A generator's propane use depends on how much power you're drawing from it, not on its maximum rating. A 20 kW standby generator running a fridge and a few lights burns far less than it would at full load. So the first number to know is your actual load.
A gallon of propane holds about 91,452 BTU (EIA). If a generator turns about 20% of that into electricity, a 5 kW load burns about 0.93 gallons an hour, about 22 gallons a day; a 20 kW load burns about 3.7 gallons an hour. Your generator's spec sheet lists its actual consumption at half and full load, which is the better number to use.
Propane use by load
Gallons per hour = load in kW × 3,412 BTU per kWh ÷ (91,452 BTU per gallon × generator efficiency). The table assumes 20% efficiency; small generators running far below their rating are often less efficient than that.
| Load | Gallons per hour | Gallons per day |
|---|---|---|
| 2 kW | 0.37 | 9.0 |
| 5 kW | 0.93 | 22.4 |
| 7.5 kW | 1.40 | 33.6 |
| 10 kW | 1.87 | 44.8 |
| 15 kW | 2.80 | 67.2 |
| 20 kW | 3.73 | 89.5 |
Your load is what's running: a fridge, a furnace blower, lights and a sump pump might total 2 to 5 kW. Our generator sizing calculator adds up appliances for you.
Half load vs full load
Spec sheets list propane draw at 50% and 100% load because consumption climbs with output. The table below is an example computed from the formula on this page at 20% efficiency, not a reading from any real spec sheet; a manufacturer's own figures are the ones to trust.
| Generator rating | Half load | Full load |
|---|---|---|
| 14 kW | about 1.3 gal/hr (7 kW) | about 2.6 gal/hr |
| 22 kW | about 2.1 gal/hr (11 kW) | about 4.1 gal/hr |
Half load uses close to half the propane, which is why a lightly loaded generator stretches a tank. Full load roughly doubles the hourly burn, so a long outage near full output drains even a large tank quickly.
How long a propane tank lasts
Propane weighs about 4.2 pounds per gallon, and large tanks are filled to about 80% (an NFPA 58 safety limit) to leave room for expansion. At a 5 kW load:
| Tank | Propane | Runtime at 5 kW |
|---|---|---|
| 20-lb cylinder | about 4.7 gallons | about 5 hours |
| 30-lb cylinder | about 7.1 gallons | about 8 hours |
| 40-lb cylinder | about 9.5 gallons | about 10 hours |
| 100-lb cylinder | about 23.6 gallons | about 25 hours |
| 250-gallon tank, filled to 80% | 200 gallons | about 9 days |
| 330-gallon tank, filled to 80% | 264 gallons | about 12 days |
| 500-gallon tank, filled to 80% | 400 gallons | about 18 days |
| 1,000-gallon tank, filled to 80% | 800 gallons | about 36 days |
Cylinder labels are the weight of propane they hold, so dividing by 4.2 pounds per gallon gives the volume: a 30-lb cylinder holds about 7.1 gallons, a 40-lb about 9.5. Large residential tanks are sized by total volume, and the 80% fill rule leaves the 250 and 330-gallon tanks with 200 and 264 usable gallons.
Runtime = gallons ÷ gallons per hour. Halve the load and the runtime roughly doubles.
Whole-house standby: per-day and multi-day use
Whole-house standby units are often sold around 14 kW and 22 kW, but those are maximum ratings, not what the house actually pulls. At full load the 14 kW class burns about 2.6 gallons an hour, close to 62 gallons a day (2.6 × 24). The 22 kW class at full load burns about 4.1 gallons an hour, close to 98 gallons a day (4.1 × 24).
Most homes draw well below the rating once the big startup surges pass. A home averaging 10 kW through an outage burns 1.87 gallons an hour, about 45 gallons a day from the load table above. Over a three-day outage that is about 135 gallons (1.87 × 72).
A 250-gallon tank filled to 80% holds 200 gallons, so it covers that three-day draw with room to spare. A 40-lb cylinder holds only about 9.5 gallons, roughly five hours at the same 10 kW load, so small cylinders suit camping or short outages rather than whole-house backup.
What it costs to run
Cost per hour = gallons per hour × your propane price. At an example price of $3.00 a gallon, a 5 kW load costs about $2.80 an hour, or $67 a day. Propane prices vary by region and season; your supplier's quote is the number to use.
Cold weather
Propane leaves a tank as vapor, and in cold weather a small cylinder can't vaporize it fast enough for a large generator. The tank pressure drops and the generator can lose power even with propane left. Large tanks hold more liquid surface and cope better; for cold climates, size the tank for winter loads.
Propane vs. other fuels
Gallon for gallon, propane holds less energy than gasoline (about 120,000 BTU) or diesel (about 137,000 BTU), so a generator burns more gallons of propane for the same output: about 1.3 gallons of propane for each gallon of gasoline, and 1.5 for each gallon of diesel. Propane stores for years without going stale, which is why many standby generators use it. Natural gas generators draw from the utility line instead; a gallon of propane equals about 88 cubic feet of natural gas.
Safety
Run portable generators outdoors only, at least 20 feet from the house, with the exhaust pointed away from doors and windows, as the CPSC advises; generator exhaust carries deadly carbon monoxide. Have standby generators installed with their required clearances.
Frequently asked questions
How much propane does a generator use per hour?
About 0.9 gallons an hour at a 5 kW load and about 3.7 at 20 kW, assuming 20% efficiency. Check your generator's spec sheet for its figures at half and full load.
How long will a 500-gallon propane tank run a generator?
About 18 days at a steady 5 kW load, from a tank filled to 80% (400 gallons). A heavier load shortens that.
How long does a 20-lb propane tank last on a generator?
About 5 hours at a 5 kW load, and longer at lighter loads.
Is propane more expensive than gasoline for a generator?
Per hour it often is, because a generator burns about 1.3 gallons of propane for each gallon of gasoline. Propane's advantages are long storage life and cleaner running.
How long will a 100-gallon or 250-gallon propane tank run a generator?
A 100-gallon tank filled to 80% holds about 80 gallons, roughly 3.5 days at a steady 5 kW load. A 250-gallon tank holds about 200 gallons at 80%, roughly 9 days at the same load. Heavier loads shorten both in proportion.
How much propane does a whole-house generator use per day?
At full load a 14 kW unit burns about 62 gallons a day and a 22 kW unit about 98, but most homes draw far less once startup surges pass. A home averaging 10 kW uses closer to 45 gallons a day. Size the tank for a realistic multi-day load rather than the nameplate rating.
Sources & References
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