On a property without natural gas service, propane is usually the answer, and the tank is usually the part that gets decided last and thought about least. It should be the other way round: on propane, the tank is the runtime.
Runtime is a tank question
A generator's fuel consumption is published as a rate — gallons per hour at half load and at full load. Multiply that by the hours you want to keep running and you have the tank size you need, minus the portion of the tank you cannot use.
That last part surprises people. A tank is filled to roughly 80% to leave room for expansion, and drawing it down to empty is not something you want to plan for. The usable volume is meaningfully less than the number on the side of the tank.
Cold weather takes some of it back
Propane has to vaporise to be burned, and vaporisation draws heat out of the liquid and the tank wall. A tank that is small, or low, or very cold will struggle to sustain the vaporisation rate a generator under load needs — exactly when you are asking the most of it.
This is why tank sizing on a rural property is not simply arithmetic on gallons. Surface area and fill level matter, and a hard freeze is when the difference shows.
Decide the runtime first
The useful conversation runs backwards from the outage you are preparing for. Two days of ice, or a week? Whole house, or the loads that matter? Those answers set the tank, and the tank sets the delivery schedule you will live with for the next twenty years.
We size the fuel system as part of the design rather than leaving it to whoever sets the tank. More on the trade-offs in propane vs natural gas.