Yes, quite correct, - I thought that way too. However, when anchoring out a lot like in the Bahamas, we discovered that our usage pattern shifted. We do not run the genset overnight preferring short generator runs to minimize noise for ourselves and anchorage neighbors. Since we have to run the generator for most meals and only have a small solar array, we tend to do everything at once for short duration: stove, +/- oven, laundry, AC, water maker, water heater, max battery chargers, charge all devices in addition to ongoing fridge & ice maker. Surprising how quickly it adds up. YMMV.
I think this is important - how you operate your boat when at anchor. If you are a generator-on all the time boat for air conditioning, etc, loads can be spread out over time resulting in a lower, but more consistent load. HVAC plus one other large load (oven, water maker, hot water heater, etc.) will probably define your max need.
But if you run mostly on batteries while at anchor, generator time will be when you run all those heavy loads, so power demand will be much more concentrated over a short time. You will be charging batteries, plus trying to do laundry which also means heating hot water, maybe running a water maker, and maybe doing some cooking with electric appliances. And if you are in a warmer climate, you might also run some HVAC to cool the boat before bed time. That will generate a lot of load over a few hours.
With LFP batteries, we can now run something like the oven for an hour or two to cook a meal. But that means recharging is all the more demanding, and with LFP, and you can comfortable do it in 2hrs if you have enough charger capacity. If we are charging batteries, doing laundry, heating hot water, and running a water maker, I can easily max out our 25kw generator. And that's without HVAC. I typically only need to run for 2-3 hrs a day, but in that time it's working hard.
The key here is that the more battery capability you have, the more concentrated, and intense you generator run time becomes. So if that's how you operate, and the direction you are moving with your power system, be wary of undersizing your generator.
