Slightly different output values - Exact same Solar Panels

KnowledgeSponge

Is it normal for solar panels of the exact same manufacturing batch to have slightly different outputs?

If so, what is the acceptable variance / tolerance range?

Cariboocoot

Re: Slightly different output values - Exact same Solar Panels

All electronic components (which is what solar panels are) have tolerances of +/- X%. Typically it's 10%. Really good stuff may be 5%. Low priced stuff 20%. Resistors even have an indicator band for this.

When you put panels together either in series or parallel they are going to "even out" somewhat. In actual operation they rarely produce rated values anyway.

KnowledgeSponge

Re: Slightly different output values - Exact same Solar Panels

Cariboocoot wrote: »

All electronic components (which is what solar panels are) have tolerances of +/- X%. Typically it's 10%. Really good stuff may be 5%. Low priced stuff 20%. Resistors even have an indicator band for this.

When you put panels together either in series or parallel they are going to "even out" somewhat. In actual operation they rarely produce rated values anyway.

ok. thx.

I didn't go cheap on the panels. I bought German made Q-cell panels.
So far only one is off and it's about 7%

westbranch

Re: Slightly different output values - Exact same Solar Panels

when you buy a "140W" panel it can be anywhere from say +-5%, they all differ, so as low as 133W and 147W.
From the flash tests on mine they were all at or above the 140W mark...

Vic

Re: Slightly different output values - Exact same Solar Panels

My belief is that for PV Modules, almost all manufacturers do a 100% productlon line Flash Test on each/every module. This allows them to be graded by output after the manufacturing process is complete -- this would often be in addition to a 100% flash test of each/every PV Cell.

So, in the case of PVs, one would expect that the output values for the same model number PV would track very closely from one module to another, although at a significant reduction in actual output vs rated value, due to the heating effects of the sun's radiation.

Opinions, Vic

NorthGuy

Re: Slightly different output values - Exact same Solar Panels

I have two sub-arrays. Usually they're within 100W of each other (3%).

jaggedben

Re: Slightly different output values - Exact same Solar Panels

Nowadays many manufacturers (but not all) are promising -0/+5%.

Also keep in mind that unless you test panels under incidental conditions, (temperature, angle, etc.), you will get other factors that cause varying output. I recently did an install where we had two strings of the same size with different orientations. (We used a Power-One inverter with two separate inputs). When I tested the strings they had a difference in voltage of around 10%, and for a second I wondered if a whole panel was missing from one of the strings. But then I realized that one string was in much more direct sun at that moment, and the glass was much hotter to the touch. That explained why that string had a much lower voltage, and the system worked fine.

KnowledgeSponge

Re: Slightly different output values - Exact same Solar Panels

Could the relative humidity affect output?

30% vs 86% for example.

Cariboocoot

Re: Slightly different output values - Exact same Solar Panels

KnowledgeSponge wrote: »

Could the relative humidity affect output?

30% vs 86% for example.

Collectively (all panels) yes. Individually it had better not.
Humidity will affect the insolation.

inetdog

Re: Slightly different output values - Exact same Solar Panels

jaggedben wrote: »

Nowadays many manufacturers (but not all) are promising -0/+5%.

Also keep in mind that unless you test panels under incidental conditions, (temperature, angle, etc.), you will get other factors that cause varying output. I recently did an install where we had two strings of the same size with different orientations. (We used a Power-One inverter with two separate inputs). When I tested the strings they had a difference in voltage of around 10%, and for a second I wondered if a whole panel was missing from one of the strings. But then I realized that one string was in much more direct sun at that moment, and the glass was much hotter to the touch. That explained why that string had a much lower voltage, and the system worked fine.

Usually when the insolation (and therefore the temperature) are that different, one of them is producing far less current than the other and so the effect of the voltage imbalance into a single MPPT input would likely be small. If you have some relative power numbers, it would be interesting.

KnowledgeSponge

Re: Slightly different output values - Exact same Solar Panels

Cariboocoot wrote: »

Collectively (all panels) yes. Individually it had better not.
Humidity will affect the insolation.

I left out some info.....

If you tested one panel on Tuesday.....30% humidity, full sun...no clouds.

Then, you tested that SAME panel WEDNESDAY...86% humidity, full sun..no clouds....

Under those circumstances would it be likely or unlikely to see a difference in output?

Cariboocoot

Re: Slightly different output values - Exact same Solar Panels

KnowledgeSponge wrote: »

I left out some info.....

