Generator Fuel Needed for an Outage: Plan for 24, 48, or 72 Hours

Researched and reviewed by Dustin Sosko
Published: September 28, 2026

How much fuel should you keep for an outage? Start with your generator’s gasoline consumption at the load you expect to run, then multiply it by your planned running hours. A generator running 12 hours a day needs fuel for 12, 24, or 36 engine hours across a 24, 48, or 72 hour outage.

The calculator below estimates gasoline burned, fuel to have on hand after your chosen reserve, and additional tank refills. If you plan to use propane, use our propane generator runtime calculator and its outage planning extension instead. Gasoline and propane runtime figures are not interchangeable.

Gasoline outage planner

How much gasoline will your generator need?

Use a gasoline consumption rate or your exact model's full-tank runtime at a stated electrical load. The three results assume the same running hours each day.

Generator specification How is fuel use specified? Gasoline tank capacity (gallons) Published full-tank runtime (hours) Load for that fuel figure (% of gasoline running watts) Planned average load (% of gasoline running watts) Generator running hours per day Reserve kept unspent (% of fuel on hand) Gasoline already in generator tank (gallons)

Custom figures: check your model's manual or measured consumption. Use gasoline figures only.

The planned average load must match the stated load. If it differs, find a manufacturer figure or measure consumption at the load you expect. Starting surges and changing loads are not modeled.

A planning estimate, not a guaranteed runtime. Check the model's maintenance schedule for prolonged use. Operate portable generators outdoors and follow the manufacturer's fueling instructions.

How to choose the right fuel figure

Check the manual or manufacturer specifications for your exact model. Enter either its gasoline consumption in gallons per hour or its gasoline tank capacity and published full-tank runtime. Record the electrical load attached to that figure, commonly 25% or 50% of gasoline running watts.

The included model examples are reference inputs, not recommendations to buy these generators:

Generator	Manufacturer’s gasoline figure	Implied consumption
Honda EU2200i	0.95-gallon tank; 8.1 hours at 25% load	About 0.117 gal/hour
Champion 201531	1.54-gallon tank; up to 10 hours at 25% load	About 0.154 gal/hour
WEN DF480iX	1.9-gallon tank; up to 7 hours at half load	About 0.271 gal/hour

These are gasoline figures published by the respective manufacturers. The implied hourly rates divide tank capacity by the stated runtime; they are not independently measured consumption rates. Your results will change with load and operating conditions. The calculator stops if your planned load differs from the load attached to your fuel figure, because it cannot reliably infer consumption at another load.

Diagram showing that a fuel figure published at 25% load applies to a 25% planned load, but cannot be used to estimate 50% load — Use a consumption figure measured at the load you expect; the planner does not extrapolate between loads.

What “fuel to have on hand” and “refills” mean

The calculator uses these steps:

Expected burn: gallons per running hour × running hours per day × outage days.
Fuel to have on hand: expected burn ÷ (1 − reserve percentage).
Additional refills: the number of tank additions needed after counting gasoline already in the generator. A partial final fill counts as one refill.

For example, if your generator uses 0.5 gallon per hour and runs 12 hours per day, a 48 hour outage involves 24 running hours and an expected burn of 12 gallons. With a 10% reserve kept unspent, you would plan to have 13.33 gallons total, including any gasoline already in the generator.

Example chart comparing 6, 12, and 18 gallons burned with 6.67, 13.33, and 20 gallons to have on hand for 24, 48, and 72 hour outages — Example only: 0.5 gallon per running hour, 12 running hours per day, and a 10% reserve.

Refill count is a handling estimate. Turn the generator off and let it cool before each refill. It does not mean you must pour a full tank’s worth every time.

Example showing 2 gallons already in a generator tank, two full 2-gallon refills, and one partial 0.67-gallon refill — The final partial fill still counts as a refill event.

Plan running hours before buying fuel

Running hours are different from outage hours. You may choose scheduled generator periods to cool a refrigerator, charge devices, run a well pump, or replenish a portable power station. Enter the hours the generator will actually operate each day, up to 24.

Keep the load assumption realistic. A refrigerator compressor, air conditioner, pump, or battery charger can change electrical demand while the generator runs. If you have only a manufacturer’s 25% load runtime but expect a materially heavier load, find a better fuel figure or measure consumption under your planned setup before relying on a multiday estimate.

If your generator runs on propane

Use the existing propane runtime calculato r to estimate propane use at your model and expected load. Its outage extension converts that result into propane needed and additional cylinders for 24, 48, and 72 hours. Enter the fuel contents of each cylinder: a cylinder sold as a 20-pound size may contain less than 20 pounds of propane when exchanged.

Generator safety during an outage

Diagram showing a portable generator outdoors at least 20 feet from a home, with exhaust directed away — Keep the generator outdoors and exhaust away from the home; the drawing is not to scale.

Operate a portable generator outdoors, at least 20 feet from the home, with exhaust directed away from windows, doors, and vents. Keep working carbon monoxide alarms in the home. Never refuel a running generator; turn it off and allow it to cool first. Follow your generator manual for fuel storage and maintenance during extended operation.

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