Anyone else finding their MPPT controller massively underperforming on overcast UK days?

Picked up a Victron SmartSolar 100/30 earlier this year and paired it with two 200W panels wired in series on the roof of my Sprinter. On a decent sunny day it's brilliant — hitting around 350-400W input no bother and keeping my 200Ah LiFePO4 topped up nicely. No complaints there.

But on your typical grey British day (which let's be honest is most of them), I'm seeing inputs of 20-30W at best, sometimes less. I get that diffuse light is rubbish, but my mate with an older PWM setup and similar panels reckons he's pulling proportionally similar figures, so I'm not convinced I'm getting the MPPT advantage I paid for. Wondering if it's a shading issue, panel angle, or just the reality of UK cloud cover.

Has anyone done proper side-by-side testing or logged data over winter months? I've got the VictronConnect app so I can pull history graphs — happy to share screenshots if that helps diagnose what's going on. Just want to know if this is expected behaviour or if there's something worth tweaking.

@EwanLamb yeah this is totally normal for the UK tbh. Overcast days kill your voltage more than anything — panels in series helps keep Voc up which is good, but diffuse light just tanks the wattage regardless.

Worth checking your MPPT logs in the VictronConnect app though. If it's showing low panel voltage rather than low current, could be a dodgy connection or partial shading dragging the whole string down (series wiring is sensitive to that).

Realistically on a proper grey British day I'm seeing maybe 15-20% of rated output on my shepherd's hut setup. That's just life up here. Factor it into your daily budget and you'll stress less about it.

Worth adding from my static caravan setup — even on properly gloomy days I'm often surprised how much comes through if the panels are clean. Dirty panels in diffuse light genuinely lose you a significant chunk compared to full sun conditions where you'd barely notice the grime.

Also worth checking your battery state of charge when you're seeing low numbers. If the bank is already near full, Victron will throttle back absorption — looks like underperformance but it's actually the controller doing exactly what it should.

Pull up the VictorConnect history and check whether it's actually limiting due to absorption/float rather than panel input. That caught me out for weeks before I spotted it. The app graphs make it pretty obvious once you know what you're looking for.

Hey @EwanLamb, one thing worth checking is whether your panels are actually dipping below the controller's start voltage threshold on those really gloomy mornings. The SmartSolar needs a minimum input before it'll even begin tracking, and with a series string you might think you've got enough voltage but the MPPT algorithm can take a while to lock onto the proper operating point in diffuse light. Also worth having a look at your absorption and float settings — if they're tuned for summer peak performance they can sometimes be a bit aggressive and cut the harvest short on lower-output days. The VictronConnect app is brilliant for this, just pull up the history tab and see what the panel voltage was doing throughout the day. Overcast UK skies are brutal but there's usually a bit of room to squeeze more out through the settings. 🙂

Really interesting thread this. One thing I'd add @EwanLamb — have you had a look at the VictronConnect app to check your absorption and float voltage settings? On overcast days the controller can sometimes spend ages sitting in bulk trying to squeeze every bit out of what little's coming in, which is actually correct behaviour, but it can look like underperformance when really it's just working with what it's got.

Also worth checking your panel connections haven't developed any resistance over winter — even a slightly loose MC4 connector can cost you a surprising amount in low-light conditions when every watt counts. Had this exact issue on my own van last November and couldn't work out why numbers seemed off until I reseated everything. Made a noticeable difference!

Great thread, @EwanLamb. Something nobody's mentioned yet — panel orientation and angle make a surprisingly big difference on overcast days in the UK. Unlike direct sunlight where you're mostly chasing the sun's position, diffuse light on cloudy days comes from the whole sky dome, so a more horizontal tilt can actually capture more of it than a steep angle optimised for summer sun. Worth experimenting if your roof mount allows any adjustment.

Also, are your panels getting any partial shading from roof fixtures, aerials, or even the Sprinter's high roof profile at low sun angles? Even slight shading on series-wired panels absolutely hammers output — one shaded cell can drag down the whole string. Might be worth checking throughout the day rather than just at noon. The VictronConnect history graphs @DebbiePowell76 mentioned would help identify if that's the culprit.

Great points from everyone already. One thing I'd add to what @ValleyNomad mentioned about orientation — on overcast days in the UK you're essentially dealing with diffuse light coming from the whole sky rather than a direct source, so you actually want maximum exposed panel area rather than chasing a sun angle. A flat or near-flat mounting can genuinely outperform a tilted setup on a heavily clouded day.

Also worth bearing in mind @EwanLamb — 350-400W from two 200W panels on a good day is actually pretty respectable performance and suggests your setup is fundamentally sound. Don't be too disheartened by overcast figures; even quality panels can drop to 10-20% rated output in thick British cloud. That's just physics unfortunately, not a controller issue!

Boat life taught me that UK "overcast" covers everything from "a bit grey" to "basically underwater," and my Victron handles them all differently — diffuse light from a white sky can actually scatter decent wattage if your panels aren't shaded by a rogue seagull or, in @EwanLamb's case, a Sprinter roofbar.

@MarineKaren is right — "overcast" is almost meaningless as a descriptor. That said, one thing nobody's touched on: check your actual Voc at low irradiance. MPPT algorithms can struggle to find the true power point when the I-V curve flattens out in diffuse light. In the SmartSolar app, watch whether the controller is hunting. Also worth verifying your panel wiring connections — high-resistance joints that barely matter at 400W become proportionally significant at 40W. I've seen dodgy MC4 crimps rob 20-30% on dull days on my narrowboat system when sunny-day figures looked perfectly fine.