Size a propane tank for a generator from the generator's full-load propane consumption on its spec sheet, the longest outage you want to cover, and the coldest temperature you expect. Generac, for example, recommends at least a 250 gallon tank for its home standby generators and counts only about 60% of a tank as usable, because tanks are filled to a maximum of 80% and should not run below about 20%.
The short answer
A propane tank for a generator is sized from three numbers: how much propane the generator burns at full load, how many hours you want it to run without a delivery, and how cold it gets where the tank sits. Your generator's manual sets the minimum, and your propane supplier and installer do the final sizing; the steps below let you check their recommendation against the manufacturer's own figures. This guide uses Generac's published documentation as its worked example because it spells the method out. If you own another brand, use that maker's manual and spec sheet, because the figures differ.
Start with the manufacturer's minimum
Generac's installation manual for its 10 kW to 28 kW air-cooled home standby generators gives a recommended minimum LP tank size of 250 US gallons and tells installers to contact the LP provider to size the tank correctly. The same manual permits vertical tanks, which are measured in pounds, if they are correctly sized for the generator, and says not to connect the generator to a 20 or 30 lb tank. Generac's support site gives the same 250 gallon minimum for its home standby generators. Treat the minimum as a floor, not as the right size for your house.
Step 1: Read fuel consumption from the spec sheet
A standby generator's spec sheet lists propane consumption at half load and at full load, usually in cubic feet per hour and gallons per hour. Generac's installation manual publishes approximate figures by model and says to use the spec sheet or the unit's fuel data decal for specific values. Plan around full load: Generac states that the fuel supply and pipe must be sized at the 100% load rating.
| Generac air-cooled model | LP at half load (gal/hr) | LP at full load (gal/hr) | LP at full load (cu ft/hr) |
|---|---|---|---|
| 10 kW | 0.94 | 1.60 | 58 |
| 14 kW | 1.76 | 2.92 | 106 |
| 18 kW | 1.65 | 2.87 | 105 |
| 22 kW | 2.45 | 3.71 | 135 |
| 26 kW | 2.05 | 3.95 | 144 |
These are Generac's approximate values. Your unit's spec sheet and data decal govern.
Step 2: Count only the usable propane
A tank never holds its full rated capacity in propane. Generac's installation guidelines state that the typical maximum fill level for LP tanks is 80% and the lowest recommended operating level is 20%, which leaves about 60% of the tank's capacity to use when calculating run time. The bottom 20% is not wasted: Generac says roughly that much is needed for the liquid to keep turning into vapor.
| Tank size | Available for run time (about 60%) |
|---|---|
| 250 gallons | 150 gallons |
| 500 gallons | 300 gallons |
| 1,000 gallons | 600 gallons |
Step 3: Estimate run time
Generac's sizing method starts by multiplying the generator's full-load consumption in gallons per hour by the longest run time you want. Turned around, run time is usable gallons divided by gallons per hour. For a generator burning 3.71 gallons per hour at full load, the approximate figure Generac lists for its 22 kW air-cooled model:
- 250 gallon tank: 150 divided by 3.71 is about 40 hours at full load, or about 61 hours at the half-load rate of 2.45 gallons per hour.
- 500 gallon tank: 300 divided by 3.71 is about 81 hours at full load, or about 122 hours at half load.
- 1,000 gallon tank: 600 divided by 3.71 is about 162 hours at full load, or about 245 hours at half load.
These figures assume the tank starts at 80% and feeds nothing but the generator. Real loads move between the two rates, so use the full-load figure for planning. If a furnace, water heater or range shares the tank, Generac's manual tells installers to verify the tank can supply both the generator and the other appliances, so subtract their use before you count hours.
To work backward from a target, multiply full-load gallons per hour by the hours you want, then divide by 0.6. For example, 3.71 gallons per hour for 72 hours is about 267 gallons; divided by 0.6 that calls for a tank of at least about 445 gallons, so a 500 gallon tank.
Step 4: Check vaporization in cold weather
Storage is only half the question. A standby generator runs on propane vapor, and the liquid in the tank has to boil to make it. Generac's guidelines explain that only the liquid in contact with the tank wall absorbs heat from outside, that cold weather reduces a tank's vaporization capacity because less heat is available, and that frost on the tank further inhibits the heat transfer. A tank can hold plenty of propane and still not deliver vapor fast enough on a very cold day.
