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I have a dumb question about panel output...

LVLAaron

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Jul 16, 2021
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I have two 50 watt Rengogy mono panels + Victron 75/15 MPPT controller.

A year or two ago, I wired the panels up in series, stuck them in the sun and was collecting 95-97 watts.

I put the panels away for a while.. I moved, built a new shop, life, etc.

I have them out on a clear and sunny day and get MAYBE 60 watts out of them.

So I just tried to connect them in parallel and I get 49-50 watts on the nose, 3.3 amps.




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What are the potential things I am doing wrong? Why do my panels suck?
 
Depends on temperature.

If you keep them cool and then bring them out into the sun real quick you'll get pretty close to rated power for a bit.

Once they get hot that output drops.

I get 30-35w out of my 50w panel consistently.
 
It's currently 98 degrees.

Sounds like I need some of those big boys from Signature Solar
 
It's currently 98 degrees.

Sounds like I need some of those big boys from Signature Solar
98 degrees on its own is noticeable but the real temperature issue is the panel itself.

A black panel in the sun gets *hot*. That's where most of the loss is, barring angular placement.
 
When its 100 out and not a cloud in the sky and the sun at high noon, I measured my flat glass panels at 134. I measured my flexible panel at 154.

If this is a test, someone mentioned taking a hose to it and watering the panels down to cool them off. He saw a noticeable raise in output, but within 20 minutes the panels heated back up.

EDIT: In my signature block there is an off grid calculator. That is supposed to take into account things like that. You should be able to use that to get an approximate output.
 
#1 reason for reduced PV output is battery near full charge, doesn't want as much current.
#2 could be less sun intensity. Seems clear, but enough something in the air could be attenuating light.

And of course, panels have to be aimed at the sun.

"95-97 watts" is better than expected most of the time for 2 x 50W (STC) panels. More like 85W would be expected. But 60W is too low to be just temperature. Data sheet should give temperature coefficient of power.
 
I got the battery low enough to get the victron to be in "Bulk" mode so there shouldn't be a limitation. I hosed the panels for a minute with cold water and they got up to 60 watts total... :/
 
Probably something obscuring sunlight. Smoke from fires? Visibility in my area is somewhat obscured now. A few days ago, the sun and sky were the wrong color.
I wouldn't expect crystalline panels to degrade in storage.
Since two in parallel worked, you could try one at at time, see if identical.
But since 60W in series, 50W in parallel, they may not have enough voltage headroom to be at maximum power point, so not a great test.
If you have a DMM with sufficient current rating, can measure Isc and Voc. But that doesn't guarantee good Vmp & Imp, due to several failure mechanisms.
Just be patient, see if it behaves differently in the future?
 
Air is clear - I'm in the midwest. It's hot as hell, but the skies are blue. I've tried them one at a time, one panel is always 50 percent of the total. Nothing funny going on with one single panel.
 
I'm still betting on illumination.
But if you think these panels aren't working right, you could get yourself a 24V panel as a replacement and see what percentage of rated output it delivers.
Then, assuming you can fit all of them in your application, put the two 12V in series with each other and the 24V one in parallel with them. (best if 2x Vmp of the 12V panel is similar to Vmp of the 24V.

I've got two strings of panels in parallel and have observed one is 3/4 the current of the other, according to clamp ammeter. After I replace fuses in my DMM I plan to do some tests. I have several oil-filled radiator heaters, and I think by using them in series or in parallel I can load either an entire string or a single panel to approximately maximum power point, and read Vmp & Imp. (In series, I need to make sure all similar resistance so it doesn't put excessive voltage on one.)

I'm just trying to find the odd man out, so I don't need one standard sun.

Yours are probably supposed to be 50W at 17V. Maybe a headlamp bulb would last long enough for testing. But what you really need to know is whether you've got 1000W/m^2 sun.

Some panels have degraded badly, but that takes some kind of exposure.
 
I'm still betting on illumination.
But if you think these panels aren't working right, you could get yourself a 24V panel as a replacement and see what percentage of rated output it delivers.
Then, assuming you can fit all of them in your application, put the two 12V in series with each other and the 24V one in parallel with them. (best if 2x Vmp of the 12V panel is similar to Vmp of the 24V.

