Glad to see sailorchic chime in.
First, this statement is not germane to your situation:
"If I run a BTU to kW conversion, I get that 1kW equals 3412btu..."
Second, most of the (primarily high speed) light duty generators marketed for recreational use are not designed for continuous duty. I am not sure how frequently, and for what period of time, you intend to operate air conditioning while aboard, but if planning to operate operate for extended periods you will need to be more discerning regarding your choices.
Operating any generator at over 80% of name plate capacity will be stressful, and likely shorten the
service life of the unit. At the same time, operating at low loads - less than say 30% of capacity - will also risk damage to the generator.
I would proceed by having an energy assessment done by
three experienced vendors (those who
service and sell generators) and see what their size recommendations are.
In my professional career I would occasionally specify a generator, though more frequently be responsible for the
fuel tank or gas piping for the unit. That experience had only slight relevance to owning and operating my own 4.4kw generator.
The challenges I face are keeping a sufficient load on the generator when operating the unit to charge the house
batteries. As the
battery charge rate lowers, the load on the generator also lowers. Our solution is to time
battery charging to coincide with making coffee, operating the microwave, vacuum, power tools, etc. - anything to prevent "underloading" the unit.
There are downsides to oversizing a generator. More cost upfront to you. More weight and space requirements. The challenge of keeping a sufficient load on the generator (what if you decide to just run one AC unit?).
Operating requirements for the AC units will be listed in the Owners Manual and/or the name plate on the unit: RLA (rated load amps), FLA (full load amps), etc. Use these for calculating electrical load, not what the rated
cooling capacity of the unit might be. Add the sea
water pumps also.
These are expensive units. Don't screw up the selection. I know of one Nordhavn owner who has a 12kw generator that he never uses, instead using his
Yanmar 3GM30 wing
engine as an alternative energy source.
By the way, since Cummins took over Onan, my understanding is that they have made changes to their units, particularly the countries of origin (COO) for the components. I am not certain whether the units you are considering are utilizing Chinese engines, but I have heard reports from users complaining about Taiwanese carburetors, for example, on gasoline powered units.
Good luck with your search.
P.S. You have recommendations in posts above ranging from 4 kw to 11 kw. That alone should send you to an experienced vendor for assistance determining required size