You really need to be more specific, about how long you are planning to be away, what electrical items you want to use and how, and are you really only thinking of main season caravanning.
Anyway to try and help:
To be self sufficient through the main season, ie away as long as you like, and be a moderate user of TV excetra, I think the consesus of opinion is that in the UK you need at least a 50 Watt panel.
If it includes the shoulder seasons and is still long stay then it needs to be 85 Watts.
If its also winter then all I can add is my 85 Watt is not enough to be self sufficient, the evenings are so long requiring light and a bit of entertainment and the day time to charge is too short.
Still talking the main season. If its extending a week end by a few days then anything "helps", but clearly the more the better.
It also then depends on how big a battery you have and critically how much TV you need.
IMO its pretty pointless buying below 40 Watts; below that you have to be restricted in your power use, time away and be lucky all the time with the weather.
If its for servicing a battery in storage then these small 10-15 Watt units are ideal.
This is from years of experience first with a 13 Watt panel and three years now with an 85 Watt panel.