One other thing to keep in mind when sizing a portable generator to run your tank is the heat requirement. In the winter, during a protracted outage, unless the generator is big enough to run the house heat, you’ll need a lot more tank heater wattage than for normal conditions. Depending on how far the ambient falls, as much as 3x.
I would doubt many people keep that much spare on hand ..... heck, maybe nobody. My tank needs 1,000 watts and I have another 1,000 in spares on hand, but even that might be inadequate.