Mixing 12V and 24V panels on same MPPT — anyone actually done this long-term?

So I've got a bit of a mess situation on my static caravan roof. Currently running a Victron SmartSolar 100/30 on a 24V system, with two 200W 24V panels wired in parallel. Works fine. But I've been given — yes, given, can't say no — four 12V 100W panels from a mate who upgraded. Trying to work out whether there's any sensible way to bring them into the same controller or if I'm just creating problems.

The maths seems like it could work if I wire the 12V panels in series pairs to get them up to roughly 24V nominal, which would put Voc somewhere around 44-46V per pair. That's within the 100V input limit on the 100/30. But then combining those series strings with the existing 24V panels in parallel — the Voc mismatch is probably going to cause the MPPT to compromise. Victron's documentation isn't exactly forthcoming on mixing panel specs and I've gone down a rabbit hole on their community forum without a clear answer.

Wondering if the smarter move is just a second MPPT for the "new" 400W string entirely. I've got a Renogy Rover 40A sitting in a box doing nothing from a previous project — 12V/24V auto-detect, 100V input. Could run that into the same 24V battery bank. Has anyone run two MPPTs into the same Fogstar Drift lithium bank without issues? The BMS comms side of it is where I get a bit fuzzy.

@VanRhys the panel voltage labelling is a bit misleading tbh — what matters is the actual Voc and Vmp figures, not the "12V" or "24V" marketing label.

Most panels sold as "12V" are actually ~36V Voc, and "24V" ones are ~45V Voc. So mixing them in series or parallel isn't really about the label, it's whether the voltages actually match.

On my narrowboat I ran mismatched panels through a Victron SmartSolar for a couple of years — worked fine because the MPPT just finds the best operating point regardless.

Key things to check:

• Voc must stay under your controller's input limit (100V on yours)
• Parallel strings should have matching Vmp, otherwise the stronger panel gets dragged down
• Fuse each string separately

What are the actual spec sheet numbers on the panels you're looking at adding?

ChloeFisher72 | 847 posts | ⭐ Trusted Member

@VanRhys the key thing with the Victron 100/30 is that it doesn't care whether your panels are "12V" or "24V" — those are just nominal ratings. What matters is the actual Voc and Vmp figures. As long as your total array Voc stays under 100V and your combined Isc doesn't exceed 30A, you're theoretically fine mixing them in series or parallel configurations.

That said, mixing panels with significantly different Vmp values in parallel is where it gets messy — the lower Vmp panel effectively drags the higher one down. Series is usually cleaner if the voltages stack sensibly.

I ran a mixed array on my static for about 18 months without drama. Just plug your actual panel specs into the Victron MPPT calculator before committing to anything. What are the specific panels you're looking at adding?

BreezySkipper | Posts: 847 | Location: Norfolk Broads

@VanRhys the short answer is yes, absolutely doable — your Victron doesn't actually care whether panels are "12V" or "24V" labelled, it just sees the actual Voc and Vmp coming in. Those nominal voltage ratings are largely a marketing hangover from PWM controller days.

What does matter is keeping your combined string Voc safely below that 100V maximum. Add up the open-circuit voltages of all panels you're planning to connect and make sure you've got headroom, especially on cold mornings when Voc climbs noticeably.

Parallel wiring mismatched panels can cause some efficiency losses if the Vmp values differ significantly — worth checking those figures specifically rather than just the nominal voltage labels. What panels are you looking at adding? If you share the spec sheets I can help you crunch the numbers. 👍

EssexCruiser | 312 posts | 📍 Essex (obviously)

Done exactly this on my garden office setup — chucked a spare 12V 100W Renogy on a 24V string and the Victron SmartSolar didn't even flinch, just quietly got on with harvesting whatever it could like a good little Dutchman. The real gotcha nobody mentions is that mismatched Vmp means the MPPT compromises on the whole string, so your shiny new panel is basically running at the pace of the slowest one — like putting a sports car behind a tractor on the A12. Worth running Victron's MPPT calculator first to check your combined Voc doesn't exceed 100V on a cold morning, because a frozen Essex sunrise will push those figures higher than you'd expect.

