I way oversized ours for emergency power at our house, 22KW 230V figuring 2 things - 1, if I needed to run everything at once (including the wall heaters), and the well pumps kicked on, would it cover it?, and 2, if I was running the generator at low capacity (say 2500 watts) it would be barely sipping fuel.
Both scenarios played out, one winter we needed the wall heaters, electric oven, well pump, and shop... no problem. When not, however, it would stretch the 350 gallon propane tank almost a month.
Side info - One, we went with Propane as we had the tanks installed, and while propane is not as thrifty as, say, diesel, it also doesn't grow algae and fungus... since we were setting up for a potential "dimished grid situation", we wanted enough to keep the power on for quite a while if needed, ergo it lasts indefinitely without degradation. Also I already had experience with a propane portable genset at altitude, so knew the consumption issues and that running a correctly tuned genset with little to no drag induced from the rotor/stator interaction from having low demand (the more demand = the more drag from the generator = more fuel needed to spin it)...