Two rules take a third of the tank away
Before any arithmetic, the capacity you can actually use is smaller than the label, for two separate reasons.
The 80% fill rule. Propane tanks are filled to 80% of their water capacity, leaving room for the liquid to expand as temperature rises. This is a safety requirement, not a delivery shortcut — a tank filled to 100% on a cold morning would vent on a hot afternoon.
The 20% floor. Below roughly a fifth full, vapor pressure falls and delivery becomes unreliable, particularly in cold weather. Most suppliers schedule refills at 30% and treat 20% as the practical bottom.
| Tank | Filled to 80% | Usable down to 20% |
|---|
| 120 gal (upright, "420 lb") | About 96 gal | About 72 gal |
|---|
| 250 gal | About 200 gal | About 150 gal |
|---|
| 500 gal | About 400 gal | About 300 gal |
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| 1,000 gal | About 800 gal | About 600 gal |
|---|
Usable figures are 80% fill minus a 20% reserve, both calculated on the nominal water capacity. Your supplier can confirm the exact figures for your tank.
So a 500 gallon tank — the common residential standby size — realistically gives you about 300 usable gallons between deliveries.
How fast a generator drinks it
Consumption depends on load far more than on the machine's rating. A generator carrying a 3 kW critical load burns roughly the same whether it is a 10 kW or a 20 kW unit — a larger engine is somewhat less efficient at light load, but the load does most of the work.
| Load | Rough consumption | Days on 300 usable gal |
|---|
| Critical circuits only, 3 kW average | Around 1.5 gal/h | About 8 days continuous |
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| Half load on a 14 kW unit, 7 kW | Around 2.5 gal/h | About 5 days continuous |
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| Whole house with air conditioning, 12 kW | Around 4 gal/h | About 3 days continuous |
|---|
| Weekly exercise cycle, all year | Small per run, but 52 runs | Several gallons a year before any outage |
|---|
Consumption figures are rough planning ranges for air-cooled residential standby units. Your manufacturer publishes a table of gallons per hour at half and full load for your specific model — use that, not this.
The important pattern: load shedding buys days. A house that backs up critical circuits rather than the whole panel triples its endurance on the same tank.
Sizing the tank to the outage you expect
Work backwards from the longest outage you want to cover without a delivery.
A day or two: a 120 gallon tank is adequate for critical loads. This is the common answer where outages are short and frequent.
Three to five days: 250 gallons on critical loads, or 500 if the whole panel is backed up. This covers the storm scenario most households are actually planning for.
A week or more: 500 gallons minimum, and 1,000 if you are remote or on a route that gets plowed last. The constraint stops being the tank and becomes whether a truck can reach you.
If the house already uses propane for heating, cooking and hot water, size for the household load plus the generator rather than the generator alone. A tank that comfortably heats a house in February empties much faster if a generator is also drawing from it.
Cold weather is the part people miss
Propane is stored as a liquid and burned as a vapor, and the tank has to vaporize it continuously. That process draws heat from the liquid and from the tank walls.
Two consequences that matter in exactly the weather you bought the generator for.
Vaporization rate falls as temperature drops. A tank can hold plenty of propane and still fail to deliver enough vapor to run a generator at load in deep cold. This is a surface-area problem, which is why the fix is a larger tank rather than a fuller one.
A low tank is worse. Less liquid means less wetted surface area to vaporize from, which is part of why the 20% floor exists.
If you are somewhere that sees sustained sub-zero temperatures, raise this explicitly with your installer and your propane supplier. The answer is usually a larger tank than the runtime arithmetic alone suggests, and it is a design decision rather than something to discover in January. The cold weather page covers the same physics for portables.
Owned or leased, and why it matters here
Residential propane tanks are commonly leased from the supplier who fills them, and that arrangement has a consequence worth knowing before you size anything.
A leased tank usually ties you to that supplier, which means you cannot shop the fill price. An owned tank lets you buy from whoever is cheapest, at the cost of owning a pressure vessel with inspection obligations.
Neither is wrong. What matters for this page is that the tank size may be constrained by what your supplier offers on lease, and that is worth establishing before the generator is specified rather than after.
Practical habits
- Top up before storm season, not during. Delivery demand spikes exactly when everyone wants fuel.
- Ask for keep-full service if your supplier offers it. It removes the decision entirely.
- Read the gauge as a percentage, not a level. It shows percent of water capacity, so 40% on the dial is half of your 80% fill.
- Know your consumption figure. The manufacturer publishes gallons per hour at half and full load; write it on the inside of the panel door.
- Shed load early in a long outage. Turning off the air conditioning on day one is what makes day five possible.
The short version
- 80% fill, 20% floor. A 500 gallon tank is about 300 usable gallons.
- Load sets consumption, not the generator's rating. Shedding load triples endurance.
- 250-500 gallons covers the three-to-five day outage most people are planning for.
- Cold weather needs a bigger tank, not a fuller one. Raise it with the installer.
