How do I test new solar panels?

[SolarHead]

I got new solar panels in last week. How (should) I test each one before installing them?

 

[OffGridInTheCity]

I set each one facing the sun - e.g. noon'ish @ 45-60 degree angle in spring-thru-fall should get 75-90% of expected power - and then use a voltmeter/clamp-meter to
1) Measure the open circuit voltage - should around the specs +/- a few volts. Say the specs are 39v Voc then 36v to 40v kind of thing means they are healthy, all diodes are working etc.
2) Short circuit the panel leads and do a clamp-meter amp test. Say the specs are 9.8a and you get 9a or 8.5a - then good. A stray cloud could dramatically change amps as it floats by.

The above is not a precise test so much as a general test to assure yourself that the panels are healthy / worth putting up. All the panels should measure roughly the same. Remember that variations in angle, a cloud passing by can affect volts and amps - but you'll get the drift after you do a few.

If you have a good, known reference panel, you could compare your new panels against a reference for even more precise assurance. For example, in the pic below I had a 'new' 285w panel and compared it to used Suntech panels doing the 2 tests above and doing volts x amps for power as the comparision since the Voc was 38v on one vs 44v on the other.

 

[SolarHead]

Thanks for the reply. So........ a short circuit test means taking the + and - leads on one solar panel and connecting them to each other, then clamping my multimeter clamp around the wire to get an AMP reading. Correct? I kept reading and seeing "Short Circuit" on things but have never understood what it meant. When does a panel ever short circuit itself (other than during a test) ? Doesn't seem like a real world test since they are never short circuited, or at least I don't think they are. I suppose in the combiner box if a wire got sliced by accident and they touched but sounds like that would arc and be a really big problem especially if you have multiple panels going to the combiner box. I would have thought plugging a panels 2 wires into each other would have caused damage to a panel.

 

[OffGridInTheCity]

Thanks for the reply. So........ a short circuit test means taking the + and - leads on one solar panel and connecting them to each other, then clamping my multimeter clamp around the wire to get an AMP reading. Correct?

Yes

I kept reading and seeing "Short Circuit" on things but have never understood what it meant. When does a panel ever short circuit itself (other than during a test) ?

It doesn't. But it's OK in a solar panel case because it's not like a battery that can jump to 100(s) of amps - a panel just maxes out at it's max amps - e.g. 9a or whatever it's rated for.

Doesn't seem like a real world test since they are never short circuited, or at least I don't think they are.

It is a real world test - it shows the MAX the panel can deliver.

I suppose in the combiner box if a wire got sliced by accident and they touched but sounds like that would arc and be a really big problem especially if you have multiple panels going to the combiner box. I would have thought plugging a panels 2 wires into each other would have caused damage to a panel.

It does not hurt the panel at all because the max amps is what the panel is designed to deliver continuously - e.g. won't burn up wiring or diodes or hurt the silicon wafers.

**I know it's weird** - but really, it's OK

 

[Bud Martin]

Isc is part of the test spec that is done by shorting the output of the panel together.

Testing PV Modules - SunWize | Power Independence

Testing PV Modules is efficiently done by checking both the open circuit voltage (Voc) and short circuit current (Isc) in full sunlight conditions.

www.sunwize.com

When performing the Isc test you should cover up the panel with blanket or cardboard before and after performing the short circuit test since you can have pretty good arc occurs.

 

[circus]

When buying my panels, I was lucky, found my old cheap multimeter could measure up to 10 amps. They would sell a lot more meters if they included a text book instead of a post-it note.

 

[SolarHead]

It is a real world test - it shows the MAX the panel can deliver.

okay not trying to be complicated, but where does cold temperature come into play? I mean if I get a short circuit test, is that the highest the panel can ever produce, or can it go higher in cold weather (I've read or heard where panels produce more power the colder it gets which makes me wonder why they don't make white solar panels and not black but that's another subject/question).

 

[SolarHead]

They would sell a lot more meters if they included a text book instead of a post-it note.

Now I think they don't include a post-it note on some products. You get a business card size card with a QR code on it, which is smaller than most post-it notes.

 

[SolarHead]

So, the ISC is short-circuit (with connectors connected) , and the Voc is voltage (with connectors not connected).

