Fuel
What size propane tank does a standby generator need?
Two answers, and most people only ever get the first one. How many gallons the tank holds decides how long it runs. How fast the tank can boil those gallons into vapor decides whether it runs at all on the coldest night of the year.
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 propane expands with temperature and the tank needs headspace for vapor. 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.
That single adjustment is where most back-of-envelope estimates go wrong, and it goes wrong in the direction that leaves you short.
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. Real outages almost never hold a generator at its rating — a correctly sized unit spends the event cycling well below it, because the air conditioning and the heat are not both running flat out for four days. The full-load row is the floor, not the forecast.
The practical read for a rural property: 250 gallons is thin for a multi-day winter event, which is precisely the event the generator exists for. Five hundred is the size most people end up wishing they had started with, and the cost difference at install time is far smaller than the cost of adding capacity later.
Gallons are only half of it
Everything above is a volume question. There is a second constraint that catches people out, and it is a rate question — the one that turns a tank with plenty of fuel in it into a generator that will not hold load.
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 at exactly the wrong moment:
- 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. Two 250s can outperform one 500 for this reason, which is why suppliers sometimes propose them.
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. It is the single scenario the machine was bought for, and it is the scenario in which 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. Any competent supplier sizes for this as a matter of course, and asking it in those words tells you within a sentence whether you are talking to one who does.
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 specifically that is a real advantage in the one scenario that matters most, and it is the reason to consider burying a tank beyond simply not looking at it.
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, and those are the propane installer’s responsibility rather than ours. Our side of it stops at the regulator: the pipe run from there to the generator has to carry the full-load BTU demand at the pressure the unit needs, and an undersized run produces the same symptom as an undersized tank.
What this changes about the generator
One thing worth knowing before the tank is ordered: on some models the machine 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 on the spec sheet.
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 20kW unit, 500 gallons is the size most people end up wishing they had started with. At half load — the realistic planning figure — a 500 gallon tank gives roughly 9.2 days, and a 250 gallon tank roughly 4.6 days. A 250 is thin for a multi-day winter event, which is the event you are buying the generator for.
Why does a 500 gallon tank only hold 400 gallons?
Because propane expands with temperature and the tank needs headspace for the vapor. Filling to roughly 80% is the standard, so every runtime figure should 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 number to plan on. A generator sized correctly for a house spends an outage cycling well below its rating, because the air conditioning and the heat are not both running flat out for four days.
Can a propane tank be too small even when it is full?
Yes, and this is the part that catches people. Propane leaves the tank as vapor, and it has to boil off the liquid first. That boiling happens at the wetted surface inside the tank and 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 line up during a February outage. The tank can be full of gallons and still not 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. Any competent supplier sizes for this as a matter of course, and asking the question in those terms tells you immediately whether you are dealing with one who does.
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 specifically, the buried tank's temperature stability is a real advantage in the one scenario that matters most.
Do I need propane at all if I have a gas meter?
Almost certainly not. Natural gas has no tank, no delivery, no fuel to run out of, and the mains generally hold pressure through the storms that cause outages here. 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 the vaporization against. Two conversations, one set of numbers, and no surprises in February.
Rather just talk to someone?
Leave a number and we will call you. No sales script, no obligation.
Got it. We will call you. If you would rather not wait, ring (214) 957-4307 and ask for whoever is on generators.