Delancy, it's about real estate and load profile. The loads need to be quantified on a daily watt hour basis and, ideally, matched by input from the panels on a equal watt hour basis of production per day (with some reserve if possible). There is no "rule of thumb" except, perhaps, the "mo is betta" approach. Once you've quantified the loads, you can address with inputs, possibly considering a small wind
gen for supplement to the solar. If you can find enough realestate on the boat to provide the input, then you have it licked. Choices of electrical
usage items geared towards efficiency should be the first approach, then satisfying the load requirement with solar & wind
last. Energy efficiency = cost efficiency in operation protocol. Burning diesel
or gas to make watt hours or amp hours at $5+/gal is money
better spent on photovoltaics or wind, if you can find the space to put them. Example: there are watermakers out there that make 20-30 gpd using 8-12amps or 3 to 4 amps. Choosing the more energy efficient model will allow your renewable energy generated watt hours to go 3 times as far. Same with all other electrical
usage items-lighting, radios, entertainment systems, etc. if you would like a load sheet to help you figure out the system demands, let me know and I will send you one that will walk you through the calculations.