:39 AM   #2
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"So your 100W panel could produce as much as 120W or as little as 80W depending on the temperature. The voltage and amperage perform the same way."

Isn't is more like you can get a 20% increase in power when cold?
I never get the rated output.
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Old 11-22-2018, 11:43 AM   #3
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Hi

How well your controller handles variations in voltage is very much a "that depends" sort of thing. A good controller is not a lot of money (as a percentage of the total bill) so, get a good controller.

Even with a good controller, roof mounted panels will be getting sunlight at a lower angle in the winter (when it's cold) than in the summer. Thats not going to help you get max output power. Also as very clearly noted above, the panel temperature is what matters. If you *do* get a lot of sun and solar gain takes over ... there goes your cold weather advantage.

What you want is a very clear day, a wide open location, high winds, and a trailer that's tipped a bit to the south ....

In reality, the flip side of this is probably more worth noting. On a calm day with really hot panels on the roof, power and voltage drop. If your controller is ... errr .... less than ideal, you may see a falloff from that on top of the power drop.

Bob
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Old 11-27-2018, 08:25 AM   #4
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Quote:
Originally Posted by mythbuster88 View Post
I never get the rated output.

Assuming your panels were at 77F or lower, that's because of the other two STC test constants, solar irradiance and the air mass spectrum. My post was written to offer a simple explanation of how temperature effects solar panels. And, to demonstrate the importance of keeping your panels a cool as possible during any lighting conditions.


AMSolar has a good explanation of STC and how it is used to rate solar panels. The URL is: https://amsolar.com/diy-rv-solar-ins...dpanelratings/


If you really want to get into the mathematical weeds, check out this URL: https://en.wikipedia.org/wiki/Solar_irradiance


Pat




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