Anyone else noticed MPPT efficiency drop in winter low-light conditions — or is it just my Victron 100/30?

So I've been keeping a closer eye on my setup over the past few weeks now that we're deep into the grey British winter, and something's caught my attention. My Victron SmartSolar 100/30 seems to be tracking noticeably less efficiently when the panels are putting out under about 50W — we're talking the kind of flat, overcast days where the sky is basically one giant grey blanket. I've got two 200W panels wired in series (so 24V nominal array) feeding into a 12V leisure battery bank, and on a good summer day the thing is spot on. But lately I'm seeing the MPPT hunting around quite a bit before it locks on, and sometimes I wonder if it's even finding the true peak at all.

I've been logging via the VictronConnect app and on some of these dim days I'm pulling maybe 18–22W from conditions where a quick calculation suggests I should be getting closer to 35–40W. That's not a massive loss in absolute terms but over a short winter day it adds up, especially when I'm trying to keep a 200Ah LiFePO4 topped up enough to run a 12V compressor fridge overnight.

I've read a few things suggesting MPPT controllers in general can struggle with very low irradiance because the power curve flattens out and there's less of a distinct peak for the algorithm to lock onto. But I'm not sure if this is a known quirk of the Victron specifically, a firmware thing, or just physics doing its thing. Has anyone done any proper comparisons between controllers in these conditions — Victron vs EPever vs Renogy etc?

DaiWalker61 | 847 posts | ⭐ Trusted Contributor

@SparkySailor Not just you, mate. I've noticed the same with my 75/15 — efficiency figures look a bit pants compared to summer, but honestly a lot of that is the panel voltage behaviour in cold, diffuse light rather than the MPPT algorithm itself struggling.

Worth checking your Voc at dawn when panels are cold and overcast — you might find it's actually sitting quite close to your array's maximum input voltage, which can cause the controller to back off slightly.

Also, have you looked at your bulk/absorption thresholds in VictronConnect? Sometimes the controller spends ages in float during these short days because the battery never properly cycled the day before. That can look like efficiency loss but it's really just system behaviour.

What's your panel configuration and battery bank?

SueParker64 | 312 posts | 🌊 Narrowboat Life

Living aboard on the cut, this hits close to home. Last January I was moored up near Foxton Locks for a fortnight and my Victron was doing something peculiar in those flat, overcast days — barely a whisper of current despite the panels not being shaded.

What I eventually pieced together was that the MPPT algorithm genuinely struggles when the array voltage is hovering right at the threshold. The controller almost can't "decide" where the maximum power point actually is when the curve is so flat.

Worth checking your VRM data if you're logging it — I spotted mine was rescanning the MPP every few minutes rather than tracking smoothly. That constant hunting costs you real yield. Fogstar mentioned something similar in one of their newsletters about winter behaviour.

What's your panel voltage at the array in those grey conditions, @SparkySailor?

Nige1977 | 234 posts | 📍 Array

This took me down a proper rabbit hole last January when I was trying to squeeze every watt into my garden office setup. What I found was the MPPT algorithm genuinely struggles when irradiance is patchy — it hunts for the peak but cloud edge events keep shifting it before it locks in.

Worth checking your absorption voltage settings too. In low-light my Victron was occasionally hitting absorption prematurely on the Fogstar cells, then backing off, which looks like efficiency loss but is actually the battery profile doing its job.

The real culprit for me turned out to be panel temperature — counterintuitively, cold panels on a crisp clear day actually outperformed summer. It's the thick grey soup overhead that kills you, not the cold itself.

Have you tried logging via VictronConnect over a full week? Patterns become much clearer than day-to-day snapshots.

ExSquaddie | 156 posts | 📍 Array

Worth checking your panel voltage at the controller input in those flat grey conditions — I was convinced my Victron 75/15 was playing up last February until I realised my panels were barely hitting 18V open circuit on an overcast day. MPPT needs a meaningful voltage differential to actually do anything useful.

Also had a corroded MC4 connector causing grief that only showed up properly in low-irradiance conditions — full summer sun would mask it completely. Gave everything a proper inspection with a multimeter and found a dodgy connection losing me nearly 20% of what little I was generating.

The Victron app history graphs are genuinely useful here — compare your peak wattage on similar forecast days across weeks and you'll soon spot if it's gradual degradation vs a specific fault.

DucatoSolar | 847 posts | 🚐 Van + Cabin Build

Something worth adding that hasn't been mentioned: in very low irradiance conditions, your panel's actual operating voltage can sit awkwardly close to your battery voltage, and the MPPT algorithm genuinely struggles to find a proper peak — there's barely a curve to track. I've logged this on my 100/20 across both the Ducato and the cabin setup.

One practical check: pull your VRM history (assuming you've got the dongle) and compare your peak power point voltage against battery voltage on overcast days. If they're within ~5V of each other consistently, that's your culprit — undersized array VOC for winter conditions rather than controller fault.

@ExSquaddie raises a good point on checking input voltage. I'd go further and log it over several hours using the Victron app rather than spot-checking.

SimonHenderson | 412 posts | 📍 Rural Aberdeenshire

Good thread this. One thing I'd add — have you checked your panel temperature? Counter-intuitive I know, but cold panels actually produce higher voltage, which can work in your favour with MPPT tracking. The problem in deep Scottish winters for me has been the current side being so low that the controller's tracking algorithm genuinely struggles to find a stable maximum power point — it's almost dancing around rather than locking on.

I started logging mine through VictronConnect and the difference between a crisp cold bright day versus a flat grey overcast one is stark, even at similar temperatures. @DucatoSolar's point about low irradiance behaviour is spot on in my experience.

Worth checking your absorption and float thresholds too — batteries behave differently in the cold and that can skew your efficiency readings somewhat.