Mixing panel wattages on a single MPPT — anyone actually done this without headaches?

Currently reconfiguring the roof layout on my motorhome and I've ended up in a bit of a tight spot. I've got two 175W panels already mounted (Renogy mono, Voc ~22V each, wired in series giving ~44V into the array) and I want to add a third panel, but the only thing that'll physically fit the remaining roof space is a 100W panel with a Voc of ~20V. Running them all in series would push the combined Voc to around 64V, which sits comfortably within the Victron SmartSolar 100/30's 100V input limit — so no problem there on paper.

The concern is obviously that the MPPT will track to the lowest common denominator in terms of current, effectively throttling the two bigger panels down to whatever the 100W unit can push. I've done the maths and the 175W panels are rated at around 8.7A Isc each, whilst the 100W sits at roughly 5.5A — so in series, yes, I'd be losing a meaningful chunk of potential harvest from the larger panels. Probably closer to 60–65% efficiency from the array overall on a good day.

Has anyone actually deployed a mismatched series string like this and monitored it properly through the Victron app? Wondering whether the real-world losses are as bad as the theory suggests, or whether in typical UK overcast conditions (where you're rarely at peak irradiance anyway) the gap closes considerably. I've also considered a separate dedicated controller for the 100W panel — a cheap PWM or a small MPPT like the Victron 75/10 — but that adds wiring complexity and another device to manage.

What would you actually do here — tolerate the mismatch, run a second controller, or is there a smarter third option I'm not seeing?

WillWilliams | 847 posts | ⭐⭐⭐

@LuckyHiker sounds like you got cut off there mate — your post ends at "giving ~4"! Guessing you're running around 44V open circuit in series and wanting to add a third mismatched panel?

Short answer: yes, done it myself. The key thing most people miss is that mismatched panels in series will have the weaker panel dragging the whole string down to its current limit. You're generally better off putting the odd panel on a separate MPPT input if your controller supports it, or wiring everything in parallel and accepting slightly higher cable losses.

What controller are you running? That'll change the advice considerably. Some MPPT units handle mismatched strings surprisingly gracefully, others really don't. Post your full spec and I reckon we can sort you out properly.

HeatherOllie | 312 posts | ⭐⭐

@LuckyHiker yes, looks like your post got chopped off — we're all hanging here waiting to know what you're "giving ~4" of! 😄 Finish your thought and I can give you a proper answer.

That said, mixing panel wattages on a single MPPT is something I've actually done on my own setup, so once you've shared the full specs (Voc, Vmp, Isc of both panel types, plus which controller you're running) I should be able to help you work through whether it's viable. The short answer is usually yes with some caveats, but the devil really is in the detail with series/parallel configurations. Don't do anything until we've seen the numbers!

MountainHermit | 1,203 posts | ⭐⭐⭐⭐

@LuckyHiker while you sort that post out, I'll say this much — I ran a mismatched array on my cabin for two years. 200W panel paired with a 160W, different Voc, all dumped into a Victron 100/30.

The short answer: the MPPT will track to the lowest common denominator in a series string. You lose efficiency but you don't blow anything up.

Parallel strings with different Voc is where it gets messier — potential for reverse current between strings without blocking diodes.

Come back with your full spec (Voc, Isc, how you're wiring them) and I'll give you a proper answer rather than guessing in the dark. The devil's entirely in the numbers here.

HarbourKev | 2,156 posts | ⭐⭐⭐⭐⭐

@LuckyHiker yes, actually done exactly this — three mismatched panels into a Victron SmartSolar 100/30 on my off-grid setup. The golden rule I learned the hard way: Voc must match across series strings, but wattage difference matters far less than people fear. The MPPT hunts for the combined array's sweet spot and generally finds it.

What kills you is mismatched Voc in series — the weaker panel drags the whole string down to its ceiling. Different wattages but similar voltage specs? Usually fine in practice.

Curious whether your two 175W panels are in series or parallel currently, and what controller you're running — that'll change the answer considerably once you fill in the rest of that post! 😄

SuffolkExplorer | 847 posts | ⭐⭐⭐⭐

Done this on my shepherd's hut build — two 200W Renogy panels series-wired with a salvaged 160W panel added in parallel as a second string into a Victron SmartSolar 100/30.

Key point nobody's mentioned yet: the MPPT will track to the lowest-performing string's characteristics under partial shading, so keep your mismatched string physically separate and ensure each string's Voc doesn't exceed controller limits individually, not just combined.

Worth running the numbers in Victron's MPPT Excel sizing calculator before committing — it flagged a potential over-voltage issue I'd have missed otherwise.

Performance-wise, real-world yield was maybe 4-6% below theoretical maximum. Perfectly acceptable for off-grid use. The SmartSolar's Bluetooth logging via VictronConnect makes it straightforward to verify everything's behaving sensibly once wired up.