1 or 2 MPPT controllers? And what size?

DIY
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Apologies for asking a question that might be common, but I just can’t find a clear answer, either in the documentation or on the Internet. (Lots of speculation and guesses, but no clear answer from anyone who seems to be an expert.)

I have a Tag teardrop trailer that came stock with a curved solar panel on the roof and a Victron Smart Solar 75 | 10. This setup doesn’t do much because I have a roof rack on the trailer. Even with nothing on the rack, it blocks the sun enough that the roof solar panel doesn’t generate much power. I leave it connected because it does something, but not a lot. I added a Victron Smart Shunt to this setup. I have a 100Ah 12v LiFePO4 house battery.

The trailer was used perhaps once and came with some galley modifications I would have made to a new trailer, so it was a great deal for me. Unfortunately, the galley was modified around a very inefficient 12v refrigerator. I can’t find another refrigerator that will fit in the space designed for it. Rather than modify again, the cheaper and faster option was to purchase two solar panels. These are nominally 120w, 18v (max), 6.6a (max) panels. I’ve seen them both generate 6.4 amps in ideal mid-summer sunlight, so I’m pleased. Since I often have shade problems, I have been attaching both panels in parallel to the charge controller.

Sadly, the Renogy charge controller broke, and they aren’t honoring the warranty. Time to get a Victron charge controller to match and better integrate with my existing equipment, but which charge controller and in what configuration? Questions:

  • I was thinking of getting a Victron 75 | 15 Smart Solar controller. Will that be adequate to handle both panels? Should I step up to a 100 | 20 instead?
  • I had two Renogy controllers, an MPPT (which seems to be broken) and a PWM. When I attached one panel to each controller, I noticed that the output differences between the two were negligible. However, when I attach both panels in parallel to a single controller, MPPT or PWM, full sun, and no shade issues on either panel, the total output is less than using two controllers by 3% to 10%. Is that typical? Should I buy two 75 | 15 (or even 75 | 10) controllers?
  • If the advice is to stick with just one controller, with the 75 | 15 be adequate or should I step up to the 100 | 20?
  • I might go with the 100 | 20 anyway in case I purchase a more powerful solar panel in the future. Will the 100 | 20 be inefficient with my current panels?

Responses appreciated. Thanks.

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Hava a look at the Victron MPPT Calculator linked below. This will answer most of your questions.

Note, Victron controllers need 5V above battery to start so Voc needs to be ideally 20V or higher, so you may need panels in series.

2 controllers is always better as it will optimise both panels to maximum power. If they see different shading or direction using one controller means the optimum may be twice the maximum power of the worst panel. But if panels face the same direction then one controller will suffice.

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