Generac publishes vaporization rates for tanks at a 20% level, the smallest wetted surface it allows for:
| Tank size | At 40°F (BTU/hr) | At 20°F (BTU/hr) | At 0°F (BTU/hr) |
|---|---|---|---|
| 250 gallons | 507,600 | 338,400 | 169,200 |
| 500 gallons | 792,540 | 528,360 | 264,180 |
| 1,000 gallons | 1,416,960 | 944,640 | 472,620 |
To compare, convert the generator's full-load demand to BTU per hour. Generac's spec sheets give the conversion for LP: multiply cubic feet per hour by 2,500. A generator using 135 cubic feet per hour at full load needs about 337,500 BTU per hour. By these figures, a 250 gallon tank at the 20% level only just covers that at 20°F, and both the 250 and 500 gallon tanks fall short at 0°F, while a 1,000 gallon tank covers it. A fuller tank vaporizes faster, so the table is a worst case, and Generac says to consult an LP dealer for temperatures below 0°F. In cold climates, this step is where a propane supplier's judgment matters most.
Placement, codes and who does the work
Generac's manual requires its generators to be installed in accordance with NFPA 37 and NFPA 70 and any other federal, state and local codes for minimum distances from other structures. The tank itself falls under NFPA 58 and your local code. Distances vary by jurisdiction, so your installer and local authority set them, and Generac advises contacting local fuel suppliers or the fire marshal to verify the rules. Generac also states that connection of the fuel source must be done by a qualified professional technician or contractor. Gas piping, regulators, leak testing and the first fill are jobs for licensed professionals and your propane supplier, not a homeowner project.
Choosing the tank
With the numbers in hand, choose the placement and rating. Underground vs above ground propane tanks compares placements, and ASME vs DOT propane tanks explains the certifications. Tanks on this store at or above Generac's 250 gallon minimum include:
- 250 Gallon Above Ground Propane Tank, $1,250.00
- 320 Gallon Horizontal Above Ground Propane Tank - ASME, $1,800.00
- 500 Gallon Above Ground Propane Tank - ASME, $1,650.00
- 1000 Gallon Above Ground Propane Tank, $3,500.00
- 250 Gallon Underground Propane Tank - ASME, $1,700.00
- 500 Gallon UG Propane Tank - ASME, $3,800.00
Tanks ship empty by freight; see shipping and delivery. Installation by a licensed local installer can be added at checkout on some tanks, and the first fill is done by your propane supplier. If you are deciding whether to own the tank at all, read should I buy or lease a propane tank, and see propane tank cost for the full budget. The size chart lists dimensions, and you can contact the store to talk through a size.
Quick checklist
- Find the full-load LP consumption on your generator's spec sheet.
- Decide the longest outage you want to cover without a delivery.
- Multiply the two to get the propane you need.
- Divide by 0.6 for the minimum tank size, and never go below the manufacturer's minimum.
- Add any other appliances that share the tank.
- Check the tank's vaporization rate at the coldest temperature you expect.
- Have your propane supplier and licensed installer confirm the size.
Common questions
What size propane tank do I need for a whole-house generator?
Generac recommends at least a 250 gallon tank for its home standby generators, and the right size above that depends on full-load fuel consumption, the run time you want and the coldest temperature you expect. Multiply full-load gallons per hour by the hours you want, divide by 0.6 to allow for the 80% maximum fill and 20% minimum level, then check cold-weather vaporization. Your propane supplier should confirm the final size.
How long will a 500 gallon propane tank run a generator?
About 300 gallons of a 500 gallon tank is usable for run time, based on Generac's 80% maximum fill and 20% minimum operating level. At the 3.71 gallons per hour Generac lists for its 22 kW air-cooled model at full load, that is roughly 81 hours, or about 122 hours at its half-load rate. Smaller generators run longer, and other appliances on the same tank shorten the run.
Can I run a standby generator on 20 lb or 100 lb propane cylinders?
Not a Generac home standby generator on 20 or 30 lb tanks: its installation manual says not to connect one, and it recommends a 250 gallon minimum. The manual does permit vertical tanks measured in pounds if they are correctly sized for the generator, which is a calculation for your propane supplier. Smaller tanks have less wetted surface area, which Generac says lowers the rate at which they can vaporize propane.
Why can't a generator use all the propane in the tank?
Because tanks are filled to a maximum of about 80% and should not run below about 20%, roughly 60% of the tank is usable. Generac's installation guidelines give those figures and say about 20% of the tank capacity is needed for the liquid to turn into vapor. Running lower shrinks the wetted surface area, which reduces how fast the tank can supply vapor, and that matters most in cold weather.
Can my generator share a propane tank with my furnace?
It can if the tank is sized for both. Generac's air-cooled installation manual tells installers to verify the LP tank can supply both the generator and other household appliances, and its Protector-series guidelines call for the generator to have its own dedicated fuel supply from the primary regulator. Add the furnace's consumption to the generator's for run time and vaporization, and leave the piping design to your installer.
Who should connect the propane tank to the generator?
A qualified professional. Generac states that connection of the fuel source must be done by a qualified professional technician or contractor, and gas piping, regulators, leak testing and the first fill are work for licensed installers and your propane supplier. The National Fuel Gas Code, NFPA 54, requires the piping to be checked for leakage immediately after gas is turned on into a new system.