Backup Generator Runtime Calculator

Runtime is calculated as fuel tank gallons divided by the fuel consumption rate, so tank size sits directly in the numerator of that division -- doubling tank capacity roughly doubles runtime at the same load and fuel type, holding everything else constant. A change in generator wattage or connected load also affects runtime, but only indirectly by changing the consumption rate, not by scaling the calculation directly the way tank size does.

A larger generator has more rated capacity in kW, and fuel consumption scales with that capacity even when the generator isn't running at full load -- a bigger engine simply burns more fuel per hour to produce power than a smaller one does, all else equal. So if you size a generator larger than your connected load actually requires, you gain headroom for future loads but give up some runtime per gallon compared to a generator sized closer to your actual needs.

Fuel consumption rate is determined entirely by generator capacity, fuel type, and load percentage -- how fast the engine burns fuel while running -- and has nothing to do with how much fuel is available to burn. Tank size only determines how many hours that consumption rate can be sustained before refueling, which is why it shows up in the runtime calculation but not in the gallons-per-hour figure.

Diesel generators typically consume the least fuel per kW of output (roughly 0.055 gal/hr per kW at 50% load in this model), followed by gasoline (about 0.075 gal/hr per kW), with propane the least efficient by volume (about 0.10 gal/hr per kW) due to its lower energy density -- though propane offers longer shelf life and doesn't degrade in storage the way gasoline can. The right choice depends on your priorities: diesel and gasoline generally offer more runtime per gallon, while propane trades some efficiency for fuel that stores indefinitely without stabilizers.