I just noticed on my panels spec sheet, they say Fuse Rating of 20, I assume that's a 20amp fuse , I'm using 15A fuses, is that going to be okay? I have fuse holders in my combiner boxes.

 

[OffGridInTheCity]

I just noticed on my panels spec sheet, they say Fuse Rating of 20, I assume that's a 20amp fuse , I'm using 15A fuses, is that going to be okay? I have fuse holders in my combiner boxes.

In my setup, I'm configured with 3s5p running to each combiner box with 15a circuit breakers. Each set of wires (and circuit breaker) represents 3s1p string. I have 5 x string = 5 x 15a circuit breakers in each combiner box. 15a is plenty because the panels max out <10a. When you put panels in series, the voltage goes up but the amps remain the same.

Assuming you also run Xs series of panels -> combiner box, then the max you'll have is 9.351a and a 15a fuse/breaker will be OK. There's no magic here. A 9.351a panel is not going to produce any more than that

okay not trying to be complicated, but where does cold temperature come into play?

Extremely cold temps like -50F/-45C can cause the voltage to rise significantly to the point where it exceeds the max voltage your charge controller allows - and blow it up. So there's typically a coefficient specification to help you calculate this so you size your strings (of panels) correctly for your charge controller. This is not about power - its about the max voltage you're charge controller is designed to handle before it blows up.

A charge controller will also have a max power rating - but if you panels produce a bit more than the max, the charge controller will typically ignore it rather than blow up so it's not a critical in the same way as the voltage.

In my case, I have Midnite Classic 150 charge controllers - with a max voltage of 150v. My 3s strings produce 120v leaving 30v headroom. If I were to do 4s I'd be at 160v and be too hi. So 3s it is and my temps are mild so it's not significant with a 30v headroom.

To give you a sense of how cold it has to get for this to be significant - notice in this Midnite Sizing Tool example with -0.33 and -0.45 coefficients that the VMP goes from 91.8 -> 114.6 at -30C and the VoC goes from 112.8 -> 133.2 at -30C.


You can play with the tool using you're own specs and see what it shows... https://www.midnitesolar.com/sizingTool/

in cold weather (I've read or heard where panels produce more power the colder it gets

It's true, panels are more likely to hit 100% of specs if it's cooler - because they're tested (to get the specs) at cooler temps. When it's 120F/49C on the roof, the efficiency goes down - maybe 10% of so.

which makes me wonder why they don't make white solar panels and not black but that's another subject/question).

Yes sir - there's much discussion on things like this and way to cool the panels etc etc... but invariably they cost $ and it's likely cheaperto add an extra panel to make up for the 10% loss. It's when you have space constraints (can't add panels) that this topic becomes more interesting?

 

[SolarHead]

Assuming you also run Xs series of panels -> combiner box, then the max you'll have is 9.351a and a 15a fuse/breaker will be OK. There's no magic here. A 9.351a panel is not going to produce any more than that

I think the coldest it gets around here would be -5F but thats rare, most winters get down to around 5F but usually 10F to 20F is coldest of winters, we typically see about 30F all winter. Some nights might dip to 12F. Sounds like I dont need to worry about cold temps in my design or configuration (sounds non issue based off what you mentioned above).

Here's what I got (4 strings of 9). I hope I'll be okay with 15A fuses.
Each combiner box is 4 fuse holders total, which is 2 fuses (one fuse on each black) + 2 fuses (one fuse on each red) , within each combiner box. I have a Sol-Ark so each MPPT can handle up to 500v , and since its a transformerless inverter I could NOT ground my PV wires in the combiner boxes so the black goes thru fuses, and the red goes thru fuses, they combine then run underground via 2" conduit to my basement where the inverter is located. Its about a 200 foot run from ground mount to my Sol-Ark.

 

[SolarHead]

Forgot to mention, when I view real-time stats on the Sol-Ark screen, I see each MPPT at about 8A so I figure its about right, and the 15A in the combiner boxes should be okay.

