Generator runtime calculator
A 3,500W portable at half load gets about 3.1 hours per gallon of gas, so the usual 4 gallon tank is 13 hours, overnight with a wake-up refill. Set your unit, load, and fuel below and plan the cans.
The model, in one paragraph
Portable gasoline generators deliver about 5.5 kWh of electricity per gallon at half load. Run them light and the per-gallon figure drops to about 4.0 (engines idle badly); run them near rated and it rises to about 6.2. Propane carries 80% of gasoline's energy per gallon, and a 20 lb grill cylinder holds 4.7 gallons. Hours = gallons × kWh-per-gallon ÷ load in kW. That's the whole model, and it reproduces published spec runtimes within a half hour on the units we checked it against.
Quick reference at half load
| Setup | Runtime |
|---|---|
| 2,200W inverter on 1 gal gas | 6.1 hours |
| 3,500W portable on its 4 gal tank | 12.6 hours |
| 5,000W portable on 5 gal gas | 11 hours |
| 7,500W portable on 20 lb propane | 5.5 hours |
| 12,000W portable on 8 gal gas | 7.3 hours |
Questions people ask
How long will a generator run on 5 gallons of gas?
About 16 hours for a 3,500W portable at half load, and roughly 7 hours for a 7,500W unit at half load. Load is the whole story: the same 5 gallons lasts nearly twice as long at quarter load. Our model delivers about 5.5 kWh per gallon at 50% load, which matches published spec-sheet runtimes within a half hour.
How long does a 20 lb propane tank run a generator?
A 20 lb cylinder holds 4.7 gallons of propane, worth about 80% of gasoline's energy per gallon. On a 3,500W dual-fuel unit at half load that is roughly 12 hours, a night and a morning. Grill tanks are a storage convenience, not a capacity upgrade; for multi-day outages people plumb in a 100 lb cylinder or the house's 250 gallon pig.
Why does my real runtime beat the spec sheet?
Because your load is lighter than the 50% the spec assumes. A fridge cycles; a furnace blower runs minutes per hour. A generator loafing at 25% load sips fuel, though it also wastes some (engines are least efficient lightly loaded, which is why our per-gallon figure drops at 25%). Inverter units with eco throttles stretch light-load runtime dramatically.
Should I store gas or propane?
Propane, unless you burn through gas regularly anyway. Gasoline goes stale in months even stabilized, gums carburetors, and evaporates; propane keeps indefinitely and the cylinder is its own containment. The tradeoff is about 10% less power on propane and cold-weather pressure drop below roughly 20°F on small cylinders.