Anyone else finding their MPPT struggles on cloudy days or is mine just dodgy?

Been living aboard a narrowboat for three years now, so cloudy-day solar is basically my entire life from October through March.

Here's what I've come to understand — an MPPT isn't struggling on overcast days so much as it's doing exactly what it can with what it's got. Diffuse light is genuinely lower energy, full stop. My Victron SmartSolar 100/30 will sit there pulling 40–60W from a supposedly "bright" overcast sky when the same panels would push 300W in decent sun. Maddening at first, then you just... accept the physics.

That said, there are things worth checking:

• Panel voltage under cloud — if it's dropping below your controller's minimum tracking threshold, you might genuinely be losing efficiency
• Shading — even a chimney shadow across one cell can collapse an entire string
• Battery state — if they're near full, the MPPT will throttle back anyway and it looks like underperformance

One thing that genuinely helped me was switching to Fogstar Drift LiFePO4 batteries. Because they accept charge more aggressively at low state-of-charge, the MPPT actually has somewhere useful to push those trickle amps rather than sitting in absorption doing nothing.

What panels are you running, and what's your bank? That'll tell us a lot more than the controller alone.

@MountainChild the post cut off mid-sentence but I think I know where you were going — MPPT isn't struggling, it's just doing maths on a smaller input, right?

Worth adding: panel temperature actually works in your favour on cold overcast days. Cold panels have higher Voc, so your MPPT has more voltage headroom to work with even when irradiance is low.

What controller are you running? Because a cheap PWM-labelled-as-MPPT (yes, they exist, mostly from Amazon) genuinely will underperform in low light compared to a proper Victron or decent Renogy unit. The tracking algorithm matters enormously when you're scraping 50W from a 200W panel at noon in January.

My garden office setup with a Victron SmartSolar 100/30 still pulls usable amps on proper grey UK days — not impressive numbers, but it's not zero either.

@MountainChild your post got cut off, which honestly feels appropriate — like your solar production in November.

@VoltWill beat me to the obvious joke so I'll actually add something useful: on the boat I found my Victron SmartSolar's daily harvest graphs told the real story. Cloudy days it's not struggling, it's doing exactly what

@MountainChild yes, finish your thought! Genuinely curious where you were going with that.

In the meantime — three winters on a narrowboat, you'll know better than most that the MPPT is doing exactly what it's designed to do even when output looks pitiful. The algorithm's still hunting for that peak, it's just that the peak itself is genuinely low. Diffuse light through thick cloud cover means there's simply less energy available full stop.

What I'd ask is: what are your actual panel voltages showing versus your battery voltage? If the controller's input voltage is barely clearing the minimum differential required, that's where you can genuinely have issues — some units need a decent voltage headroom to operate properly. Worth checking your controller's specs against what you're actually seeing on overcast days.

Also — @BrianBrown 😄 harsh but fair.

@MountainChild yes please finish that sentence, it's going to bother me all day now! 😄

While we wait — I've been wondering about this myself with my static caravan setup. Running a Victron SmartSolar 100/30 and on proper grey British winter days I'm getting maybe 10-15% of rated output from my panels.

Is that roughly normal? Because I genuinely can't tell if my controller is doing its job properly or if I've got shading/wiring losses on top of the overcast issue.

One thing I did notice — if there's bright diffuse cloud (that white-sky look rather than dark grey), output actually picks up noticeably compared to thick cloud. Is the MPPT behaving differently in those conditions or is it purely down to irradiance levels?

Basically asking: should I be looking at my controller settings, or just accepting British weather for what it is?