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

Running a bit of an experiment on the narrowboat and thought I'd share before asking the question. Currently got a Victron SmartSolar 100/30 feeding a 200Ah Fogstar Drift LiFePO4. Up until last month I had two 175W panels wired in series — tidy, predictable, happy days.

Picked up a decent second-hand 250W panel (same Voc, roughly — 40.2V vs 39.8V) and added it in parallel with one of the 175W units rather than buying a matched pair. Total array is now around 600W across a slightly mismatched parallel string. The Victron Connect graphs look... fine? Pulling decent amps on sunny mornings, no error codes, absorption hits when it should.

The textbook answer is obviously "match your panels," and I know the 250W is effectively dragged down to behave like a 175W when paralleled because the string voltage is dominated by the lower Isc panel. So I'm probably leaving 50–75W on the table in ideal conditions — not the end of the world on a canal boat where I'm rarely solar-limited in summer.

What I'm actually curious about is whether anyone's run mismatched parallel strings through a single MPPT over a year or two and noticed any degradation, weird thermal behaviour, or whether the controller just quietly compensates and you never really notice. Static caravan crowd especially — you lot tend to leave arrays unattended for months at a time which is where I'd worry most.

Foggy91 | 847 posts | ⚡ Solar Evangelist

@Scouse sounds like a familiar situation! I've been mixing a 200W and a 100W panel on my Victron 75/15 for about 18 months now, both wired in parallel. The key thing I found is that the MPPT just sees a combined string voltage and hunts for the best overall power point — it doesn't care that one panel is physically smaller, as long as the Voc ratings are reasonably matched.

Where it gets interesting is partial shading. The smaller panel can drag down the whole array if they're in series. Parallel kept things much happier for me practically speaking.

What panels are you actually running and how are they configured? That'll make a big difference to whether this is going to work cleanly long-term. The 100/30 has plenty of headroom current-wise so you've got that going for you at least.