Any explanations for production loss at solar noon?

TL;DR: Is this reduction in peak power a result of my panels getting hotter? I didn't check voltages, so don't know if there was clipping going on at the inverter level. We have another glorious day today, hotter than yesterday, and my production curve is already just-slightly-lower than the day before.

Yesterday was a beautiful day. Perfect weather. I think it hit 80 degrees ambient. I looked at my production, and noticed the peak was... flat. That horizontal line is 2 kW.



The day before was cloudy in the morning. The clouds prevented good production, but it would routinely go higher than the day before, and the peak wasn't flat. When the clouds finally parted, it was glorious production until my batteries filled.

Probably.

PV peak performance is often lower in summer than it is in spring/fall due to the higher cell temps, BUT you get more total kWh/day because of longer solar exposure.

Check your cell temps. They're probably in the 40-50°C range, which will typically shave about 8% off your peak.

Measuring as simple as using a handheld device like this?

plympton said:

Measuring as simple as using a handheld device like this?

View attachment 214483

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Yep. I wouldn't trust a long range reading... want to be up close and personal. Check backside too

During this time of the year, from like 11 AM until about 1:30 PM the angle of the sun on the panels is near optimal and so you are not going to get a sharp peak or spike, more of that continuous flatter power output you are showing. That's fine.

In direct sun at ambient 80 degrees, I suspect your panel surface will reach about 135-140 degrees. I have had mine measure about 145-150 in San Diego in direct sun on 100 degree days.

If you don't want to spend the $$$ on the IR thermometer (although it's a nice tool to have for other stuff), put a big load on your system (so the array is at max. output) and cool the panels with some water at some point near solar noon. They are rated for this (rain).

Watch your PV output before and after.

You will be surprised and a little dismayed at the loss of output due to high temps. It was a "holy s**t! moment for me when I did it many years ago when I was just starting out.

Bird landed on panel?

It's a very small dip.

Oh! Maybe MPPT entered periodic search mode restart, spent some time exploring worse power points.

Finally, even on a "blue sky" there are differences in atmospheric moisture content that I don't think we can really differentiate with our eyes. So maybe just slightly worse illumination.

hwy17 said:

Bird landed on panel?

It's a very small dip.

Oh! Maybe MPPT entered periodic search mode restart, spent some time exploring worse power points.

Finally, even on a "blue sky" there are differences in atmospheric moisture content that I don't think we can really differentiate with our eyes. So maybe just slightly worse illumination.

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Small, but noticeable. All this solar stuff is mostly a hands-on exploration to learn the nuance of it all - so pouring over the data is really interesting to me. Seeing the effect of shade, heat, etc. I'll start tracking temp vs. output on these really nice days and see what the difference is. Makes me wish I kept my "spare" panels!

plympton said:

Small, but noticeable. All this solar stuff is mostly a hands-on exploration to learn the nuance of it all - so pouring over the data is really interesting to me. Seeing the effect of shade, heat, etc. I'll start tracking temp vs. output on these really nice days and see what the difference is. Makes me wish I kept my "spare" panels!

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Do you have a weather station yet? I use an Ambient Weather one that has solar irradiance readings. Tracking that separately would surely provide more interesting graphs to look at.

v_green57 said:

If you don't want to spend the $$$ on the IR thermometer (although it's a nice tool to have for other stuff), put a big load on your system (so the array is at max. output) and cool the panels with some water at some point near solar noon. They are rated for this (rain).

Watch your PV output before and after.

You will be surprised and a little dismayed at the loss of output due to high temps. It was a "holy s**t! moment for me when I did it many years ago when I was just starting out.

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They're awesome devices. And great cat toys. I was curious if anyone employed active cooling, but of course, it's been discussed. I was thinking a thermostatic valve attached to line pressure water and a simple mister, but sounds like more hassle than it's worth.

Wasn't prepared for the drop in production so soon in the year. Makes me wonder what it'll be like when we hit > 100 F ambient!

hwy17 said:

Do you have a weather station yet? I use an Ambient Weather one that has solar irradiance readings. Tracking that separately would surely provide more interesting graphs to look at.

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Not yet, but could be in my future. I do have an irradiance meter I can break out to get some manually collected data. Helps I can work from home on the nice days!

plympton said:

Not yet, but could be in my future. I do have an irradiance meter I can break out to get some manually collected data. Helps I can work from home on the nice days!

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Mine's the WS-2902 or very similar to it, and I'm happy with it. 3 years now still going fine. I set it up to report to wunderground so I can just check my wunderground station page for historical data and graphs. The base station wifi setup is a little finnicky but functionally works fine once set up. My outdoor is 100ft from where the living room base station lives and never has signal problems.

v_green57 said:

If you don't want to spend the $$$ on the IR thermometer (although it's a nice tool to have for other stuff), put a big load on your system (so the array is at max. output) and cool the panels with some water at some point near solar noon. They are rated for this (rain).

