First, I think that there are two reasons gensets are so large. First, because it's much, much easier to make the genset larger than it is for the manufacturer to try and educate the average user how to manage the load. Second, because gensets are usually speced by electrical engineers, who just add up the max load, apply a 120% safety margin, and use that as the spec.
Can you get away with less? You bet. I grew up in an isolated research camp, where we had two 20Kw gensets (specified by by an electrical engineer) and a 4Kw (which was supposed to be temporary). In practice, we ran the 4Kw 3000 hours a year and the 20Kw about 250 hours a year. Yes, you have to watch the load, but that was pretty easy to learn.
One incredibly simple rule is to not use electricity to create heat. Burn propane or diesel. That will take care of a great deal of the load right there.
On my boat (a 60 footer) I use diesel heat and propane for cooking. All lights are 24 volt (although in hindsight I'd use 12). I do use an electric hot water heater, mostly for convenience. I have no AC (I'm in the PNW).
I have a 6Kw genset (50A at 110v), a 4Kw inverter, and a single 30A, 110V shorepower cable (3.3Kw). Through careful load arrangements and a smart inverter, I find I have no problems at all with load. It helps that my inverter is quite smart, it will both cut back the charger and assist when the load becomes too high.
The only situation that causes problems is when I connect to shore power and both the battery charger and hot water heater call for power. However, I rigged up a selector switch that lets me run the hot water heater through the inverter, which is smart enough to dial back the charger when the hot water heater calls.
There is only one real limitation in my system, and that is that I have to be satisfied with a 110V Splendide washer/dryer. I definitely do miss a nice big 6Kw 220V electric dryer (and FYI, this is the main reason we ran our 20Kw when I was growing up, too). I've considered installing a propane dryer, maybe I will if I ever move aboard in the PNW.
YMMV, but it works for me. I have no idea at all why you would need 20Kw.
Scott Welch
Island Eagle