We've had solar for about 8 years.
Do not count on the panel's listed rating to have anything to do with it's actual output except on a cloudless sunny day (& no shadows on the panel) around noon. There will be plenty of days when you will not get near the 100 watts at any time of the day. Count on an average of between 20 and 35% actual output over the long run, unless you are sailing in a desert or somewhere else very sunny and unusually cloudless.
Planning your solar array is just plain math. You must figure how much electricity you will use, in watts or amps, then how much solar power you will need to stay ahead of your needs. Remember, once you have free power, it's awful easy to add more devices, so perhaps plan for 10 to 20% more consumption than you currently are considering.