iostrym avatar image

is it possible to explain my why do we talk about 12V, 24v or 16V solar pannel ?

I don't understand these voltage values because these values are quite never provided by manufacturers. (sometimes by vendors).

I have been told that we got this voltage values based on the cell number.

36 cells => 12 V

60 cells => 16 V (not sure)

72 cells => 24V

but I don't understand what could we do with these values. is it "approximatively" the voltage of the MPPT voltage ? I 'm not sure as my victron 100/20 always indicated Vmax value around 36V-39V for my 24V solar panel.

Solar Panelvoltage
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3 Answers
JohnC avatar image

Hi @iostrym

A throwback to the bad-ole-days, but still commonly used by cheap 'camping' kit and still seen in early Victron literature. When PWM cc's were the norm (and still are with cheapos), a '12V' batt had to be mated with a '12V' panel, although neither operated at 12V. It was a nominal thing.

The cell size was usually '5 inch', and common then to go by cell numbers as they were all muchly the same. When 60 cell panels came in they were usually bigger '6 inch' cells, and although most probably fitted in as '24V' panels, the V-nomenclature was changing.

Nowadays, there's cells of all types and sizes, and you really have to look at the panel specs to determine how to handle them. And mppt has largely taken over from pwm in serious systems, so matching the panels is a different process.

But it's a fair question, as confusion can still arise. Let's imagine a Victron buyer has just been recommended and received a 100/20 mppt, and he wants to charge a 24V batt. He's on a budget and a mate has just offered him 5x '12V' panels free. If he were unaware he might say no. But 4x in a single string might suit him admirably, 5x likely too many, as a typical one might run ~22Voc with no load.

And if you can't see the sticker on the back, you'd test them in full light. In your case, you might notice under full charge the Vmp tracked at (say) 36Vmp, but with no load it might rise to even 44Voc.

You can keep calling it a '24V' panel if you want, and I know what you mean. But the term isn't really meaningful once you've stepped up to mppt.

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iostrym avatar image

Thanks for answering. Yes I have 12v batteries (I'm thinking about connecting serially my two batteries to have 24v and being not limited to the 20A of the 100/20. But only if I don't have enough power this way. Because I would need to by a new 220v transformer.

By the way how do you calculate the 290w ? I would have though 240W max ? (12*20...)

Was I wrong in my supposition that connecting both solar panel can't be worse than having only one (the new one or the old one) ?

Another idea that I had is to reuse my stecca charger that is pwm and connect only my old solar panel on it. As it is a 24vif I have now 24 v batteries it won't be too much waste. But here again 24v batteries whereas Vmp is around 36v it could be also a waste of power....

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The 290W comes from the battery V under charge, which isn't 12V, but more like 14.4V at Absorb level. That works out to 288W, but I rounded it to ~290W. Victron actually have 290W in their spec sheet, unsurprisingly hey.

Yeh, even unmatched panels will produce more, just not optimally. Akin to matched panels, but with one shaded. The mppt will do it's best to maximize the output.

And even your Steca pwm will produce into 12V batts from a '24V' panel. Again, not optimally, but in poor light mightn't actually lose too much. Overall though, perhaps you'd you'd lose half to as much as two thirds of potential W. But better than nothing.

Changing your batt V is a big step. Consider it very carefully..


yes for the 14.4v voltage...

about mmpt that will do its best to maximize output.. I wonder one thing... lets assume that vmp1 = 10V and vmp2 = 15V and Vmp2 is for a 500 Wc pannel and Vmp1 is for a 100 WC pannel. global voltage vs power curve will have two "top" one top around Vmp2 and one top (small one) around vmp1. is there a risk that mmpt will track the wrong "top". I don't know how works mppt algorithm but maybe these mppt algo are no made to deal with "two top" curves ? if one "top" is discovered maybe it will be taken as the Vmp voltage whereas is it not the good one. what do you think ?

what you said about my stecca is true, it is the mistake that I have done when I bought it. 24V pannel on 12V pannel. Now with my victron MPPT I have X2 my power per day (sunny weather)

when you said "consider it very carrefully" does it mean that this step is "tricky/dangerous" and that I could make a wrong choice or do you mean that it could be a very good choice and I need to focus on it very soon ? (as you already notice my english is not very accurate). On my point of view, if I didn't have my transfor problem that don't support 24V, I would go on 24V right now to minimize current on batteries. But to be honnest I don't know how batteries will react serially connected. (currently two 12 V batt in parallel)

iostrym avatar image

thanks @JohnC a lot for your detailled answer.

Then I guess it is no more usefull to look for pannel with same cell number to know if different solar pannel will match together. right ?

I have an 10 years old solar pannel : Solartech, sunrise 185WC :

Voc = 44.21V

Ioc= 5.59A

Vmp = 35.8V

nb cell =72 => "24V"

I wanted to add a new one in parallele on my victron smartsolar 100/20

I was said by a vendor to buy a 24V too and buy and old "techno" solar pannel so that specification would be the same. it's a shame because for the same price of an old 200WC solar pannel I could buy a victron bluesolar 360WC !!! So I would like to be sure that I need to look for "techno" compatibility.

As I understand your mail, cell number is no more an indication of compatibility. Then what should I look in characteristic to know if my new solar pannel with match best with my old one ?

If I have correctly understood MPPT process, the goal is to find a solar pannel that may have approximatively the same Vmp tracked at 100% sun exposition ?

What is the "sensibility" of the approximation of Vmp ?

for instance : mine is 35.8 V

if the new one is 35.85 is it ok ? 35.9 ? 36V ? what would be an "correct" voltage approximation that would lower too much power provided by each pannel ? Because I don't know voltage precision impact on intensity. Maybe at 35.9 there is a -40% power (exageration).

Maybe by experience you know that +/- 0.1V arround Vmp would provide correct results.

Can I trust only Vmp value to do the match ? maybe Vmp value will match at 100% sun light but when 50% sun light Vmp will be completely different ?

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There's a whitepaper, freshly dated and likely a revamp of something older, but still relevant if you care to delve.

There's other factors come into it too, like temp and light intensity, so matching different brands gets a bit mushy. The mppt will track the best combo from both panels, so if they were 2Vmp apart in specs, then might only be 1V away from the optimum of each individual one.

And indeed, in poor light Vmp can drop markedly. Mine halves in the worst of that, but still produces.

And if you're experimenting, your Victron doesn't track mpp continuously, more like every 10 min, so be patient.

From another post, you have 12V batts. So your 100/20 might never exceed ~290W, and may spend much of it's time far below that, in far-from-specsheet-conditions.

A lot of these older panels are being removed from older domestic installations holus-bolus for various reasons, and are often available very cheaply.