SolarJunkie | 1,247 posts | 📍 Undisclosed

@VanRhys people gloss over the critical bit here: mismatched Voc values. Your 24V panels will typically sit around 37-45V open circuit. Shove a "12V" 100W panel in the same string and you're looking at ~22V Voc — the MPPT will be hunting between two completely different power curves and settling on a compromise that satisfies neither.

Parallel strings can work if Voc values are reasonably close (within ~10%), but a genuine 12V panel alongside a 24V panel isn't that scenario. You'll bleed efficiency constantly, not just occasionally.

Done a similar bodge myself on the shepherd's hut before I knew better. Lost roughly 18% yield compared to matched panels. Not theoretical — logged it through the Victron app over six weeks.

Buy a second matching 200W panel or sell the 12V one. False economy otherwise.

VanNicola | Posts: 2,103 | 📍 Somewhere on the cut

Ran a similar bodge-job on the boat for two winters — inherited a 175W 12V panel when a marina mate upgraded his setup, bolted it alongside my two 24V Renogy panels on the same Victron SmartSolar 100/50.

The MPPT just sees string voltage and current, doesn't care about the panel's rated system voltage. What @SolarJunkie said about Voc is the bit that'll catch you out — add those figures carefully before you commit any cables.

Mine pulled noticeably less than theoretical peak, but free watts are free watts on a cloudy British afternoon.

MidlandsOffGrid | Posts: 634 | 📍 Staffordshire

Worth adding to what @SolarJunkie touched on — the MPPT doesn't care about the panel nominal voltage at all, it only sees the array voltage coming in. What actually matters is that your combined string voltage sits comfortably within the controller's input window (12V minimum, 100V max on your 100/30) and that your total wattage doesn't exceed the 30A charge current limit at 24V system voltage.

Where people come unstuck is assuming panels will contribute equally when they're from different manufacturers with slightly different Vmp values. The MPPT will find the best compromise point, but you'll rarely extract full theoretical power from both simultaneously. For a static caravan that's probably acceptable — you'll still get decent output rather than leaving capacity completely unused on the roof.

What's the Voc and Vmp on the spare panels you're considering adding?

Davo49 | Posts: 891 | 📍 Array

Running a shepherd's hut with a mismatched panel situation myself — inherited two 12V 150W panels when I bought the plot, then added proper 24V panels later. What actually sorted my thinking was the EV charging side of things: you cannot afford a flaky harvest when you're trying to push meaningful kWh into a vehicle overnight.

Ended up putting the 12V panels on a completely separate Victron 75/15 feeding into the same battery bank. Two controllers, clean wiring, zero compromise. Costs another £60-ish but the reliability gain over two years has been absolutely worth it.

MultiPlusFan | Posts: 1,247 | 📍 Array

My Victron SmartSolar didn't care about the mixed voltage drama — it cared deeply about my mismatched Voc nearly kissing the input ceiling during a surprisingly sunny February in Worcestershire, which is basically a solar eclipse most years. Check your combined open-circuit voltage against the 100V input limit before you wire anything, because "emergency backup" stops being funny when your MPPT becomes a very expensive paperweight. 🔥

RhysPalmer | Posts: 312 | 📍 Wales

The key thing nobody's mentioned yet — check your Voc figures carefully before mixing. With panels in parallel, your combined Voc only needs to stay within the controller's input limits, but mismatched Vmp values mean one string will drag the other away from its optimal operating point. You won't damage anything on a decent MPPT, but you'll leave real watts on the table. Run the numbers through Victron's MPPT calculator first. What spec are the 12V panels you're considering adding, @VanRhys? That'll determine whether it's actually worth the hassle.