Anyone else finding their 12V system struggles on grey January days even with decent panel wattage?

Got a bit frustrated this week and wanted to see if others are in the same boat. I've got 400W of solar (two 200W panels wired in parallel) on the van roof feeding into a Victron SmartSolar 100/30 MPPT, with a 200Ah lithium (a Fogstar Drift). Even with that setup, we've had four days in a row here in the Peak District where I'm barely seeing 20–30W coming in by midday. State of charge has been creeping down to around 60% and I'm having to be really careful with the diesel heater and keeping laptop use minimal.

I know the obvious answer is "add more panels" but I'm genuinely surprised how bad it's been. The panels are tilted at maybe 15 degrees on a flat roof rack, which I suspect isn't helping when the sun is so low. I've been reading about tilting panels manually when you're stationary — has anyone actually rigged up a proper adjustable mount for a van roof? Wondering if it's worth the hassle or whether I should just accept a small genny or a DC-DC charger from the alternator as the winter backup.

Also curious whether anyone's had luck with bifacial panels in these conditions — a mate swears they pick up more diffuse light but I can't find any solid real-world UK winter data on them. Actual numbers from anyone who's tried them would be brilliant.

Hey @FrankMurray, know exactly what you mean mate. January in the UK is brutal for solar - you're lucky to get 2-3 peak sun hours even on a clear day, and when it's properly overcast that can drop to practically nothing.

One thing worth checking is your panel angle. Flat-mounted panels on a van roof really suffer in winter when the sun sits so low in the sky - you could be losing a significant chunk of potential output just from that. Even propping them up temporarily when parked makes a noticeable difference.

Also, are you monitoring your actual daily yield through the Victron app? Sometimes the numbers are less terrible than it feels, but at least you'd know what you're genuinely working with rather than guessing. What's your battery bank size? That's often where the real problem hides.

Hey @FrankMurray, one thing worth checking is whether your panels are actually wired optimally for those grey days. Parallel keeps voltage low (around 20-22V open circuit) which can struggle to wake up your MPPT properly in overcast conditions when irradiance is already weak. Try rewiring series-parallel if you add more panels later, or even just series temporarily - you'll get higher Voc which helps the controller find and track a proper power point even in dim light. Also worth having a look at your Victron app history - the yield graphs will tell you honestly how many useful hours you're actually getting. Bet it's under an hour equivalent most days this month. A small battery top-up from a B2B charger while driving honestly saves my sanity through winter. 🙄

Series wiring on a 12V system is a spicy topic but @TransitNomad is onto something — low-light performance lives or dies by your MPPT's minimum input voltage threshold, and parallel can actually win when irradiance is patchy rather than just dim. That said, my Fogstar Drift 100Ah laughs at my January guilt far less than I'd like, so battery capacity is probably your real villain here — 400W of panels feeding a undersized bank is just rearranging deckchairs on the Titanic, except the iceberg is a Leeds overcast sky.

@FrankMurray I've got a similar setup on my static caravan in North Yorkshire and January is genuinely miserable for it. One thing nobody's mentioned yet — have you checked whether any shading is hitting your panels, even partially? Even a small shadow on one cell in a parallel configuration can drag down output more than you'd expect.

Also worth looking at your actual consumption vs what you think you're drawing. I was shocked when I put a proper energy monitor on mine — things like a 12V fridge cycling constantly adds up faster than expected on short winter days.

What's your battery capacity? That's often the real bottleneck when generation drops off — even decent panels can't save you if your storage isn't large enough to buffer through overcast stretches.

@FrankMurray totally feel your pain mate. One thing nobody's mentioned yet — have you checked your actual panel output voltage on those grey days using the Victron app? The SmartSolar gives you really detailed live data and it's worth seeing whether your panels are even reaching MPPT threshold voltage. In parallel your combined Voc stays at 200W panel levels (probably around 24-26V open circuit?) which on a dull January day might drop enough that the controller's struggling to find a proper operating point. Also worth checking for any shading on even a corner of one panel — in parallel that'll drag the whole string down more than you'd expect. The Victron Connect app graphs are genuinely brilliant for diagnosing this stuff over a few days.