Fuel
What size propane tank does a standby generator need?
For a Cummins 20kW unit, a 500 gallon tank (about 400 usable gallons) runs roughly 221 hours at half load and a 250 gallon tank roughly 110 hours. The tank also has to vaporize propane fast enough on a cold night, which is a separate check with your propane supplier.
This page assumes propane is already the answer. If you are still deciding, that is a different question — natural gas vs propane vs diesel covers it, and for most DFW homes with a meter at the house the answer is natural gas.
Start with 80 percent
Propane tanks are filled to about 80% of their nameplate size, because liquid propane expands as it warms and the tank needs room for that. Every runtime number below is calculated on that basis. A 500 gallon tank is a 400 gallon tank for planning, a 250 is a 200, and a 1000 is an 800.
How long each tank actually runs
Consumption at each load step is published by Cummins. These are those figures converted to gallons, against the three tank sizes normally offered.
| Model | Load | Gal/hr | 250 gal tank | 500 gal tank | 1000 gal tank |
|---|---|---|---|---|---|
| Cummins RS13A 13kW on LP | Half | 1.40 | 143 hrs ~6 days | 285 hrs ~11.9 days | 571 hrs ~23.8 days |
| Full | 2.28 | 88 hrs ~3.7 days | 175 hrs ~7.3 days | 351 hrs ~14.6 days | |
| Cummins RS17A 17kW on LP | Half | 1.73 | 116 hrs ~4.8 days | 231 hrs ~9.6 days | 462 hrs ~19.3 days |
| Full | 2.80 | 71 hrs ~3 days | 143 hrs ~6 days | 285 hrs ~11.9 days | |
| Cummins RS20A 20kW on LP | Half | 1.81 | 110 hrs ~4.6 days | 221 hrs ~9.2 days | 441 hrs ~18.4 days |
| Full | 3.08 | 65 hrs ~2.7 days | 130 hrs ~5.4 days | 260 hrs ~10.8 days |
Calculated at 80% fill and 36.39 cubic feet per gallon, the conversion printed on the Cummins sheet. Consumption figures are the published LP values at each load step.
Plan on the half-load row. A correctly sized unit spends most of an outage below its rating. The full-load row is the shortest runtime you should expect.
On those figures a 250 gallon tank gives a 20kW unit about 4.6 days at half load before a delivery is needed. Weigh that against how quickly a propane delivery can reach the property during a regional storm.
Gallons are only half of it
Everything above is a volume question. The second constraint is a rate: how fast the tank can turn liquid into vapor, which can leave a generator unable to hold load with fuel still in the tank.
Propane is stored as liquid. It has to boil into vapor before the engine can burn any of it, and that boiling happens at the wetted surface inside the tank, the area where liquid is touching steel. It has a maximum continuous rate, and three things push that rate down:
- Cold slows the boil. Vaporization depends on heat moving from the tank wall into the liquid, and a February night is short of heat.
- A low tank has less wetted surface. Down to the last quarter, the liquid touches much less steel than it did full.
- A physically small tank has less surface to begin with.
Those three line up during a multi-day freeze: the tank is colder than when it started and emptier than when it started, and the generator has been running the whole time. That is where a volume-only calculation fails.
The question to ask your propane supplier. Ask for the continuous vaporization rate of the tank they are proposing, at winter design temperature, against the generator’s full-load BTU demand. Not the gallon capacity, the rate.
What a freeze does to the rest of the installation — the battery, the oil, the cold weather kit — is a separate subject and has its own page.
Above ground or buried
Buried holds temperature. Ground temperature barely moves compared with air temperature, so an underground tank vaporizes more consistently in a freeze. For generator duty that is the main reason to consider burying one.
Above ground is cheaper and easier to live with. Lower install cost, no cathodic protection, simpler service, and you can see the gauge and the fittings. It also sits in the weather, which is the trade.
Either way the tank has its own clearances from the house, from ignition sources and from the generator, set by the propane company that installs it. The propane company or a licensed plumber also runs the pipe from the regulator to the generator, which has to carry the full-load BTU demand at the pressure the unit needs; an undersized run produces the same symptom as an undersized tank. Our electricians do not do gas work.
What this changes about the generator
On some models the generator makes more power on propane than on natural gas. The RS20A is the example — it is a 20kW unit on LP and a 18kW unit on natural gas. If you are on propane you get the larger figure, which is the one in the model name.
That matters here because the tank is sized against consumption, and consumption is a function of the load you actually put on it rather than the badge on the enclosure. If you have not settled the size yet, the sizing calculator comes first — the kW answer is what the tank gets sized against.
Questions
What size propane tank do I need for a whole house generator?
For a Cummins 20kW unit, at half load a 500 gallon tank gives roughly 9.2 days and a 250 gallon tank roughly 4.6 days, from the published consumption at 80 percent fill. Pick the tank against the longest outage you want to ride out without a delivery, then confirm with the propane supplier that the tank can vaporize fast enough at winter temperature.
Why does a 500 gallon tank only hold 400 gallons?
Because liquid propane expands as it warms, so the tank is filled to about 80% to leave room for that expansion. Every runtime figure should therefore be calculated on 80% of the nameplate size. A 500 gallon tank is a 400 gallon tank for planning purposes, a 250 is a 200, and a 1000 is an 800.
How long will a 500 gallon propane tank run a generator?
About 130 hours at full load on a 20kW unit, and about 221 hours at half load — roughly 9.2 days. Half load is the more realistic planning figure, because a correctly sized generator spends most of an outage below its rating.
Can a propane tank be too small even when it is full?
Yes. Propane leaves the tank as vapor, boiled off the liquid at the wetted surface inside the tank, and that has a maximum rate. A physically small tank has less wetted surface, cold slows the boil, and a tank down to its last quarter has less liquid touching steel. All three can line up during a winter outage, and a tank with fuel in it can then fail to deliver vapor fast enough to hold the engine at load.
What should I ask the propane supplier?
Ask for the continuous vaporization rate of the tank they are proposing, at winter design temperature, against the generator's full-load BTU demand. Not the gallon capacity, the rate.
Above ground or underground?
Underground tanks hold a steadier temperature, which helps vaporization in a freeze, and they are invisible. They cost more to install, need cathodic protection, and a leak is harder to find. Above ground is cheaper, easier to service and easier to inspect, but it sits in the weather. For generator duty, the buried tank's steadier temperature helps most during a freeze.
Do I need propane at all if I have a gas meter?
Usually not. Natural gas needs no tank or delivery, and runs as long as the utility keeps pressure in the main, which is not guaranteed: in the February 2021 freeze Atmos Energy shut off gas to homes near Joe Pool Lake in Grand Prairie for about three days. Propane is the answer when there is no gas main at the address. Which fuel to choose is a separate question and it has its own page.
LP consumption figures from Cummins Power Generation spec sheet NAS-6254-EN (A056T581), RS13A / RS17A / RS20A / RS20AC, retrieved 2026-08-19. Runtimes are calculated from those figures at 80% tank fill and are planning estimates, not guarantees — actual consumption depends on the load you run.
Not sure what size tank your place needs
We size the generator and tell you the full-load demand in BTU, which is the number your propane supplier needs to size the tank and its vaporization rate against.
Rather just talk to someone?
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Got it. We will call you. If you would rather not wait, ring (214) 957-4307 and ask for whoever is on generators.