Mixing panel wattages on a single MPPT — anyone actually done this long-term?

Got a bit of an odd situation with my shepherd's hut build. I've got two 200W panels already wired up to a Victron SmartSolar 100/20, running sweet as a nut. But I've managed to get hold of a 175W panel dirt cheap (same Voc, roughly — 37V open circuit vs 38V on the existing two) and I'm tempted to bung it in series-parallel with the others rather than buying a second controller.

The maths seems fine — total array would be 575W, still within the 100/20's input limits, and the voltages are close enough I'm not losing sleep over it. But I keep reading conflicting things about whether the MPPT will properly track when the panels have different Pmax points. Some say it just finds a compromise and you lose a bit of efficiency, others say it's basically fine in real-world conditions.

Has anyone actually run a mismatched array for a season or more and got proper data out of it? I've got VRM logging on the Victron so I could compare before/after reasonably accurately. Wondering if it's worth the faff or if I should just sell the 175W on and keep the array clean.

AngliaNomad | 847 posts | Midlands

@Gill1982 Yes, done exactly this on my narrowboat setup for about three years now. Mixed a pair of 250W panels with a lone 160W and it's been absolutely fine through the controller.

The key thing to watch is your string configuration rather than worrying too much about the wattage mismatch itself. If you're wiring in parallel (which sounds likely given your 100/20), just make sure the Voc figures are reasonably similar across your panels, otherwise the lower-voltage panel can drag performance down somewhat.

Worth checking your total array wattage doesn't push the controller beyond its limits too — the 100/20 at 12V is only rated around 240W safely, so three panels could be nudging it depending on your battery bank voltage.

What voltage are you running the hut at? That changes the answer quite a bit.

StuThompson | 312 posts | South Wales

@Gill1982 Worth double-checking the Voc of that 175W panel before you do anything else. If you're already near the 100V limit on your SmartSolar with two 200W panels in series, adding another could push you over — especially on a cold morning when Voc spikes. Parallel is almost certainly the safer route here.

The mixed wattages themselves aren't really the issue — the MPPT will find the best compromise point across the string. You'll lose a tiny bit of efficiency but nothing dramatic in real-world terms.

What's the make/model of the 175W? Some of the older panels have slightly different temperature coefficients which can complicate things marginally, but honestly for a shepherd's hut it's unlikely to matter much. Post the spec sheet if you've got it.

VanKen | 234 posts | Array

My static has three different wattage panels on one SmartSolar and the only thing that's suffered long-term is my ability to explain it to anyone who asks — the Victron just quietly gets on with it like a champ. 🤷