During winter it needs to be larger than you might think, because the sun angle is low, so comes through more atmosphere, is much more affected by tree etc shadows, the skies are more frequently overcast, the duration of light is short and in many cases the panel is left horizontal.
I have helped somebody who keeps his undercover with solar kit to charge it from outside the building. He had an alarm and there were a few trees on the boundary. I concluded 40 Watts would be enough, mounted at 50 deg to the horizontal, facing south. After pricing it out he spent a bit more and went over the top at 100 Watts, but it has performed well.
I feel nowadays buying less than 40 watts is not cost effective with the panels being so cheap. You must fit a good controller.
I have just fitted one to my MH its a Sterling unit when the leisure reaches 12.3 volts it diverts charge to engine battery.
Three wires, 2 in line fuses bit of extension lead to fit
I currently have 13.4 volts showing on the leisure battery and 12.9 volts on the engine battery
All supplied by 120 watt panel and a cheap 10 amp regulator.
wish i had done it years ago no battery charging and boiling batteries.
If you haven't fitted a solar panel i recommend a motorcycle trickle charger they keep the battery topped up without using a high charge
You will need power nearby cost about £13.00
I wouldnt bother with these small dash board size solars they are only good in the summer for carparks while people jet off on holiday.
As others say winter light will just produce milliamps which wont be enough if you have an alarm fitted.
Thanks for the advice, it has to be a solar charger as I've no electric socket near to where I park my van.
I'll look at the unit that you've fitted, sounds ideal.