If you tested one panel on Tuesday.....30% humidity, full sun...no clouds.

Then, you tested that SAME panel WEDNESDAY...86% humidity, full sun..no clouds....

Under those circumstances would it be likely or unlikely to see a difference in output?

Every possibility that the same panel on two different days will give two different readings if some atmospheric condition such as humidity has changed significantly between the two days.

Humidity will scatter some of the sunlight, preventing it from acting on the panel (even though you may not notice it visibly).

Blackcherry04

Re: Slightly different output values - Exact same Solar Panels

You'd probably see a difference in 10 Minutes from the first reading to a second reading on the same day.

Cariboocoot

Re: Slightly different output values - Exact same Solar Panels

Blackcherry04 wrote: »

You'd probably see a difference in 10 Minutes from the first reading to a second reading on the same day.

Definitely: angle of the sun changes = panel output changes.
Same for temperature, high level atmospheric conditions, the Deathstar passing in front of the sun ...

BB.

Re: Slightly different output values - Exact same Solar Panels

To measure solar intensity... You can take a panel (or a small panel) and put it on a current meter (or a low ohm resistor and milli-volt meter) and measure the current.

Silicon solar cells are pretty linear with respect to solar input and their output current (within ~10% linearity as I recall).

Measuring Vmp/Imp is a bit more variable. For the most part, Vmp is a function of solar cell temperature and Imp is a function of solar intensity (assuming that you have weak sun rise direct sun on the panel at a minimum). If you don't have enough sun/light, then Vmp will collapse (low Voc).

-Bill

NorthGuy

Re: Slightly different output values - Exact same Solar Panels

BB. wrote: »

To measure solar intensity... You can take a panel (or a small panel) and put it on a current meter (or a low ohm resistor and milli-volt meter) and measure the current.

You can also purchase a light meter or luxmeter and measure insolation directly in W/m². If you get 1000, your panels should produce rated output (corrected for cell temperature). You will know right away if your panels produce what they are supposed to.

KnowledgeSponge

Re: Slightly different output values - Exact same Solar Panels

Ok, lesson learned.

I learned (like you guys said) that the SAME panel will change it's output depending on 1000 variables.

I took all my panels out today and one by one, tested them. They were all Exceptionally close in output.

But here's the clincher.....over the 30 minutes it took to test them, (I un-boxed and re-boxed them one by one because they haven't been installed yet) they slowly increased output.
This was around 11:30am. So the sun was still reaching it's maximum for the day.

The readings were.....

21.4v -- 5.62A
21.6v -- 5.58A
22.0v -- 5.64A
22.2v -- 5.75A

This told me the quality control was probably pretty good on these

Blackcherry04

Re: Slightly different output values - Exact same Solar Panels

I looked at my output for the day. The peak just doesn't last that long, it's up hill, a rest at the peak and down the other side. It was a little wiggly today, some days it will last a little longer. I put the time I broke 5000 watts so you can get a idea how long things last.

10:05 5051 Watts
12:55 6890 Watts
1:00 6915 Watts Peak
1:05 6907 Watts
4:10 5112 Watts

Attachment not found.
.

inetdog

Re: Slightly different output values - Exact same Solar Panels

KnowledgeSponge wrote: »

Could the relative humidity affect output?
30% vs 86% for example.

Other than high humidity air having a higher heat capacity and therefore maybe cooling the panels better for the same air temperature and wind speed, no. Did you measure the panel temp?
Once you get above 100% the rain or fog will interfere with light reaching the panels.

inetdog

Re: Slightly different output values - Exact same Solar Panels

Cariboocoot wrote: »

Humidity will scatter some of the sunlight, preventing it from acting on the panel (even though you may not notice it visibly).

Water droplets in the air will scatter light, as will dust particles, etc.
Water vapor (individual molecules) in the air will change the index of refraction and may absorb some frequencies of light more than dry air, but there will be no scattering.

Cariboocoot

Re: Slightly different output values - Exact same Solar Panels

I wish I were young enough to know everything.

solar_dave

Re: Slightly different output values - Exact same Solar Panels

Cariboocoot wrote: »

I wish I were young enough to know everything.

Reminds,me of,the,line, At 18 I was amazed at how dumb my old man was, and was amazed how much he learn by the time I turned 25.

inetdog

Re: Slightly different output values - Exact same Solar Panels

Cariboocoot wrote: »

I wish I were young enough to know everything.

Only Fizzics. And almost everything else. :-)
But I think I am starting to forget things. Not quite sure, of course.