Watch your PV output before and after.

You will be surprised and a little dismayed at the loss of output due to high temps. It was a "holy s**t! moment for me when I did it many years ago when I was just starting out.

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I have contemplated solar sprinklers, for this reason. (Recycled rainwater)
If and when I decide that I need more production in the summer.

timselectric said:

I have contemplated solar sprinklers, for this reason. (Recycled rainwater)
If and when I decide that I need more production in the summer.

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Are you worried about evaporation leaving behind deposits? That can hurt production, too.

While rain water is theoretically distilled and pure, it does pick up airborne particles and dust that washed off your roof.

Given the price of panels, a few extra seems like the cheapest solution provided you have the rack space.

Mike C.

plympton said:

They're awesome devices. And great cat toys. I was curious if anyone employed active cooling, but of course, it's been discussed. I was thinking a thermostatic valve attached to line pressure water and a simple mister, but sounds like more hassle than it's worth.

Wasn't prepared for the drop in production so soon in the year. Makes me wonder what it'll be like when we hit > 100 F ambient!

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You will need to use distilled water or your panels will become encrusted with some nasty hard-to-remove mineral deposits fairly quickly.

edit:

Mike C. posted the same time as I did about this...

mciholas said:

Are you worried about evaporation leaving behind deposits?

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Nope
Not any more than a typical rainy day.

I have plenty of production. This is just another option for the future, maybe.

For $hits and giggles, and being a man of science in cargo shorts, I just hoisted a ladder up to the panels and gave them a good spray. Works! Not a ton of water, just a spritz for a minute or so. Looks like it has about a 10 minute effect.


Went from 1892w to 2036w - a noticeable bump. What's curious to me is it appears there's a new baseline at 1953w. Maybe it sprayed away some contamination? Dunno. Neat stuff.

plympton said:

They're awesome devices. And great cat toys. I was curious if anyone employed active cooling, but of course, it's been discussed. I was thinking a thermostatic valve attached to line pressure water and a simple mister, but sounds like more hassle than it's worth.

Wasn't prepared for the drop in production so soon in the year. Makes me wonder what it'll be like when we hit > 100 F ambient!

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Actually, those who have been in the solar game for a while are all too aware of the 'hot panel' dropoff, it is something that has to be factored into the offgrid calculations where I live (N/E Australia) where summer temps in excess of 40C or 100F are common for much of the summer months (just when demand is highest)
My 'peak' times are spring and autumn, with summer and winter pretty close to being the same...
By the time summer rocks around my panels (which have a listed -0.4% drop in power per degree Celsius rise) have dropped to under 200W from their STC ratings of 250W!!!
So basically most of the summer, I have lost about 1/5th of my arrays output...- just when I want to run the A/C...
I have actually measured my panels temps in summer at over 80C/176F, and thats a LOT of -0.4%'s adding up, with the panel being 55C above the STC specs...

So much for all the non solar 'why don't they fill the Sahara desert with solar panels and make tons of electricity' questions so commonly heard....
Literally the last place on earth to actually put them lol (thermal solar yes, PV- definitely not...)

Bop said:

My 'peak' times are spring and autumn, with summer and winter pretty close to being the same...
By the time summer rocks around my panels (which have a listed -0.4% drop in power per degree Celsius rise) have dropped to under 200W from their STC ratings of 250W!!!
So basically most of the summer, I have lost about 1/5th of my arrays output...- just when I want to run the A/C...
I have actually measured my panels temps in summer at over 80C/176F, and thats a LOT of -0.4%'s adding up, with the panel being 55C above the STC specs...

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Well, running the calcs it totally makes sense now.

Power Loss=(−0.36%/°C)×((68°C)−(25°C))×(400 Watts) = -62 w/panel loss
-62*6 (panels) = 371 w loss. Nameplate: 2400w, corrected to be about 2048w.

I love me some ChatGPT. I've heard it's particularly bad at math!

Power Loss=(0.36%/°C)×((68°C)−(25°C))×(400 Watts)

Power Loss=0.0036×43×400

Power Loss=619.2 Watts

Actual Power Output=400 Watts−619.2 Watts=−219.2 Watts

So, for an ambient temperature of 68°C, the power loss due to heating would be 619.2 Watts. Therefore, the actual power output of the panel would be 400 Watts−619.2 Watts=−219.2 Watts

However, please note that a negative power loss doesn't make physical sense. It implies that the panel is generating more power than its rated power, which is not possible. In such cases, it's likely there's an error in the calculation or the given parameters.

