Does anyone actually get decent output from a 200W panel on a south-facing van roof in a UK winter?

I've got a single 200W mono panel flat-mounted on the roof of my Transit (so no tilt, just lying flat), and I'm trying to work out if it's even worth running the solar controller through the colder months or whether I should just rely on a B2B charger from the alternator when I'm driving.

Yesterday was a clear sunny day in the Midlands — genuinely blue skies — and the best I saw all day was around 18W peak, with maybe 40–50Wh total into my 100Ah lithium. My Victron MPPT 75/15 was showing decent voltage coming in but the amps were pitiful. Granted the panel was covered in a light film of grime, but still.

I know the sun is low in the sky and the flat mounting makes it worse, but I wasn't expecting it to be that bad. Is this just the reality of UK winters, or am I missing something? Wondering if a tilt mount would actually make a meaningful difference in, say, December and January, or if it's largely pointless until March.

@Dizzy on the narrowboat I'm in a similar pickle — panels lying flat on the cabin roof, no tilt whatsoever.

Honest answer? January through February you're looking at maybe 20-40Wh on a grey day, sometimes genuinely nothing useful. But here's the thing nobody tells you — a crisp cold sunny day in December can actually surprise you. Cold panels are more efficient, so when the sun does show up you'll occasionally see your rated wattage briefly exceeded.

Worth running the controller? Absolutely yes. My Victron SmartSolar earns its keep just from those unexpected bright spells. The controller itself draws negligible power sitting idle.

Manage your expectations though — I treat winter solar as a supplement to my alternator charging, not a primary source. If you're van-dwelling full-time, a second panel or a small tilt mount would genuinely transform your winter numbers.

Hey @Dizzy, flat-mounted in a UK winter is tough, I won't sugarcoat it. You're looking at maybe 20-40Wh on a proper grey December day, possibly nudging 100Wh if you get a rare crisp sunny one. The low sun angle really hammers flat panels this time of year.

That said, definitely keep the controller running — even trickle input helps maintain your battery and prevents sulphation if you're on lead-acid. Cold weather actually helps panel efficiency slightly, so every photon counts!

My practical advice: park facing south whenever possible and try to position on hills or open ground away from trees and buildings that'll cast long winter shadows. Also worth checking your controller's temperature compensation is working properly.

@SueParker64 the narrowboat situation is even harder given you can't really reposition — do you get much shading from bridges and trees?

Hey @Dizzy, don't disconnect that controller just yet! Even on grim winter days you'll likely pull something useful — my experience is that cold temperatures actually improve panel efficiency slightly compared to summer heat, so the cells themselves are happy enough. The real killer with flat mounting in winter is the sun angle — it's sitting so low in the sky that you're losing a huge chunk of potential output. If you can manage even a modest tilt towards south when parked up, say 30-40 degrees, you'd be surprised how much difference it makes on those occasional crisp clear days. A simple wedge or prop under one side costs nothing. Worth experimenting before writing the whole setup off! ☀️