I've got two strings of panels in parallel and have observed one is 3/4 the current of the other, according to clamp ammeter. After I replace fuses in my DMM I plan to do some tests. I have several oil-filled radiator heaters, and I think by using them in series or in parallel I can load either an entire string or a single panel to approximately maximum power point, and read Vmp & Imp. (In series, I need to make sure all similar resistance so it doesn't put excessive voltage on one.)

I'm just trying to find the odd man out, so I don't need one standard sun.

Yours are probably supposed to be 50W at 17V. Maybe a headlamp bulb would last long enough for testing. But what you really need to know is whether you've got 1000W/m^2 sun.

Some panels have degraded badly, but that takes some kind of exposure.

I'm just playing with these. I am considering a 1200 watt ground mounted system... so I'm not terribly concerned with these two... They're just packed away in my "bug out" trailer 99 percent of the time.
 
Check out SanTan Solar for panels (or other new/used liquidators in your area if you can avoid shipping.)
Don't necessarily buy the lowest $/watt.
If your system will have batteries rather than grid tie, I suggest going oversize on the PV array and undersize on battery, and regulate down charge current to optimum rate. The extra PV can feed inverter if system is well enough integrated (communication or in one unit.) PV is cheap, and batteries are expensive. 2400W of panels might be $800, for instance.
 
400 watt panels at 1200 watts gives you only 3 panels. Only set up option is 3S or 3P. If set up in 3S shade on one panel will hugely limit production. Ground mount with a shadow from a tree crossing is an example. 3P would limit this, but at the expends of working with in excess of 30 amps, so you’d lose the ability to use 10 AWG wire and MC4 COnectors.

Getting one more panel, you can go 2S2P.

I have 33 cell panels, and when I shaded two cells in a series of three panels, I lost about 1/3 of the production. When I shaded most of the panel, power output dropped by 2/3 or 3/4.

I’m not sure how much power you need or how much shading is on the panels.

My local Craigslist has truckloads of panels for sale. Other locations are not so lucky.
 
400 watt panels at 1200 watts gives you only 3 panels. Only set up option is 3S or 3P. If set up in 3S shade on one panel will hugely limit production. Ground mount with a shadow from a tree crossing is an example. 3P would limit this, but at the expends of working with in excess of 30 amps, so you’d lose the ability to use 10 AWG wire and MC4 COnectors.

Getting one more panel, you can go 2S2P.

I have 33 cell panels, and when I shaded two cells in a series of three panels, I lost about 1/3 of the production. When I shaded most of the panel, power output dropped by 2/3 or 3/4.

I’m not sure how much power you need or how much shading is on the panels.

My local Craigslist has truckloads of panels for sale. Other locations are not so lucky.

Good point. Thank you!
 
I have two 50 watt Rengogy mono panels + Victron 75/15 MPPT controller.

A year or two ago, I wired the panels up in series, stuck them in the sun and was collecting 95-97 watts.

I put the panels away for a while.. I moved, built a new shop, life, etc.

I have them out on a clear and sunny day and get MAYBE 60 watts out of them.

So I just tried to connect them in parallel and I get 49-50 watts on the nose, 3.3 amps.




View attachment 61334



What are the potential things I am doing wrong? Why do my panels suck?
Check how many series cells the panels have. 15.02v sounds like 30 or 32 cell panels. Common for Renogy to do. Not enough overhead voltage to run an MPPT controller with 13.6v battery voltage. Controller may be dropping back to PWM mode of operation and with their series inductor they have more loss then regular PWM controller. or controller cannot find the true mppt point on panels..

Need 36 cell panel minimum to run a 12v mppt system. Also mppt not worth the extra money for such low wattage panels.

Stick with series connections on panels.
 
I have learned a bit about panels and heat. My first 100 watt panel was having a good day here in Monterey if I could pull close to 75 watts but the average being more like 50 - 60 watts, I used to say that I guess the only way you get a 100 watts out of a panel is in Death Valley...in August. But heat being bad ju ju for panels, that apparently is not true. I wonder what the true best case conditions are to actually get 100 watts out of a 100 watt panel. Time to read up and see.
 
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