Solar power and car batteries

I have a small, unpowered garage that my 996 will likely live in for winter. I’d like to keep the battery topped up (and if possible conditioned).

We don’t have many hours of sunlight in the U.K. during winter but I’m hoping that the little we do have would be sufficient to trickle charge the battery. 

Would these parts work do people think?

https://www.sunshinesolar.co.uk/prodshow/Sunshine_Charge_Regulator_12V___7AMP/REG7.html 

https://www.sunshinesolar.co.uk/prodshow/Sunshine_Solar_Battery_Saver_Maintainer_HEAVY_Duty__15_Watt/saver15.html

I use this: https://www.amazon.com/gp/product/B00PFG56ZS/ref=oh_aui_detailpage_o09_s01?ie=UTF8&psc=1 I've had it in a small skiff to maintain the battery for a couple of years. Chinese but it's still working and about $35.00

That looks interesting but possibly a little small - we average 2 hours of sunshine through winter here. 

I suspect I might need a much larger panel to get sufficient power in order to charge the battery.

What you linked to is probably overkill. A battery tender outputs something like .5 amps, so a few hours of that per day should be plenty to keep a battery charged. 

I have just ordered this: https://www.kuranda.co.uk/20w-solar-panel-sfp020ma-solar-technology

20 watt should do good for your climate. I use a 20 watt and it keeps a family car with quite a large parasitic load topped up year-round, even in winter when we have a very short window of sunlight and cloud cover. 

It only has to keep up with the parasitic load. That shouldn't need be all that big. If you really want to know for sure, use an ammeter to measure how many milliamps the car sucks down when it's off. Multiply that by 12 (24 hours divided by 2 hours charging time) and that's how many milliamps you need to provide while charging to keep up with the drain.

I'll bet it's pretty darn close to 0.5 A, or about 1Ah of total charge.

Our motorhome came equipped with a rooftop 5 watt solar panel which is connected to the chassis battery with a controller for the purpose of keeping battery charged during storage and keeping up with parasitic drain. The chassis battery is a large Group 27. Newer model motorhomes of the same brand changed to a 15 watt panel. I talked to a factory tech and he said the 5 watt was marginal in all but the sunniest condition. Certainly don’t know a lot about solar charging but as long as the panel is hooked to a controller bigger should not be a problem.

Bear in mind that the rated output of most solar panels is under conditions of perfect blazing sun at noon at the equator. Real world outputs are much lower. That said, a 20 W panel should be lots for your application. I use a 40 W to keep up the battery in my old Ferguson tractor that I use for ploughing the driveway, and most of the winter it's cloudy and half of the time it's snow covered. It seems to be overkill in that application, but other than cost, there's no downside to overkill for a solar panel (within reason).