comparing PV panel performance

[arborlinden]

Being new to PV and trying to understand my new installation I have been looking at the contribution each panel has made to the cumulative output.
18 identical panels obviously can't be expected to have exactly the same output but I ask if any member has calculated the variation between sibling panels on the same string with no detectable shading or positioning differences?
My 18 panels contribute approximately 1/18 the total output this being around 5.55% each of total. The worst performing panel over the short life of the system (21 days) has contributed .93% less than the best performing panel.
I've no idea whether that is good or bad and would welcome comparison figures from other systems and the views of experienced members

[DanKegel]

JPM has done some serious measurements, see

http://www.solarpaneltalk.com/showthread.php?22057-The-dirt-on-my-panels-after-6-months&p=163608&viewfull=1#post163608

He wrote, for instance:

6.) I'd estimate, as have others I've studied who do this sort of thing for a living, that the biggest uncertainty is the panel temp., with a good portion, but not all of that temp. uncertainty due to the variability of the wind vector. As one may suspect, the measured temps. of each panel are slightly different, with "upwind" or leading edge panels being cooler than downwind panels. My IR ther. claims +/- 1 deg. C. accuracy. The panel temps. can vary as much as 5-6 deg. C from one side of the array to the other, less in light wind, more in stronger wind, not much at all in still air and all over the place in gusty/variable wind. Adjacent panel temps. are usually within about 1-1.5 deg. C. of one another.

So think about temperature differences, and look at your panels' datasheet to see how temperature affects output.

He also cautions that measuring to within 1% is more difficult than it sounds.

[Naptown]

What you are seeing is panel mismatch. All panels have a power tolerance.eaning that they will produce under test conditions + - a percentage of the nameplate wattage.
Don't ever expect except on rare occasions to ever see nameplate wattage.
Further the monitoring is only generally accurate within about 5%
What you are seeing is within accepted parameters.

[J.P.M.]

JPM has done some serious measurements, see

http://www.solarpaneltalk.com/showthread.php?22057-The-dirt-on-my-panels-after-6-months&p=163608&viewfull=1#post163608

He wrote, for instance:


So think about temperature differences, and look at your panels' datasheet to see how temperature affects output.

He also cautions that measuring to within 1% is more difficult than it sounds.

I have a string inverter. So, I have no idea of what individual panel power output might be as I'm measuring/recording things. As Naptown notes, Panel mismatch is one reason power outputs will be different between panels. It sounds like the OP has a micro inverter equipped system.

What I'm doing has little to do with this thread. I'm trying to understand how and at what rate my panels may foul, and perhaps get a bit of insight to some of the mechanisms that may contribute to the nature and the rates of that fouling.

These attempts at learning are done for my own reasons/enjoyment. I put updates on this forum for those who may be interested in the information. That information has little, if anything, to do with this thread.

FWIW, measuring to 5-10% on an absolute basis is probably a pipedream. I'm mostly looking for patterns and to see if I might be able to spot some trends and adjust the method in response.

[posplayr]

Being new to PV and trying to understand my new installation I have been looking at the contribution each panel has made to the cumulative output.
18 identical panels obviously can't be expected to have exactly the same output but I ask if any member has calculated the variation between sibling panels on the same string with no detectable shading or positioning differences?
My 18 panels contribute approximately 1/18 the total output this being around 5.55% each of total. The worst performing panel over the short life of the system (21 days) has contributed .93% less than the best performing panel.
I've no idea whether that is good or bad and would welcome comparison figures from other systems and the views of experienced members

I'm not quite understanding the percentages. Are you saying that the spread in measured performance is .93/5.55=16.7%?

Based on sample size that to be approximating a 2 sigma distribution spread then you have 4.2% (1 sigma) unit to unit variation.

I don't know what panel you are talking about so I just grabbed a spec sheet from here

http://pdf.wholesalesolar.com/module%20pdf%20folder/CHSM6612P-305_Specs.pdf?_ga=1.95436063.269488771.1434675899

It shows a 46 +/-2 degC operating temperature variation (figure 2 sigma)

Based on the MPP temperature sensitivity of -0.45%/DegK you could expect +/-0.9% (2 sigma) variation per this manufacturers specs.

There is no other manufacturer specification on variability at least for this product (assuming others are similar) and there is a gap of almost 10:1 in 4.2% variation measured and .45% quoted.

If it is a temperature variation then expect a 10 degreeC variation for 1 sigma.

Go get a temperature gun and measure the variations in panel temperature to see if that would explain some portion of what is going on.

[sensij]

@posplayer
you might want to try reading that spec sheet again. Those panels have a +/-3% STC power tolerance. Many of the better panel companies will have +X W / -0 W tolerance.

@OP
Here is a link to my public SolarEdge page. I have about 2% difference in lifetime (~3 mo) production between my best and worst panel. I don't really have much shade right now, but for a few weeks in there I had some evening shade that would hurt a couple of the panels more than others. Panel power tolerance and thermal effects probably explain most of the observed difference, or possibly it is just calibration error in the optimizer data. SolarEdge is not really accurate better than +/-5%.

If your installer gave you the ability to run reports, you can dive off into the weeds look at the difference in panel production throughout the day. It should help you see if the production difference is consistent (suggesting calibration or mismatch), or varies as a function of time of day, temp, wind, or the other variables that might affect it.

[posplayr]

@posplayer
you might want to try reading that spec sheet again. Those panels have a +/-3% STC power tolerance. Many of the better panel companies will have a +3% or +5% tolerance, with a -0% lower limit.

