"How long will my tank last?" gets a lot of answers that start with the generator's size. Size matters less than you'd think. What decides runtime is how much power you actually draw, how much propane you really have, and, in winter, whether the tank can turn liquid into gas fast enough. Here are the numbers for the setups most people have.
Portable Generator on Propane Cylinders
For a typical 3,500 W dual-fuel portable (rated about 3,150 W on propane), which runs 10.5 hours on a 20-lb tank at half load. A 20-lb "grill tank" holds about 4.7 gallons.
| Cylinder | Light load ~800 W | Half load ~1,600 W | Full load ~3,150 W |
|---|---|---|---|
| 20-lb (4.7 gal) | 16 hr | 10 hr | 6 hr |
| 30-lb (7.1 gal) | 24 hr | 16 hr | 9 hr |
| 40-lb (9.4 gal) | 32 hr | 21 hr | 12 hr |
| 100-lb (23.6 gal) | 3.3 days | 2.2 days | 31 hr |
A fridge, a few lights, phone chargers and a furnace blower average well under 1,600 W, so for most outages the half-load column is a conservative guide. Plan on at least two or three 20-lb tanks a day if you run the generator around the clock.
Whole-House Standby Generator on a Tank
For a 22 kW air-cooled standby generator using Generac's published propane consumption (2.16 gallons an hour at half load, 3.68 at full), plus a 9 kW unit for comparison. Bulk tanks start at an 80% fill and stop at 20%, so a 500-gallon tank gives about 300 usable gallons.
| Propane supply | 22 kW at 25% 5.5 kW | at 50% 11 kW | at 100% 22 kW | 9 kW at 50% 4.5 kW |
|---|---|---|---|---|
| 100-lb cylinder | 17 hr | 11 hr | 6 hr | 24 hr |
| 250-gal tank | 4.5 days | 2.9 days | 41 hr | 6.2 days |
| 500-gal tank | 8.9 days | 5.8 days | 3.4 days | 12.5 days |
| 1,000-gal tank | 17.9 days | 11.6 days | 6.8 days | 25.0 days |
If your house averages a quarter to a half of the generator's output, a 500-gallon tank covers about six to nine days of continuous running and a 250-gallon tank about three to four and a half — one reason 500 gallons is a common choice for standby generators, especially when the same tank also feeds a furnace or water heater.
A bigger generator isn't always better. At the same 4.5 kW load, a 9 kW unit burns about 1.0 gallon an hour and the 22 kW about 1.3, because the larger engine burns more just to keep running.
Why a Tank Can Stop Working Before It's Empty
Propane leaves the tank as vapor, boiling off the liquid with heat drawn from the surrounding air. In freezing weather that boil-off slows, and it slows further as the tank empties because less liquid touches the steel. Flame Engineering's chart for a 100-lb cylinder shows a full one supplying about 113,000 BTU an hour at 0°F and 167,000 at 20°F; half full, about 64,000 and 94,000.
A 22 kW standby at half load needs about 198,000 BTU an hour. A single 100-lb cylinder can't supply that in freezing weather, so the generator surges or shuts down with propane still in the tank. The fixes are more cylinders manifolded together or a bulk tank: Elliott Electric's generator sizing guide puts a 500-gallon tank at over 260,000 BTU an hour even at 0°F. Small cylinders on large portables hit the same limit; frost on the outside of the cylinder is the warning sign.
What a Day of Generator Power Costs
At $2.50 a gallon for propane and $1.70 a therm for natural gas:
| Generator | Propane at 25% | at 50% | Natural gas at 50% |
|---|---|---|---|
| 9 kW standby | $45/day | $60/day | $33/day |
| 16 kW standby | $75/day | $114/day | $82/day |
| 22 kW standby | $84/day | $130/day | $78/day |
That works out to roughly 50¢ to 85¢ per kWh at typical loads, three to five times grid electricity. Generators are expensive insurance, which is a reason to size your load — not the generator — carefully. The generator propane runtime calculator works out hours, days, cost and cold-weather vapor supply for your own setup.
If you'd rather ride out a short outage without a generator, see how fast your house cools with the power outage cooling calculator.