In short, a typical small boat may require around 300-500 watts of solar panels, a medium-sized boat might need 500-800 watts, and a large boat could necessitate even larger solar panel capacity.
We usually measure or convert the watts into amps of solar panels to figure out how much current (amps) is being stored in the battery. Or we measure the amperage of the solar panel output to select the wire sizefrom solar panels to.