Run time on stored fuel is arithmetic, and the two inputs are usable tank volume and consumption at the load the house will actually impose.
The calculation
hours = usable gallons ÷ gallons per hour at the expected load
Usable gallons is the tank capacity times about 0.8, because propane tanks are filled to 80% to allow for thermal expansion. A 500-gallon tank holds roughly 400 usable gallons.
Gallons per hour comes from the manufacturer's specification, which publishes consumption at 50% and 100% load. Interpolate for anything between.
Estimating the load honestly
This is where people go wrong, in both directions.
The generator is rarely at full load. A house on backup, with cooking and laundry postponed and half the lights off, typically sits between a quarter and a half of the generator's rating.
But it is not at zero either, and a household that carries on as normal — running the air conditioning, the dryer and the oven — will be near the top.
The honest planning number for most households is half load, and the calculation is worth running at both a quarter and full to see the spread.
Natural gas
There is no tank and no calculation. The run time is indefinite, limited only by the engine's maintenance intervals — oil and filters after a stated number of running hours, which a genuinely long outage can reach.
The dependency is on the gas utility continuing to supply, which is normally reliable and is not guaranteed after an earthquake or during work on the main.
Sizing a propane tank
Work backwards from the outage being planned for.
A household preparing for a three-day winter event at half load on a mid-sized air-cooled unit needs a substantial tank, and a barbecue cylinder is not in the conversation.
500 gallons is the common residential standby tank. 1,000 gallons is chosen where deliveries are difficult, where the climate is severe, or where the tank also supplies heating.
Delivery during an event
The point that gets overlooked: in a widespread outage, every propane customer in the area calls the same supplier in the same week.
A tank sized so that a refill is needed on day three is a tank that will not be refilled on day three. Size for the whole event.
Filling before the season — before winter, before hurricane season — is the practical version of this, and most suppliers offer it.
Cold weather vaporisation
Propane must vaporise inside the tank, and the rate depends on the wetted surface area and the temperature.
A tank low on liquid in severe cold may not vaporise fast enough to feed a generator's demand, which shows up as the engine faltering under load. It is another argument for a larger tank kept above a reasonable level rather than a smaller one run down.
Reducing consumption during an outage
Shed what can be shed. Water heating, air conditioning, the dryer.
Postpone cooking and laundry, or do them at a time when nothing else large is running.
Let the house drift a few degrees, which reduces the heating or cooling run time directly.
Each of these extends the run time proportionally, and together they can turn three days of fuel into five.
Reading the tank gauge
The dial reads percentage of capacity, not gallons.
A 500-gallon tank showing 30% holds about 150 gallons — and since the tank was only ever filled to 80%, that 30% is 30% of 500, not of 400. Getting this wrong in the direction people usually do means believing there is more fuel than there is.
Portable generators
The same arithmetic with much smaller numbers. A portable unit's tank is a few gallons and the run time is measured in hours, which is why refuelling is part of the routine.
Refuelling a hot engine is a genuine hazard, and the correct procedure is to shut it down and let it cool. Households running a portable unit through a multi-day outage do this many times, in the dark, in the weather that caused the outage — which is the strongest practical argument for the standby alternative.