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It seems the calculations are correct. However, a negative power loss doesn't make sense physically. It's possible that the ambient temperature or other parameters might need to be adjusted.

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You think!?!?

And now I'm regretting not running water to my solar panels when I had that trench open. Oops. I wanted passive solar hot water at some point, too. Daaaaaang.

"It implies that the panel is generating more power than its rated power, which is not possible."

Of course that is possible.
Why do you suppose NEC says to size fuse and wire for 1.56 Isc?

plympton said:

Well, running the calcs it totally makes sense now.

Power Loss=(−0.36%/°C)×((68°C)−(25°C))×(400 Watts) = -62 w/panel loss
-62*6 (panels) = 371 w loss. Nameplate: 2400w, corrected to be about 2048w.

I love me some ChatGPT. I've heard it's particularly bad at math!



You think!?!?

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So yours is pretty close to mine- about 20% lost in summer because of the heat... (mines a little worse, but my airtemps run for weeks well over 40C during summer...)
At least I don't live in Marble Bar here- their highest temp in summer was 52C (125.6F)

Not a big fan of ChatGPT- it likes giving answers that sound authoritative, but are often completely wrong (especially when it comes to technical stuff....)

Hedges said:

"It implies that the panel is generating more power than its rated power, which is not possible."

Of course that is possible.
Why do you suppose NEC says to size fuse and wire for 1.56 Isc?

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Panels can most certainly generate more power than their rated output...
(it is one of the reasons I push 'derate your PVmax voltage on charge controllers to only 80% of the max voltage figure they list' so much...)
I have seen my own panels exceed their 'max peak output by over 10%-15% for brief periods of time... the worst time is summer, on a cool day immediately after it has been raining, and with light 'grey full sky overcast' with breaks in the clouds...
This can lead to the 'perfect storm' of 'bad' (too good) power production, with cool panels (ie more production), summer (best solar insolation) and more light on the panels than normal (if you get a 'shaft' of sunlight through a break in the clouds then you get the normal solar insolation from the direct sunlight, but you ALSO get diffuse 'extra light' from the clouds- which spike the voltages and power by that 10-15%...
If you do as I do and derate the combined panel Voc's by 20%- well everything's still under the PVmax rating, and it just happily grabs that extra power and stores it if possible...
If you do like so many do and 'crowd the limits' of that max voltage in particular- well if you are only say 3% under the maximum allowable voltage- and the arrays voltage rises by 10-15%....

Well...

'magic smoke' comes out...

If you look at any panels specs- you will see that all the figures listed are at 'STC only" ie standard test conditions....

Vary ANY of those conditions- and the rated output (and voltage and current) changes- panels get hotter, they make less, light gets brighter, they make more, AM (atmospheric mass) get less they make more (so a panel at sealevel makes less than the same panel on the top of a high mountain... or say summer when the sun path is 'higher' than in winter when it is closer to the horizon- thats a 'thicker' length of air it has to go through- which is why summer is (usually) considered the best time for maximum production... one AM is the height from sealevel to space so the lower the sun is in the sky, the longer path it has to take through air, and the less solar insolation it provides...)

Those with large bodies of water or tall buildings with lots of glass windows can have real issues with this (more light from both direct light and reflections can seriously boost the arrays voltages for some parts of the day) and if again you are 'crowding the limits' for your controller- this can cause premature failure...
It is also why even in Antarctica, us Aussies use solar at our bases- yes the solar insolation is lower, but when your panels are running at -30C- that brings the power right back up again, add in near or vertical panels (low sunpath) and direct light plus reflected sunlight off that lovely white snow- solar will work a lot better than many people think...

Bop said:

you ALSO get diffuse 'extra light' from the clouds- which spike the voltages and power by that 10-15%...
If you do as I do and derate the combined panel Voc's by 20%- well everything's still under the PVmax rating, and it just happily grabs that extra power and stores it if possible...
If you do like so many do and 'crowd the limits' of that max voltage in particular- well if you are only say 3% under the maximum allowable voltage- and the arrays voltage rises by 10-15%....

Well...

'magic smoke' comes out...

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Voc (STC) up to about 85% of SCC limit should be OK (whatever temperature tempco calculations indicate.)

Power might rise 15% with that extra light, but that is current not voltage. PV array is diodes, and current leaking through them is exponential function of voltage.

The voltage still rises with increased light levels- more slowly, but it does still rise...

Worse, many people use Voc and Vmp interchangeably, (some even use the 'sales talk' of '12v panels', '24v panels', and use THAT as their basis for the PVmax rating....)


Some of the things I have seen done...
urgh....