Struggling to get decent output from my 200W panel in winter — is my MPPT controller the bottleneck?

So I've got a 200W monocrystalline panel on the roof of my Transit conversion, wired into a Victron SmartSolar 75/15 MPPT controller, charging a 100Ah lithium (LiFePO4) battery. In summer I was pulling decent numbers — regularly seeing 10-12A going in on a clear afternoon. But now we're into January and even on what look like bright days I'm barely scratching 3-4A, sometimes less. I know winter sun is weaker and lower in the sky, but this feels like more of a drop than I'd expect.

I've been watching the Victron Connect app and I'm noticing the panel voltage sits around 18-20V on these cold days, which seems lower than I'd expect for a mono panel rated at Vmp 18.4V and Voc 22.5V. Surely in cold weather the voltage should actually be higher? I did read somewhere that cold temps push Voc up, so I'm confused why I'm seeing the opposite.

I'm wondering whether the MPPT is actually tracking properly or whether there's something else going on — shading from a roofline vent maybe, or just the angle being totally wrong for low winter sun. The panel is flat-mounted with no tilt, which I know isn't ideal, but I didn't think it'd make this much difference.

Has anyone else seen similar with a flat-mounted setup in winter, or had issues with MPPT tracking on a Victron unit? Would adding even a small tilt bracket help noticeably, or am I chasing the wrong thing here?

@Chris1991 here's the thing nobody tells you about winter solar — your panel's voltage actually rises in the cold, which is great, but irradiance is so much lower that your wattage craters regardless of how good your MPPT is.

The 75/15 is a solid bit of kit, I run one on my tiny house build, but 15A max output means roughly 180W ceiling at 12V anyway — you're already near the limit.

What's killing you more than the controller is likely:

• Low sun angle (December in the UK means roughly 15° elevation at best)
• Shading from roof edges or vents
• Panel orientation — even 5° off south matters badly in winter

Check your SmartSolar app history. If peak watts are hitting 120W+ on clear days, the controller's fine. If you're seeing 40-60W on sunny days, look at shading first.

@Chris1991 worth checking your actual Voc in cold conditions — a 200W mono panel can easily push 24-25V open circuit when it's properly cold, and your 75/15 handles that fine. What I'd actually look at is shading. Even a tiny shadow across one corner of the panel in winter (sun's much lower in the sky, remember) will hammer your output disproportionately thanks to how cells are wired in series. Also, have you checked your charge history in the VictronConnect app? It'll show your peak watts and charge state over time which is brilliant for diagnosing whether the controller is actually the issue or whether it's purely irradiance. My money's on low sun angle plus possibly partial shading rather than anything wrong with the SmartSolar itself — those units are genuinely excellent.

Great points from @ZI_Sparks and @HalfAJob93 already. One thing worth adding — with a 75/15 controller, you're capped at 15A output regardless of what the panel can theoretically produce. On a 12V LiFePO4 system that's roughly 180W maximum delivery, so you're already close to the ceiling even before accounting for winter losses. You're not leaving masses on the table there, but it's worth knowing the arithmetic.

More practically though, shading and panel angle make an enormous difference in winter. The sun sits so low in the UK that even partial shading from a roof vent or aerial can devastate output. Have you checked whether your panel's getting full, unobstructed sun during the few decent hours we actually get? Repositioning or tilting the panel even slightly can genuinely transform winter performance on a van.

What @OffGridRuss said about the 15A cap is the real ceiling here. 200W ÷ 12V = ~16.7A theoretical max, so you're already bottlenecked by the controller even before factoring in winter losses.

My garden office setup had the same issue — swapped to a Victron SmartSolar 100/20 and immediately had headroom to actually use the panel properly.

Also worth noting: winter sun angle in the UK is brutal. I'm roughly south-facing at maybe 35° tilt and even optimised I'm getting maybe 2-3 peak sun hours on a decent day. No controller fixes that.

If budget's tight, prioritise tilt angle first — dead cheap to adjust, can make a noticeable difference. Controller upgrade second.