Some interesting news from a study commissioned by Hypower showed some interesting correlations between the amount of power a boat requires when it ties up for a night or two or if it is a permanently moored craft.
As you would figure, the bigger the craft the more power it required. For example, a 30-footer might require 30 amps or so of power for a night at a mooring site, while a 60 or 70-footer will likely require at least twice that.
And then there are all of the it depends items:
It depends on the amount of auxiliary equipment the boat houses plasma screen high-definition television sets and microwaves definitely are not easy on the power mainsIt depends on the amount of direct marine equipment the boat houses, such as its communications console and/or its radar-GPS unit. And, what about the good old-fashioned LORAN C, there are still many boats out there that, while they have GPS installed, still have LORAN C as a backup location deviceIt depends on the the amount of lighting the boat will need at the mooring siteIt also depends if the boat owner has managed to shoehorn a hybrid gas/electric engine into the engine bay of the craft. This seems to be the Holy Grail of many boaters to have the ability to used either gasoline for general ocean-going or lake-going use when faster speeds may be needed and gasoline is the best idea to the times when they just want to putter into their moorings with minimal headway and without the necessity of having their engines revving. An electric motor is perfect for this (interestingly, you can even take advantage of electrical regeneration everytime you put your gas motor into reverse as it will then not only reverse the boat, but will also charge the battery pack
If an owner can managed to shoehorn in such a power system and there may also be manufacturers who are doing this right now the only problem that this creates is where you put a rack of batteries and an electric motor that can generate 14 or 15 kWh. You have to put them below the waterline so that they don't disrupt the handling too much, while, at the same time, you have to place them close enough to the center line of the boat so that ultimate balance is not upset. It's a neat trick.
Getting back to the requirements of mooring sites, planners have to keep their eyes on the prize on this one as they have to ensure that just one heavy isn't pulling out all of the power their grid can offer. For example, there are some large (60 to 90 feet) craft that might require upwards of 200 amps, if all of their electronics are tuned on at the same time and that is enough to fry any marina's electrical panel. So, in planning you have to allow for the smaller craft that may only need 15 amps (a day sailor or small powerboat) and then that power requirement will be intermittent all the way up to the 90-footer that could come steaming into a berth, run out its power hookups and require, at a minimum about 100 amps and if everything is turned on 200.
If your panel is marginal in the first place, then you are chancing a meltdown you will not believe. The best idea then is to plan for the worst and expect the best. In other words, over-engineer your power supply network so that it can carry multiple 30-amp feeds with no difficulty, while, it is also handling a 90 or 100-amp feed, too.