I believe the factors that effect the size and qty of solar that you put on your cat are more to do with in no particular order.
1: How much space do I have to place the panels.
2: How many panels can I afford, remembering that the solar controllers, protection devices, cable, mechanical frame and inverter
will cost more than the panels cost. Not to mention installation
if you can not do it your self.
3: Am I going to live on the boat or just camp for short periods. Power equals comfort, you do not have to listen to noisy generators of any kind. You want a coffee turn the coffee machine on, slice of toast with that sir put the bread in the electric
toaster and throw that gas stove top bread burner away. A hot shower
perhaps, once we are back up to 100% SOC we then use the 240 volts from the inverter
to heat up the water
60 liters approx 1 hour. Then sit down and watch a movie
on the 46" Screen
4: The electrical
load on your boat will not decrease over time it will increase as you add more things.
5:The out put of the panels at mid day in full sun is not as relevant as is the output you still have at 0830 or 1630 on an overcast day. (I still see 20 to 40 amps @ 12 volts) My system in theory is capable of 150 + amps but the panels are not tracking the sun and are mounted horizontal so the reality is the best I have ever seen is 132 amps but the normal range is in the 60 to 100 amp range . Time of year, time of day, cloud cover and shadows will all degrade the output nothing will increase it other than more or bigger panels.