Solar Panel Efficiency

A few blogs back, I mentioned that I was not getting as much power from my panels as I expected.
My Dad emailed and suggested that given the warm temperatures I was seeing, he wondered if it that was the cause of the drop in output.
He also asked about the epoxy coating I added to repair the panels.
So, lets answer all his questions…. and ask a bunch more!

First up. The epoxy.
I tested the short circuit of the panels before I coated them. All were around 8.2 amps.
I also measured their open circuit voltages, all were around 35 volts.
After putting the epoxy on, they were exactly the same.
In my mind, the epoxy is a non-issue.

What a can of worms……
I found the panels spec sheet (such that it is) online.

groov solar panel data sheet questions

Ok, so lets break it down.
The red oval is the panel I have.
Now the numbers.
1. The characteristics of the panels are listed at an irradiance of 800w/m2 (more on this when we get to point 4) and a temperature of 20°C.
From my screenshot the other day, you can see that the panel is a LOT hotter than 20, so that might be a big factor. How much of a factor?

2. The spec shows that the panel output is normally 45°C. This is closer to what I am seeing… but hang on, why do they give the rated output at 20°C, but the nominal to be 45°C? Which is correct? Why the two numbers?
To make it ‘worse’, both are called the same thing…. “normal operating cell temperature”…. I am confused, how can both of those numbers be normal?

3. I read this to show the de-rating to be -0.4% per °C. So for every deg C over either 20°C or 45°C, I will lose 0.4% of my output?

4. Irradiance of 800w/m2. This is a number that is the amount of sunshine (all colors) that is shining on 1 square meter. (My panels are 1.6 meters square).
As it happens, the weather station at Opto has this value since it has a solar sensor built in.
My co-worker, and recovering software engineer, Mary wrote me some code to pull the number out of a text file on its FTP server and thus I have it in my PAC Controller and thus groov.
We are now trending irradiance on the same trend as the panels output.
groov solar irradiance
You can see that we only added the data point in the past few days, and that the marker is showing around the magic 800 mark.

So, bottom line. While I have no idea if it is 20°C or 45°C, I am collecting data and I am confident that I have all the necessary bits to work this out.

Pretty sure it will involve me standing on the roof with a garden hose water cooling my panels while collecting their data.

(Thats enough out of you lot!!).