We used to use a 80 W solar into 160 Ah, no reg. But it was when cruising and then we used the engine
only to make sure the engine
was nicely oiled and ready. No other sources. Wet, deep cycle buckets.
In port it is different - the panel overcharges (as it well should). We disconnected the panel and plugged in the 4 stage (3 stage plus supply) charger
I believe the discharge for normal (wet, deep cycle, not the newer low-discharge that you can buy from Vetus ao.) is up to 1% per day in hot climes.
This would call for 1 Ah daily charge per 100 Ah of the battery
bank. If you consider a 4-5 hr sun it means 1/5 times 12 = 3 W panel (per every 100 Ah of battery bank). There is some loss too (and 12 Volt is not enough), so say 5 W.
But have you ever seen a say 5W panel with 36 cells ..... ???
So my way would be - a bigger panel with a regulator or a trickle charger from the shore.