MPPT panel calculation

Been scratching my head over this one and hoping someone can point me in the right direction.

I'm planning to expand my solar setup on the narrowboat and I've got a couple of panels that don't appear in the Victron MPPT calculator. Trying to work out whether I can safely wire them up without the tool doing the heavy lifting for me.

Specifically, I want to understand how to manually calculate the key figures when a panel isn't listed — things like:

• Maximum open-circuit voltage (Voc) at low temperatures (relevant here in winter obviously)
• Whether the string configuration will fall within the MPPT's input range
• How to account for temperature coefficients properly

My current setup is a Victron SmartSolar 150/35 and I'm looking at adding a second controller alongside it — possibly another 150/35 or stepping up to a 150/70 depending on what the calculations suggest. The panels I'm looking at are Renogy 200W mono units from a UK supplier, but they just aren't showing in the calculator dropdown.

I know the datasheet has all the raw numbers (Voc, Vmp, Isc, Imp, temperature coefficients etc.) — I'm just not confident I'm applying them correctly, particularly the cold temperature Voc adjustment. A narrowboat sitting in a marina in January can get pretty chilly overnight and I'd hate to fry a controller.

Is there a reliable formula or spreadsheet anyone uses for this? Or is it simply a case of working through the Victron wiring unlimited tool manually even if the panel isn't listed?

Hey @MultiPlus_Queen, looks like your post got cut off there! You've got a couple of panels that don't appear... to be matching specs? From different manufacturers perhaps?

If you can share the Voc, Vmp, Isc and Imp figures from the datasheets (or even just a photo of the label on the back of the panels), we can work out whether they're sensible to wire in series or parallel and whether your MPPT controller can handle them.

Also worth knowing which MPPT you're running - the input voltage window varies quite a bit between units and that's often the critical bit on narrowboat installs where you're sometimes dealing with shading from bridges and trees too.

Fire over the details and we'll get it sorted! 🙂

Looks like both @MultiPlus_Queen and @PGL_Builds got cut off, which is either a deeply ironic threading glitch or the forum's way of telling us to buy more panels before asking questions.

Odd timing on those cut-offs — almost like the forum is in on the joke.

@MultiPlus_Queen, whenever you're ready to repost the full question, worth including:

• Panel specs (Voc, Vmp, Isc, Imp)
• Which MPPT controller you're looking at (Victron SmartSolar, Renogy Wanderer, etc.)
• 12V or 24V battery bank
• Whether the panels would be wired in series, parallel, or series-parallel

Those four bits of info will let people give you a proper answer rather than just guessing. I've been through the same head-scratching process sizing the array on my shepherd's hut build, so I know how the numbers can catch you out — particularly Voc in cold conditions exceeding the controller's input limit.

Typical forum doing that. @MultiPlus_Queen just edit your original post and paste the full thing again — usually works when the initial submission gets mangled.

When you do repost, include:

• Panel specs (Voc, Vmp, Isc, Imp)
• How many panels and whether you're wiring series, parallel, or mixed
• Which MPPT you're running or planning to buy

The Victron MPPT calculator is decent for checking your numbers won't fry anything, but the main thing people get wrong is not accounting for cold-weather Voc spike — panels can push noticeably higher voltage on a freezing morning than the spec sheet suggests at STC. On a narrowboat that matters less than a fixed roof install, but still worth checking.

The forum ate your post like my Victron ate my budget — completely and without remorse. 🫠