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How solar panels work (for dummies, volts and amps)

sunshine_eggo

Happy Breffast!
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This post is WAY less sophisticated than one might think. This deals solely with voltage and amps and how they interact (or don't matter much) in low light conditions.

There have been a handful of posts expressing concern about panels in parallel not playing well together.

This is rarely a concern.

When AMBIENT light hits panels, their voltage increases rapidly even if the light is insufficient to produce any power.

To illustrate this, I disabled my MPPT this morning to capture what happens with the voltage:

1637426064325.png

The notation is the same minute as sunrise, so anything to the left of that is before direct light hit the panels. These are 3S 72S panels in series. They tend to run Vmp around 120V.

As you can see, before sunrise, even without direct sunlight on the panels, their Voc is above the typical Vmp.

What's the point?

During daytime, even a parallel string in an array in full shade will still be capable of a Voc at or above the Vmp of the parallel string(s) and thus allow the other parallel string(s) to perform at their MPP.

Even weak light gives volts. The intensity of the light allows the panel to to produce amps.

Here's what a morning looks like when the MPPT is working to find MPP as the sun rises:

1637426645459.png
 
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This post is WAY less sophisticated than one might think. This deals solely with voltage and amps and how they interact (or don't matter much) in low light conditions.

There have been a handful of posts expressing concern about panels in parallel not playing well together.

This is rarely a concern.

When AMBIENT light hits panels, their voltage increases rapidly even if the light is insufficient to produce any power.

To illustrate this, I disabled my MPPT this morning to capture what happens with the voltage:

View attachment 72974

The notation is the same minute as sunrise, so anything to the left of that is before direct light hit the panels. These are 3S 72S panels in series. They tend to run Vmp around 120V.

As you can see, before sunrise, even without direct sunlight on the panels, their Voc is above the typical Vmp.

What's the point?

During daytime, even a parallel string in an array in full shade will still be capable of a Voc at or above the Vmp of the parallel string(s) and thus allow the other parallel string(s) to perform at their MPP.

Even weak light gives volts. The intensity of the light allows the panel to to produce amps.

Here's what a morning looks like when the MPPT is working to find MPP as the sun rises:

View attachment 72978
What panels are you running? I'd love to see the datasheet and use this to help guide my project. Thank you!

Cheers.
 
Excellent. Your chart confirms 3x ~46 = 138 is about where the Voc settles. Any idea what the temps were that day?

Cheers.
 
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Given that it was December, it was pretty cold, but I don't recall. Would expect somewhere around freezing. My panels definitely fall somewhere in the -3% portion of the rating with regards to Voc.

I started with a 150/100 and switched to a 250/100. Regardless of the actual voltages, I see temps in the single digits, and it's too much to risk.

With my hardware I would be hesitant to ride the line so closely in your case; however, if your MPPT has some built in protection and can safely go above 150V without damage, then it may be okay.
 
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Given that it was December, it was pretty cold, but I don't recall. Would expect somewhere around freezing. My panels are definitely fall somewhere in the -3% portion of the rating with regards to Voc.

I started with a 150/100 and switched to a 250/100. Regardless of the actual voltages, I see temps in the single digits, and it's too much to risk.

With my hardware I would be hesitant to ride the line so closely in your case; however, if your MPPT has some built in protection and can safely go above 150V without damage, then it may be okay.
I think you made the right call. I'd rather be 50-66% of max on any hardware spec, but I already bought and long ago installed the MPPT, so for now it's what I'm going with. I typically don't see below 20F but it's possible I'll be there someday, so maybe I need to just play it safe and go with the N Peak 2's.

I just hate to give up ~7.5% output (compared to the Alphas) on the hottest days when I'll need it the most (a/c). And the higher wind load rating (4000 vs. 2400pa) makes me think the Alphas would better withstand the many stresses even a well-installed array will experience on a lifted 4x4 RV rolling down a bumpy jeep trail.

Good feedback though. Seriously appreciate it. Thanks again.

Cheers.
 
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