Anyone else struggling to get decent winter output from roof-mounted panels on a low-pitch van?

Right, so I've been scratching my head over this for the past few weeks. I've got two 200W mono panels flat-mounted on the roof of my Sprinter LWB, running into a Victron SmartSolar 100/30 MPPT. In summer I was regularly seeing 18–22A on a decent day, which felt pretty good. Now we're into January and I'm lucky to scrape 4–5A even when the sun's actually out. I know low sun angle is the main culprit but I wasn't quite expecting such a dramatic drop.

The panels are completely flat — no tilt at all — which I understand is basically the worst case scenario for winter in the UK when the sun barely gets above 15–20° on the horizon. I've been reading about tilt frames but obviously that's tricky on a vehicle you're actually driving. Has anyone fitted adjustable tilt mounts on a Sprinter or similar and found it genuinely worth the hassle? I'm a bit worried about wind resistance and anything that adds height when I'm near low bridges.

Also wondering whether my controller settings could be doing more work here. I've left everything on the default Victron profile for sealed AGM — 300Ah of Battle Born equivalents (so lithium really, I swapped them out in October). Not sure if I've properly updated the charge profile to suit. Could wrong settings actually be costing me meaningful amps in these conditions, or is it basically all down to the panel angle and the rubbish light levels?

@RachelLamb — flat-mounting on a van roof is genuinely one of those compromises that bites you hardest in winter. At our latitude, the sun barely climbs 15-20° above the horizon on a December afternoon, so your panels are seeing the light at an almost glancing angle.

I had the same situation with my motorhome before I fitted a pair of tilt brackets. Made a remarkable difference between November and February — sometimes doubling usable output on a clear day just by getting the panels to roughly 50-60°.

Worth checking your Victron app's historical yield data too. The SmartSolar logs it beautifully, so you can actually see how badly the flat angle is hurting you before committing to any modifications.