being on a mooring doesn't preclude the occasional night anchored elsewhere.
No not at all, but I am talking about just "recharging" the batts to get the most longevity from them while on the mooring. The OP specifically talked about charging on the mooring or coming dock side to do it, which can take 10+ hours. Solar is far easier.
Ideally you want them back to 100% in the shortest time frame possible, usually 1-2 days but 3-4 days is better than never getting above 80-85%... With AGM's as the OP has these would be the "minimum" sizes needed. I would advise trying to keep the time to 100% to less than 3 days for the best longevity.
Any panel in the 1-2 day from 80% SOC size range can and will help when off cruising too. Unless you are going for multiple weeks or more, a night or two is not going to make much difference as long as you are regularly getting back to 100% SOC on a regular basis.
If you want to have your solar array get you back to a reasonable charge level when cruising, and using loads at the same time, then it needs to get quite big. This is something I doubt the OP wants to do, based on what's been written thus far. Actually it does not sound like he really want to go much bigger than about 25-30W, which is not ideal for a mooring sailed boat with AGM's, but certainly better than nothing or a 5-10W panel...
I size and install lots of solar panels for mooring "recharges". This is a different goal than being self sufficient on solar when cruising, which I also install a lot of. For mooring charging the goal is to get the banks from the "avg" state the boat returns to the mooring at to full in the fewest days possible. Often times this is around 80% SOC but it can be lower.
The average in increased bank life on the mooring sailed boats I've done has gone from 2-3 years to 5-7 years on the batteries with AGM seeing the most increase in life.. Big differences with little expense especially if the owner has $$$$$ AGM or GEL batteries.
Mooring sailed boats have a whole different set of "charging" problems than do dock sailed boats and that is where I was coming from to help the OP in his questions.