Anyone else mixing 12V and 24V panels on a single MPPT — is it actually worth the faff?

Been scratching my head over this for a few weeks now. My garden office setup currently runs four 200W 24V panels wired in two parallel strings feeding a Victron SmartSolar 100/30, which handles the 48V lithium bank (Fogstar Drift 200Ah cells in series pairs) fairly comfortably through summer. Problem is I've acquired two older 12V 175W panels from a mate who was clearing out his barn, and I'm reluctant to just leave them sat doing nothing.

The obvious option is wiring them in series to produce something approximating a 24V string and tacking them onto the existing MPPT input, but the Voc mismatch is giving me pause — the 24V panels are sitting at 45V Voc each, so the series pair hits 90V, whereas the 175W panels in series come out at around 43V Voc combined. Running mismatched strings in parallel into the controller means the higher-voltage string is going to drag the lower one up past its MPP, which as far as I can tell just bleeds efficiency. Victron's VictronConnect app does show per-panel yield if you use the newer firmware, but it won't magically fix the physics.

Has anyone actually measured the real-world loss in a scenario like this rather than just estimating it theoretically? I'm wondering if a second small MPPT — something like the Victron 75/15 — just for the two 175W panels would pay for itself in recovered yield over a season, versus the simpler but lossy parallel wiring approach. Rough numbers suggest the 75/15 is around £60–70 new, and if I'm losing 15–20% on a ~350W string that's a meaningful chunk of daily generation for the office loads and topping up the Zoe overnight.

@RoundTuit I went through almost exactly this rabbit hole when I was expanding my cabin setup last spring. Had a pair of older 12V Renogy panels sat doing nothing and tried shoehorning them into my existing 24V array.

The short version: the maths rarely works in your favour. Your MPPT will chase the Vmp of whichever string dominates, and the mismatched panels end up dragging efficiency down rather than boosting output. I lost roughly 15% compared to running them on a separate small controller.

Ended up picking up a secondhand Victron 75/15 for about £40 on eBay, dedicated purely to those orphan panels. Two controllers, clean strings, no compromise. The SmartSolar app shows both arrays separately which is actually rather satisfying to watch on a sunny morning.

Sometimes the tidy solution is the efficient one.

@RoundTuit curious what your Voc looks like once you factor in cold morning temps — that's where I got caught out on my shepherds hut build. Stacked a couple of mismatched panels thinking "close enough" and the controller was throwing fits on frosty mornings because the combined open-circuit voltage crept way higher than I'd calculated at STC.

What MPPT voltage range are you actually working within? The 100/30 has a 100V max input, so depending on how you're stringing those mixed panels the headroom might be tighter than it looks on paper.

Have you run the numbers through Victron's MPPT calculator? It accounts for temperature coefficients which makes a real difference when you're sitting somewhere that actually gets a proper winter.

@RoundTuit this is something I've been wrestling with too for my garden office — I've got a similar Victron 100/30 and was tempted to bolt on a couple of surplus 12V panels I had lying around.

What put me off was the mismatch in Vmp values creating an effective ceiling on the lower-voltage string's output. The MPPT will chase one compromise point rather than each string's true peak.

Practically speaking, did you consider just running the 12V panels through a separate smaller controller — something like a Renogy Wanderer or even a budget EPever — and feeding both into the same battery bank? Keeps everything clean and each array operating at its optimum. Slightly more wiring faff upfront but potentially meaningfully better harvest over a full season.

Has anyone actually measured real-world losses from mixed-voltage strings rather than just theorising? Curious whether the numbers justify the separate-controller approach.

@RoundTuit mixing voltages on a single MPPT is more trouble than it's worth in my experience. The controller will just pull everything down to suit the lowest-performing string — you lose efficiency across the board.

Ran into this exact headache when I was sorting panels for my shepherd's hut. Ended up keeping everything the same voltage and life got a lot simpler.

If you've got spare 12V panels kicking about, honestly just run a separate cheap MPPT for them — Renogy do decent budget ones. Keeps your Victron doing what it's good at without compromise.

The Victron 100/30 is a solid bit of kit, don't muddy it with a mismatched array 😄

@RoundTuit I've actually done something similar on my workshop setup — mixed a couple of older 12V panels in series with 24V ones to hit a workable Voc. The key thing nobody mentions is that your string current gets bottlenecked by the lowest-performing panel, so if your 12V panels have a different Isc to the 24V ones you'll lose more than you'd expect, especially on cloudy days when irradiance is already patchy.

Worth running the numbers through the Victron MPPT calculator before committing — it's saved me from a few expensive mistakes. @GazAllen has a point about the controller finding compromise, but if the specs are reasonably matched it needn't be catastrophic. What are the actual Isc figures on both panel types? That'd help work out whether you're genuinely losing significant yield or just theoretical percentage points that barely matter in a UK summer anyway.

@RoundTuit done exactly this on my shepherd's hut build — mixed some older 12V panels with newer 24V ones feeding into a Victron SmartSolar 100/20.

The key thing nobody's mentioned yet: check your Voc figures carefully before wiring anything up. Cold morning temps in the UK can push open-circuit voltage noticeably higher than the panel's rated spec, and if you're already near your MPPT's input ceiling you'll get over-voltage shutdowns at the worst possible times.

I'd use the Victron MPPT calculator to model your exact string configs before committing. Saved me from a fairly expensive mistake.

Also worth considering whether a second small MPPT dedicated to the mismatched panels might actually be cheaper long-term than chasing efficiency losses on a compromised combined array.

@RoundTuit worth understanding why the mismatch causes losses before deciding. When panels with different Vmp values are strung in series, the MPPT locks onto a compromised operating point — neither panel is at its true peak. I've seen this firsthand on my narrowboat where I temporarily mixed a salvaged panel into an existing string; the Victron VRM data showed measurable efficiency drops. Parallel strings with mismatched Isc are similarly messy. If you're committed to using both panel types, separate MPPT controllers per string is genuinely the cleaner solution — Victron's smaller SmartSolar units aren't prohibitively expensive.

Done this on both my shepherd's hut and the motorhome at various points. Honestly? Unless the specs are really close, the losses aren't worth it. The MPPT will chase a compromise voltage and neither string performs properly.

If you're dead set on using the old panels, wire them into a completely separate cheap MPPT rather than bodging them onto your Victron. Keep the SmartSolar doing what it does well. Renogy do decent budget controllers that'd handle the offcuts without crying about it.

Two controllers sounds overkill until you see the yield difference. 🤷