 

[SolarHead]

Also, what I have now is actually 32 panels, and its a mix of 18 Mission Solar and 14 Canadian Solar, tomorrow I hope weather is nice because I'm pulling the Canadian Solar off, and installing 18 of the Mission Solar , to have total of 36 (the diagram I just posted). The 14 Canadian Solar not sure what I will do with them yet. They were not same thickness, or width , and they look funny on the ground mount rails, different color in the sun too, and I decided to get more Mission Solar and make my panels all match while adding 4 more for total of 36. I figured if I gotta pull a bunch off and install 18 new, I might as well test them before installing. My first solar project and its really the only thing I am doing over, so not a huge regret but I knew I'd have to do something 2 or 3 times to get it right. If I ever do another solar project, I would definitely buy all the solar panels of the same model/brand at once, and probably even order 2 or 4 more to put into storage since they seem to discontinue models pretty quick from what I can tell.

 

[WYtreasure]

Play with one or two panels while you're testing. Try to get them pointed directly at the sun AT SOLAR NOON. Then move off a bit and watch the numbers drop. Cover a bit of the panel just to see how shade affects the numbers. I found it to be an interesting experience.

Take a peek in the junction boxes for loose connections.

 

[SolarHead]

Yea, I watched a meter on one panel one time and was surprised how sensitive that sweet spot is with tilt and direction. It was interesting to see, but also concerning because I had spent good money on panels, and there I was getting ready to bolt them down to a ground mount to one tilt angle, and one direction and never to move or adjust them again. I was like man, a sun tracker would be the crap, but they are too expensive and I read where its cheaper just to buy more panels if you want/need more power. I get that, but really, I mean, if its a sweet spot sure is a shame not to be able to point directly at the sweet spot all day long. I think that's why guys in RV's have the portable solar panels so they can move them multiple times a day and get optimal power from them as opposed to mounting them on the roof of a camper. Just have to worry with lugging them around, and theft if they are portable.

 

[OffGridInTheCity]

Yea, I watched a meter on one panel one time and was surprised how sensitive that sweet spot is with tilt and direction. It was interesting to see, but also concerning because I had spent good money on panels, and there I was getting ready to bolt them down to a ground mount to one tilt angle, and one direction and never to move or adjust them again. I was like man, a sun tracker would be the crap, but they are too expensive and I read where its cheaper just to buy more panels if you want/need more power. I get that, but really, I mean, if its a sweet spot sure is a shame not to be able to point directly at the sweet spot all day long. I think that's why guys in RV's have the portable solar panels so they can move them multiple times a day and get optimal power from them as opposed to mounting them on the roof of a camper. Just have to worry with lugging them around, and theft if they are portable.

This kind of thinking will take you into SOCD (Solar Obsessive Compulsion Disorder)! I know - I fight it all the time myself. But remember, the slightest cloud will cause a 5% drop or fog or in my area wild-fire smoke and on and on and on.... THEN you have system losses, inverter being one of the largest.

In my case I get 18,000kwh/year in PV but only 15,000kwh/year in actual consumption! That's 3000kwh/year lost

On the other hand, solar is quiet and just works, year after year and I do get the 15,000kwh from the Sun 'for free' (sort of) - so I try to fight it. Glass is 83.3% full for me - better than 1/2 full for sure

 

[SolarHead]

I wonder why something like this that's fixed wouldn't work instead of a motorized/mechanical tracker. The sun moves, but the mirrors are there designed for the suns movement. It probably wont work, sun isn't always same spot as the seasons change. Maybe Mr. Musk will figure out how to get a solar panel on a satellite and its in the sun 100% of the time, and beam the power down to Earth. This sun up , sun down thing ever since I plugged my system up 4 weeks ago is driving me bat crap. I didn't use to watch or think about clouds at all, now I hate clouds.

 

[WYtreasure]

I wonder why something like this that's fixed wouldn't work instead of a motorized/mechanical tracker. The sun moves, but the mirrors are there designed for the suns movement. It probably wont work, sun isn't always same spot as the seasons change. Maybe Mr. Musk will figure out how to get a solar panel on a satellite and its in the sun 100% of the time, and beam the power down to Earth. This sun up , sun down thing ever since I plugged my system up 4 weeks ago is driving me bat crap. I didn't use to watch or think about clouds at all, now I hate clouds.

Aren't ya glad I tossed that thought out there? It will help you sleep tonight.

@chrisski will tell you how advantageous it is to move the panels.