Izzy...for on the mooring use a 20watt panel should put in 6-8 amp hours per day or roughly 30-40 amphours a week.
Your radio,cabin lights, anchor light, bilge pump etc. probably use about that many amp hours per day you are on board so 20 watts is really pretty minimal.
You can get a much more precise idea of your electrical use by clicking on the spreadsheet link on this page:
Southeast Marine Services :: Technical Information :: Battery Bank Sizing
Hope this helps.