Calculating How Long a Generator Will Run on a Given Fuel Supply

Last updated July 2, 2026

Generator runtime calculations combine three factors: fuel tank or fuel supply capacity, the generator's fuel consumption rate at the load level it is actually running, and the type of fuel being used, since gasoline, propane, and diesel generators have meaningfully different consumption characteristics per unit of power produced. A portable gasoline generator with a 6-gallon tank consuming 0.75 gallons per hour at a 50 percent load typical of running a refrigerator, some lighting, and a few small appliances will run for approximately 8 hours before requiring a refill, while the same generator pushed to 100 percent load consuming 1.1 gallons per hour would run for only about 5.5 hours.

Dual-fuel and propane generators offer different runtime characteristics worth calculating separately, since propane generators connected to a large stationary propane tank can run for days without refueling, while portable gasoline units require manual refueling every several hours during extended use. This distinction matters significantly for multi-day outage planning, where a propane-fueled standby generator connected to a 500-gallon tank can provide reliable power for over a week at moderate loads, while a household relying solely on a portable gasoline generator needs to calculate and stockpile enough gasoline for the full expected outage duration, factoring in the practical challenge of safely storing that much gasoline.

The calculation shows generator runtime at your actual planned operating load, not the manufacturer's rated maximum, since real-world runtime can vary by 30 to 40 percent between light and heavy load operation. For extended outage planning, propane or natural gas standby generators eliminate the refueling logistics that portable gasoline generators require, making fuel type as important a planning consideration as the generator's rated capacity.