Anyone else finding their MPPT underperforming on cloudy days or is it just my setup?

Mine was doing the same sad little shuffle on overcast days until I realised I'd undersized the array — turns out my Victron SmartSolar was basically twiddling its thumbs waiting for voltage that never quite arrived! 🌥️

A few things worth checking:

• Panel voltage vs. MPPT minimum input — on cloudy days your Voc drops more than you'd think, and if you're right on the edge of the controller's threshold you'll get naff all
• Wire runs — longer cables mean more voltage drop, which hurts even more when the panels are already struggling
• Shading — even partial cloud shadow on one panel in a series string kills the whole lot

I added a third panel to my garden office setup (Renogy 200W mono, bargain off eBay) purely to give the string enough headroom on gloomy days, and honestly it made a proper difference. Now even on a properly miserable British November afternoon I'm pulling something useful rather than watching the Victron app display that heartbreaking 3W.

Also worth having a poke around in the VRM portal if you're on Victron — the historical yield data will tell you pretty quickly whether it's a voltage floor issue or something else entirely.

Curious whether anyone's tried parallel stringing specifically to address this? I keep going back and forth on reconfiguring mine but haven't committed yet. Anyone done the comparison on identical setups? 👇

@BoatPaddy had exactly this with my van build last winter. Parked up in Sussex during that grey February stretch — the MPPT wasn't underperforming, it was doing precisely what the physics demanded.

What actually shifted things for me was tilting the panels. Flat-mounted roof panels on overcast days are leaving a shocking amount on the table. Even a modest 15–20° tilt made a measurable difference on diffuse light days because you're capturing more of that scattered sky radiation rather than just direct beam.

Also worth checking your battery absorption voltage is set correctly — I'd had mine slightly low, which made the controller look lazy when it was actually just hitting target early.

What's your panel wattage vs controller rating? Sometimes the "undersized array" diagnosis is masking a VOC mismatch that's clipping your harvest before it even starts.

@BoatPaddy on my narrowboat I run a 600W array into a Victron SmartSolar 100/30 and the secret weapon on grim British days is panel orientation — even a slight tilt adjustment toward the low winter sun angle makes a surprisingly cheeky difference to those marginal voltage figures.

Worth checking your panels are actually wired for the voltage sweet spot your MPPT wants to see; too low a Voc and it's basically sat there sulking before it even tries to track. Fogstar cells are lovely but they still need the geometry working in their favour.

Also — and this one stings — make sure your battery state of charge isn't already high, because a full bank will throttle output and make the whole setup look like it's underperforming when it's actually just done its job.

@BoatPaddy worth checking whether your panels are actually wired for the highest practical Voc too. On the boat I found that running two panels in series rather than parallel made a noticeable difference on dull days — the MPPT has something to actually work with even when irradiance is poor.

Also, what's your cable run length? Voltage drop on long runs to the controller is brutal in low-light conditions when you're already scraping the barrel. I redid mine with proper 6mm² and it wasn't transformative but it helped at the margins.

The Victron kit is solid but it can't conjure watts from nothing — sometimes it really is just the physics of a grey British sky rather than a setup issue.

Great thread. One thing I'd add that hasn't been mentioned yet — have a look at your cable runs and connection quality. I lost a surprising chunk of harvest on dull days tracing it back to corroded MC4 connectors on my narrowboat. When irradiance is already low, any resistance in the circuit becomes disproportionately costly. Also worth enabling the "BatteryLife" algorithm if you're on Victron and doing adaptive charging — it genuinely helps squeeze more usable cycles during prolonged overcast spells rather than just sitting at float doing nowt. @RetiredChef your 600W setup sounds similar to mine, I'd be curious what battery bank size you're running alongside that. I found matching the array to bank capacity properly made a noticeable difference to how the MPPT behaved on marginal days here in the northwest.