How much power I need?
The answer to this question depends on what you want from your solar panels: should they only cover the batteries self-discharge when you are not on board or you want them to provide enough energy while cruising?
In the first case the answer depends on your battery type and size. Modern batteries have a very low self discharge (could be only 3% monthly). It is very easy then to calcolate how much energy you need to maintain the full capacity when not on board.
But on a cruise is different, your solar power system needs to provide enough charge to allow the batteries to run the equipment you want it to run: fridge, elecronics, autopilot, lights, etc....
And remember that you can only collect this power when the sun is shining, yet boat equipment can be used 24 hours a day.
The best approach is to make a list with all the equipments, and for each of them write down the hourly consumtions and the estimated hours of working, for example:
Fridge 4Amp/h > 8Hours a day > 32 Amp/h per day
Having only the Fridge with 100Wp solar panel I could get in the Med and in summer 40Amp/h a day: OK!
Once you get your list, the following question is how much independent you want to be. This depends on your cruising style: are you used to switch on the engine a couple of hours a day and to be often moored in a comfortable marina at night? Then 30% of the total needs could be enough......