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

So I've got a bit of an awkward situation on the narrowboat. Started out with two 175W panels wired in series feeding a Victron SmartSolar 100/30, all lovely. Then I picked up a barely-used 200W panel for a tenner at a boat jumble and couldn't say no, could I?

I've since wired all three in series — giving me a Voc of around 118V which is within the controller's 100V limit when cold, just about — but I'm aware the 200W panel is the odd one out. String theory says the weakest panel drags everything down, but in practice my daily harvest has gone from roughly 650Wh to 850Wh on a decent spring day, so something is working.

What I'm less sure about is whether I'm doing any quiet, invisible damage over time. The Victron app shows the MPPT hunting a bit more than it used to — the tracking efficiency percentage drops to around 94% on patchy days where it used to sit at 98–99%. Is that just normal behaviour with mismatched panels, or is it a sign something's struggling?

Has anyone run a mismatched string for a full season or two and got data to share? Tempted to stick a Renogy DC clamp meter on each panel feed to see what's actually happening, but curious whether there's a smarter diagnostic approach first.

HarbourKate | Moderator | Posts: 1,847

@Midge yes, running mismatched panels long-term here — three seasons on my liveaboard now. The short answer is it works, but your 200W will effectively be dragged down to match the lower-performing panels in series. Worth checking your Voc figures carefully before wiring anything up; the 100/30 has a 100V input limit and narrowboat summers can catch you out with cold-morning voltage spikes.

Personally I'd seriously consider a second small MPPT dedicated to that 200W panel running independently. Victron's 75/15 is cheap enough and you'd actually harvest meaningfully more power overall rather than letting the mismatch throttle everything. The SmartSolar app makes monitoring both controllers dead easy too.

What's the Vmp on each panel roughly? That'd help figure out whether series or parallel even makes sense here.

VoltJack | Posts: 312

@Midge I've been running a mismatched array for about two years now — a pair of 200W panels alongside a lone 160W, all through a Victron 100/50. The key thing nobody mentions is to check your Voc figures carefully before wiring in series. Different wattages are fine if the voltage characteristics are compatible. Where people come unstuck is assuming wattage differences are the main issue — it's actually the Vmp mismatch that'll cost you harvest efficiency in series strings. If you can wire that 200W as a separate parallel string into the same MPPT (assuming your controller's input current rating allows it), you'll often get better real-world results than forcing mismatched panels into series. What are the spec sheet figures on that 200W? Voc and Vmp specifically — that'll tell us whether series or parallel makes more sense for your setup.

RelayLife | Posts: 634

@Midge something worth checking that nobody's mentioned yet — with mismatched panels in series, have a look at the individual panel specs, specifically the Isc (short circuit current). Your 175W panels likely have a lower Isc than the 200W, and in a series string the current is effectively limited to the lowest panel's output. So you may not be getting as much from that 200W as you'd hope.

Worth pulling up the datasheets and comparing. If the Voc values are reasonably close but the currents differ significantly, you might actually get better results running the 200W on a separate small MPPT rather than lumping everything together. A cheap 20A unit isn't expensive and on a narrowboat the extra wiring is manageable. Just a thought before you commit to the long-term mixed setup.

Smudge_6508 | Posts: 89

@Midge worth noting that the 200W panel will almost certainly have a different Voc and Vmp to your 175W ones, so in series your string voltage will be slightly asymmetrical. The Victron will handle it fine electrically, but you'll essentially be "dragging" the MPPT tracking point away from what either panel would ideally want on its own. In practice on a narrowboat you're unlikely to notice much — the movement and shading from bridges and trees probably costs you more than the mismatch ever will!

One thing I would do is pop into the VictronConnect app and keep an eye on your yield figures for a few weeks after adding it. If you've got VRM logging set up even better. You'll soon see if something's genuinely off rather than just theoretical losses.