Anyone else finding their MPPT controller undersizing the array on cloudy days?

Been noticing this quite a bit lately, especially through this miserable grey winter we've had in Cornwall. My Victron SmartSolar 100/30 seems to harvest almost nothing on overcast days even when there's clearly some usable light about — the kind of flat white sky that should still be giving decent diffuse irradiance.

I've got a 400W array on the shepherd's hut and I'm wondering whether the issue is partly that my panels are sized too conservatively relative to what the MPPT can actually track efficiently at low light levels. I've read that some controllers struggle to find the true MPP when input voltage is low and inconsistent, which is basically every British winter morning ever.

A few questions for those who've dug into this:

• Does array oversizing actually help here? I've seen people running 150-200% of their controller's rated wattage and claiming better low-light performance
• Is this more a panel quality issue — would better low-light response panels (some of the half-cut cell ones seem promising) make a meaningful difference?
• Has anyone tried reorienting panels to catch more diffuse sky rather than optimising for direct sun angles?

I'm also running a small setup on the boat and seeing the same pattern there, so it doesn't feel like a one-off installation issue.

Would be really interested to hear what others have found, particularly anyone who's done before/after comparisons after changing their array size or controller. Does the Victron actually log enough data through the VictronConnect app to properly diagnose this, or do people tend to use something more detailed?

@CornishBoater yeah this is a known thing with MPPT in low light — the panel voltage can drop below the controller's start threshold so it just sits there doing nothing useful.

Few things worth trying:

• Tilt your panels steeper — diffuse light comes more from overhead, steeper angle helps in winter
• Check your Victron app, there's a minimum panel voltage setting — make sure it's not set too conservatively
• More panels in series bumps the Voc up which helps the controller actually wake up properly on dull days

My setup's purely emergency backup so I've got mine wired for higher voltage rather than higher amps specifically because of this issue — cloudy day performance improved noticeably.

Cornwall in January is basically a worst-case scenario for solar tbh, not much you can do about the physics of it.

@CornishBoater worth checking your panel Voc vs your battery voltage — on really flat grey days I've seen my array voltage barely creep above what the controller needs to actually kick in and start tracking.

On my cabin setup I've got a Victron SmartSolar 75/15 running 2x 200w panels and honestly in deep winter it sometimes just... sits there blinking at me doing sod all until around 10am even when it's technically "daylight."

One thing that helped me was adjusting the absorption voltage down slightly so the controller wasn't chasing a target it couldn't reach — seemed to wake it up a bit faster in low light.

Also check the VRM app if you're using it, the detailed graphs show exactly when it starts harvesting. Mine was eye-opening — losing a good couple of hours each end of the day I hadn't accounted for.

@CornishBoater something worth digging into is the panel wiring configuration — if you're running panels in parallel on a 12V system, your array voltage on a grim overcast day might barely clear the Victron's minimum start threshold.

I had exactly this on the narrowboat before I rewired two of my panels into series. Went from almost nothing harvesting on grey days to a meaningful trickle. The SmartSolar needs roughly battery voltage + a few volts overhead to actually engage properly.

Also worth enabling the "Advanced" logging in VictorConnect — look at the "Panel voltage" graph and see whether it's actually tracking or just sitting idle. Sometimes the controller is working but the harvest is genuinely that dismal (welcome to a UK winter 😅).

What's your panel layout — series, parallel, or mixed?

@CornishBoater my Fogstar-fed setup laughs at cloudy days now that I rewired to series — voltage stays respectable even when Cornwall's doing its best impression of the inside of a cloud.