When I was facing that decision I added up the amperage I was using. 4200 square feet in the house, all electric, with a (key point) 200 amp panel which also feeds the barns. On the surface a 25 KW unit would give me power to spare based on the panel main.
Actually a 10 KW would do it as I run everything through a 100 amp safety switch. Biggest power hog I have is the heat pump, pulls 60 amps at full load. So, that means I can't run the heat pump at full capacity, the water heater, the oven, the stove, the dryer, two welders, and 2 air compressors, plus a coffee pot, TV, and clothes iron. Oh, wait, I can't run all that anyway, it would trip the mains which I have never done.
I play it smart when on gen power. When the heat pump is on I keep it to things like lights, TV, coffee pot, etc. When we take showers, run the dryer, bake, etc I switch off the heat pump for that period of time. Saves a lot of fuel as the tractor isn't supplying the full 200 amps all the time. I do have it wired where the shop and barns pulls off a 50 amp aux plug on the generator rather than feeding from the main panel.
Thats a long answer to say best to take pencil and paper down to the breaker box and start adding up potential loads, then take the list and look at realistic loads when the power is off.