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wert avatar image
wert asked

Viability of a parallel-series solar array with multiple panel sizes

Hi everyone, I have 4 solar panels on my camper van, and, due to a set of specific placement requirements, 3 are of different sizes and capacities. If my understanding of series and parallel hookups is correct, I believe I ought to be able to do a parallel-series combo into one Victron 100 | 50 MPPT controller with a minimal 3w efficiency hit. I'm still learning as I go along, so I'd appreciate a sanity check and advice from the experts here.

amps figure is maximum power current, and volts are max power voltage.

Here are the panels:

120w panel

6.27A

19.14v

250w panel

13.2A

18.9v

2x 200w panels

9.71A

20.6v

I figure I can connect the 120w and 250w together in parallel to combine amperages. I lose .24v (1.5w) on the 120w panel, resulting in 368.5w output.

So: 120w+250w:

368.5w

19.47A

18.9v

Then, for the other two panels, I connect the two 200w together in parallel for 19.42A and 20.6v.

So 200w x2:

400w

19.42A

20.6v.

Lastly, I connect the two pairs in series, combining voltages. I lose .05A on the 368.5w pair, resulting in 367w. Adding this with the 400w array, I end up with 767w, losing just 3w.

Final combined array:

767w

19.42A

39.5v

I have a Victron MPPT 150 | 50 controller I was planning on using. at <40v, <20a, I'm well under that 150v, 50A figure. For some reason the online calculator is saying I should size up to the 60A-- should I be concerned? I could actually size down to a 100 | 30 and still be well within spec, right?

Also, are there any issues using MC4 parallel branch connectors and 8AWG MC4 extender cables to join the two sides in series for a 15ft run? The only limitation I hear is MC4 should be kept under 30A, which I'm still under.

And, the cable run inside the van might be 8AWG, but the individual cables attached to each solar panel might not be. Is there a risk of, say, the 250w melting something on the 120w?

Lastly, if there's anything else I might have missed, would definitely appreciate any input.

Solar Panel
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2 Answers
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klim8skeptic answered ·

@wert the individual cables attached to each solar panel might not be. Is there a risk of, say, the 250w melting something on the 120w?

Yes. You have to check the 120w panels "series fuse rating" spec. Should that figure be under the 250w panels Isc rating, you risk the 120w panel catching fire should it fail, when the 250w panel will dump all of it's energy into it.

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wert avatar image wert commented ·

Thanks for the insight! I messaged the seller asking about this figure; we'll see what they come back with.

So the fire risk is only if the 120w panel fails in a way that causes it to become the path of least resistance, right? If so, I should be fine if I attach, say, an 8A inline fuse on the positive of the 120w?

In the same vein-- my application is on a camper van roof, so it will be in plenty of varied and partially shaded situations. For the series connection, would I have to also worry about reverse current in case the front half (2x 200w) gets full sun while the rear (120w + 250w) gets nothing?

I'm guessing as a small optimization when wiring the front + back in series, it would be better to wire the positive of the rear panels to the neg on the front, as the front's individual panels, being higher amperage than the 120w weak point, would have a higher threshold of failure then?

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klim8skeptic avatar image klim8skeptic ♦ wert commented ·

@wert So the fire risk is only if the 120w panel fails in a way that causes it to become the path of least resistance, right? If so, I should be fine if I attach, say, an 8A inline fuse on the positive of the 120w?

A fuse on both pos and neg lead of the 120w panel is required. Should the panel short at the top, or bottom end of the cells.

Do note, panel retailers are not particularly reliable regarding specs. Labels are cheap and might not actually relate to panel specs. That will throw your calcs out.

my application is on a camper van roof, so it will be in plenty of varied and partially shaded situations.

Shade is bad and is to be avoided. If 1 cell in the panel is shaded and the rest are in full sun, the shaded cell will reduce the overall panel current. That will in turn reduce series connected panels current to the same shaded cell current.

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wert avatar image wert klim8skeptic ♦ commented ·

Thanks! Will definitely fuse both leads on the 120w panel. If there's anything else you could think of that I'm missing, I'll definitely appreciate any and all suggestions!

I have one final question, regarding MPPT controller sizing. I was recommended the 150V | 60A controller, but my system is only 40V 20A. Would you know if there's a reason why they'd recommend a controller so big, or can I just use a 100 | 30?

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klim8skeptic avatar image
klim8skeptic answered ·

@wert The 100-50 mppt is rated for 100v pv input, and 50a output to the battery, with a Max. PV short circuit current of 60a.

You can not go over the mppt's 100v pv input or 60a Max. PV short circuit current as the mppt can be damaged. The 50a battery output is safely limited to 50a max.

The mppt will convert your pv array from 767w, 19.42A, 39.5v pv input to 14v, 54a output to the battery. (should you have a 12v battery)

Power into the mppt pretty much equals power out to the battery. Power = Volts x Amps.

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