How do you size this? At a minimum a panel with a max output amperage of 1.0A to 3.5A can work, depending upon the size of your bank, and depending upon which charge controller you use. You can easily go beyond 20A with solar but you need some real estate to devote to it.
Ideally you want the alternator, or generator, to handle "bulk" charging and bring the bank back to 80-85% state of charge. from there you can let the solar panel do the rest and bring the bank back to 100% SOC, even when you're not there during the weekdays.
If you want to satisfy cruising needs such as refrigeration, instruments, AP etc. then your array will get quite large.
The faster you can bring the bank from 80% to 100% the less sulfation you will have and the longer battery life you'll get.
For a sizing example, lets look at a bank of 300 Ah's for a "mooring recharge".
With simple math you can see that the last 20% of capacity is 60 Ah's. However you need to take charge inefficiencies into account too so you'll really need to put back in about 70Ah's +/- to get back to 100% state of charge.
For sailboats the panels are usually left flat when your not there so that you can capture "most" of the sun. In a land based solar array the panels are fixed. In a land based array the property is not moving like a boat does, and the panels can be angled at the sun for the best overall performance.
For this reason alone land based solar calculations won't apply well to boats. On boats the panels are rarely oriented or angled at the sun for optimum solar gains.
Having the ability to "aim" the panels at the sun, on a boat, is not usually a workable solution when swinging on a mooring, or at anchor. Panel position on sailboats is a certainly a compromise but one designed to do the best we can. Obviously if you are at a dock, you can rig the panels and aim them more appropriately, but not on a mooring or off cruising where your boat will swing at will. Being able to "aim" the panels can increase the performance quite a bit so if you're at the same dock every week you may want to consider an articulating panel mounting system.
Because of these aiming restrictions you can figure on about 3.0 - 4.5 hours of full rated current output per day, on average, for the Northern climates,. Down South 4.0 - 5+ hours per day the closer to the equator you get. These are full current ratings at 25C panel temp. Of course weather can play a huge role and you could see slightly more or slightly less. Generally speaking it is best to size for worst case scenarios, as I have yet to hear anyone complain they had too much solar..
Some folks use 5 hours per day in the Northeast but after lots of monitoring of my own panels, and customers, I found that to be a little to generous. Some days it will be more some less but here in Maine 3.50 - 4.5 hours at full output, as an average sizing number, seems to work best.
So, a 300 Ah bank @ 20% down = 60 Ah's that need to be returned + charge inefficiency of about 15% = 70 Ah's total needed for a "full" bank. The math looks like this; Ah deficiency X 1.15 = needed recharge Ah's.
A 2.5A output panel X 4.5 hours per day = 12.5 Ah's/Day returned to the bank. If you have phantom/parasitic loads, like a propane sniffer or other "always on loads, subtract those and this is your "net" average/day.
Next divide 70 amp hours by 12.5 and you can see that it will take approximately 5.6 days to go from 80% SOC to full on a 300 Ah bank.
I feel that's a little long so would prefer to see a panel in the 3.5A minimum range. Bigger is always better but this becomes a "real estate" and, let's admit it, an aesthetic compromise with many boats and boat owners.. A 3.5A panel shaves a full day off the time it takes to hit full when compared to a 2.5A panel.
For cruising figure the maximum amount of time you want to run the engine or gen set per day, or every other day, then base your array size on the difference needing to be made up. For example, if you only want to run your engine for an hour per day, when you hit 50% SOC, and your alternator can get you from 50% to 70% in an hour then you have a 30% deficit to make up plus charge inefficiencies. You can size your panel to make up this deficiency....