Are you referring to this?

* Measurement tolerance +/- 3%

My assumption is that there is a 3% tolerance in the measured data on an absolute basis, but not that this would infer a 3% STC production difference panel to panel. Or is it industry norm to state unit to unit output variation as a "measurement tolerance"?



There is another spec I don't understand; can you explain?

Standard sorted output 0/+5 Wp

@OP
Here is a link to my public SolarEdge page. I have about 2% difference in lifetime (~3 mo) production between my best and worst panel.

Ironically your 2% total spread corresponds very closely to the +/- 2degC variation quoted in the data sheet and is much lower than the OP variations.

[J.P.M.]

@posplayer
you might want to try reading that spec sheet again. Those panels have a +/-3% STC power tolerance. Many of the better panel companies will have a +3% or +5% tolerance, with a -0% lower limit.

I'd also add that the NOCT mentioned has very little to do with actual operating temp. or it's variation under actual, non steady state conditions. It's a rating parameter applicable to specific conditions.

IMO, and for the OP's use, what the OP is seeing and reporting, and as Naptown seems to be writing, doesn't seem unusual.

[sensij]

I don't think the OP was implying 16% difference in the best and worst panel.

Those astronergy panels are not what I think most would consider "tier 1", and the OP's are likely to be better. The 0/5 W sorting, especially with the STC powers they offer, would generally mean that a panel testing out at 293 W would be sold as a 290 W panel, while a panel testing out at 296 W would be sold as 295 W. However, I think the +/-3% "measurement tolerance" builds in some wiggle room that would make it hard to prove that a panel didn't meet the rated STC, unless it could be measured more than 3% (of the STC rating) out of the -0/+5 W range.

[posplayr]

I don't think the OP was implying 16% difference in the best and worst panel.

Well that is what I'm waiting to hear back, it seems like a lot but it seems to be what was posted.

Those astronergy panels are not what I think most would consider "tier 1", and the OP's are likely to be better. The 0/5 W sorting, especially with the STC powers they offer, would generally mean that a panel testing out at 293 W would be sold as a 290 W panel, while a panel testing out at 296 W would be sold as 295 W. However, I think the +/-3% "measurement tolerance" builds in some wiggle room that would make it hard to prove that a panel didn't meet the rated STC, unless it could be measured more than 3% (of the STC rating) out of the -0/+5 W range.

Thanks for the explanation, and makes this even more interesting . The 5kW binning would imply a 0-5 kw uniform variation on the output of similarly rated panel. Nothing to do with quality of the panel. If you recall any quantization theory, then that implies a delta/sqrt(12) standard deviation in output power, which in this case would be: 5/300/sqrt(12)=.48% variance in binned panels based on production tests selection.

The min to max output for 20 odd panels would be +/- 2 sigma or 4*.48 approx 2% min to max. That matches your numbers again.

I don't think you can take the +/-3% tolerance CYA foot note and infer nearly that much unit to unit variation. In fact the 5Kwatt binning to 5Kwatt resolution (not accuracy) guarantees that even if the measurements are no better than +/-3% absolute.

[sensij]

I don't think you can take the +/-3% tolerance CYA foot note and infer nearly that much unit to unit variation. In fact the 5Kwatt binning to 5Kwatt resolution (not accuracy) guarantees that even if the measurements are no better than +/-3% absolute.

That would only be true if the same measuring device with a consistent calibration were used to measure all panels. Based on what is written in that data sheet (which may have nothing to do with the OP's panels), it would be possible for a true 303 W panel (measuring 3% low) and a true 282 W panel (measuring 3% high) to end up in the same 290 W bin.

While it may very well be just a CYA footnote, the data sheet allows it, and only wild guesses about what the actual calibration and panel distribution might be would allow one to narrow the range down more than that.

[posplayr]

That would only be true if the same measuring device with a consistent calibration were used to measure all panels. Based on what is written in that data sheet (which may have nothing to do with the OP's panels), it would be possible for a true 303 W panel (measuring 3% low) and a true 282 W panel (measuring 3% high) to end up in the same 290 W bin.

While it may very well be just a CYA footnote, the data sheet allows it, and only wild guesses about what the actual calibration and panel distribution might be would allow one to narrow the range down more than that.

Is your 2% your number you reported a wild guess?

With all due respect your interpretation of the +/-3% measurements error and how it might effect the performance of any particular group of binned panels would be creative (verging on indefensible) specsmanship.

[sensij]

Is your 2% your number you reported a wild guess?

Thanks for motivating me to learn how to use the ignore feature. Peace.

[posplayr]

Thanks for motivating me to learn how to use the ignore feature. Peace.

I guess the wells runs dry.

[posplayr]

I'd also add that the NOCT mentioned has very little to do with actual operating temp. or it's variation under actual, non steady state conditions. It's a rating parameter applicable to specific conditions.

IMO, and for the OP's use, what the OP is seeing and reporting, and as Naptown seems to be writing, doesn't seem unusual.

We still have not heard back from the OP if he is really seeing 16.7% min to max spread, but the binning performance would imply much lower unit to unit variation (10:1) and indicating either a combination of measurement errors or environmental to get to 4.2% 1 sigma.

OP says all panels are on one string; not sure how he determined individual panel performance.

OP says there are no other discernible physical differences, don't know if he took a IR thermometer to the panels.

If it is not unusual, and there are no marked thermal differences then it would appear to be erroneous measurements?

BTW the Binning resolution indicates the same NOCT variation through the specified sensitivity. -%/DegK