Shading on one panel tanking the whole array — MPPT not coping?

So here's the situation. I'm running a modest 400W setup on the narrowboat — two 200W Renogy panels in series feeding a Victron SmartSolar 100/30. Been working a treat all summer, but we've been mooring up near some trees lately and by about 2pm the shadow from an overhanging branch is catching the corner of one panel.

What I'm seeing in the VictorConnect app is the whole array output cratering — we're talking dropping from a solid 18A down to maybe 3 or 4A the moment that shade hits. I get why it happens in series (one panel drags the other), but I'm wondering if switching to parallel would actually save me here, or whether the lower voltage would just create a different headache with the MPPT.

Has anyone retrofitted bypass diodes or added a Tigo/SolarEdge optimiser on a small boat setup like this? Feels like overkill for 400W but the losses are pretty painful when you're trying to keep the 200Ah Fogstar lithium bank topped up through a grey October.

Curious whether anyone's actually measured a real-world improvement after switching series to parallel on a shaded array, rather than just going off the theory.

@Ozzy8 — classic series string problem, that. When one panel gets even partial shade, the whole string suffers because the current gets bottlenecked at the weakest panel.

On my static caravan I had two panels in series and a single overhanging branch caused absolute carnage on the output graphs in the Victron app.

Switched to parallel wiring and it was like night and day — shaded panel drops its own output but leaves the other completely unaffected.

Worth considering if your mooring spots are consistently causing one panel to cop more shade than the other. The SmartSolar 100/30 handles parallel absolutely fine at your voltage — just double-check your combined amperage stays within spec.

Alternatively, SolarEdge optimisers fitted per panel can help if rewiring isn't practical, though on a narrowboat I'd honestly just go parallel and be done with it.

Been exactly where you are, @Ozzy8 — had the same series setup on my narrowboat before I switched to parallel wiring. The difference was night and day on tree-lined moorings.

Worth knowing: parallel means your shaded panel drags down its own output only, not the whole string. Downside is you'll need thicker cable runs to handle the lower voltage and higher current back to the controller.

The other option I explored was a couple of bypass diodes — most Renogy panels actually have them built into the junction box already. Check yours. If they're properly functional, shading should only kill individual cell strings rather than the whole panel.

@SparkySparky is right about the series issue, but honestly on a narrowboat where you're forever nudging under bridges and overhanging trees, parallel is just the more forgiving architecture long-term.

What @CarlCole97 and @SparkySparky haven't mentioned yet — the bypass diodes inside your panels are your first line of defence here, but they're not magic. Even with them working correctly, a partially shaded cell cluster will pull the whole string's operating point down to somewhere the MPPT algorithm simply struggles to track efficiently.

On my boat I actually ran a test before rewiring — watched the Victron app during a mooring where trees were catching one panel in the afternoon. The MPPT was hunting constantly, never settling. Switched to parallel, same shading scenario, and the drop became almost negligible.

Worth also checking whether your Renogy panels are half-cut cell variants — they handle partial shade noticeably better than standard mono cells due to the split-cell architecture reducing bypass diode activation losses.

Good points from @FrankGibson on the bypass diodes. Worth adding — the Victron SmartSolar is a decent MPPT controller, but it uses a single global MPP tracker, so it'll always chase one compromise point across the whole string rather than optimising each panel individually. If rearranging the mooring isn't practical long-term, you might consider swapping to a parallel wiring configuration as @CarlCole97 suggests, which does add some cable cost/complexity but makes a real difference with partial shade. Alternatively, look into panel-level optimisers like Tigo or SolarEdge — they bolt onto each panel and essentially give you per-panel MPPT. Bit of an investment on a 400W setup, but worth knowing the option exists. What direction are the panels facing relative to where the shading's coming from?

Great thread this — @Ozzy8, one thing nobody's touched on yet is the mooring position itself. On a narrowboat you've got the flexibility to shift your position slightly or rotate the boat if the marina allows it. Even moving 10-20 feet can dramatically change your shading profile throughout the day.

Also worth checking the Victron Connect app if you haven't already — the historical MPPT data will show you exactly when and how hard the controller is struggling. You'll see it clearly in the yield graphs as a characteristic dip.

Longer term, if rewiring to parallel isn't practical right now, SolarEdge power optimisers are worth a look — they essentially give each panel its own mini-MPPT, so shading on one doesn't drag the other down. Pricier solution but very effective for awkward installations.

Great thread this. One thing nobody's touched on yet — have you considered panel-level optimisers? Something like the Tigo TS4-A-O can be retrofitted to individual panels and essentially gives each one its own mini-MPPT. Overkill for some setups, but on a narrowboat where you're constantly shifting mooring positions and dealing with unpredictable tree/bridge shade, it can make a genuine difference.

Also worth checking your cable runs — on boats I've seen dodgy crimps and damp connections add enough resistance to confuse the controller into thinking there's a shading issue when there isn't really one. @FrankGibson's point about bypass diodes is spot on, but failed diodes are sometimes misdiagnosed when the real culprit is a corroded MC4 connector. Spray some contact cleaner and reseat them before spending any money. 🙂

@Ozzy8 been there mate. My boat spent a winter under a bridge and I nearly lost my mind watching the Victron app.

Simplest fix nobody's mentioned — parallel instead of series. Yes, you lose a bit of efficiency in ideal conditions, but shading one panel won't drag the other down with it. Your 100/30 handles the lower voltage fine.

Two mins with a couple of MC4 branch connectors and you're sorted. Proper cheap solution before spending money on optimisers.

@Ozzy8 worth checking whether your Renogy panels are actually matched — grab the Voc and Isc off both labels and compare. Mismatched panels in series behave badly even without shading. Had this exact issue on my own narrowboat until I realised one panel was a slightly different batch.

Also, in the Victron app under History, check your daily peak wattage against theoretical maximum. If you're consistently pulling well under even on clear days, the mismatch theory holds up before you start spending money on optimisers.