Generator fuel run-time estimator
How long will a tank last, and how much fuel should you store for a three-day outage? Start from your generator’s own spec sheet, not a generic number.
From your spec sheet
Your outage
Estimate
Hours per full tank at your load
10.0 h
Your load is about 50% of rated output. Estimated burn: 0.40 gal/hr (spec sheet: 0.40 gal/hr at 50%).
For 3 days at 12 hours a day, plan on roughly 14.4 gallons, about 3.6 tanks.
This is a rough model: burn scales with load but never drops below 60% of the spec-sheet rate, because an engine uses fuel just to turn. Real use depends on the engine, fuel, altitude, and temperature. Keep a margin.
Why we start from your spec sheet
Generator fuel use varies a lot between engines. An inverter generator throttles its engine down at light load; a conventional generator runs at a fixed speed whatever the load. Rather than invent a gallons-per-hour figure, this tool asks for the run time your manufacturer lists, such as “10 hours at 50% load on a 4-gallon tank.” From that we get the burn rate at the spec load, then adjust it for your actual load.
The math, step by step
- Spec burn rate = tank gallons ÷ listed hours. A 4-gallon tank that lasts 10 hours at 50% load burns 0.4 gallons per hour at that load.
- Your load percentage = your average watts ÷ rated running watts.
- Adjusted burn = spec burn × (your load % ÷ spec load %), but never less than 60% of the spec burn. The floor reflects that engines use fuel just to keep turning; it is a conservative assumption, not a measured constant.
- Hours per tank = tank ÷ adjusted burn. Total fuel = adjusted burn × hours per day × days.
Your average load is lower than your peak
A refrigerator compressor cycles on and off, a sump pump runs only when water rises, and a furnace fan follows the thermostat. So the average load over an hour is usually well below the peak you used to size the generator. If you are unsure, use about half of your running total from the size calculator as a starting guess, then check real use with a plug-in watt meter or the generator’s own display.
Running in shifts saves fuel
You do not need to run a generator around the clock. A full refrigerator that stays closed holds food for several hours, and many households run the generator a few hours at a time to recover the fridge and freezer, charge devices, and pump water. Set “hours run per day” to match your plan. See how long a generator can run for maintenance limits such as oil checks.
Storing the fuel safely
Store gasoline only in approved containers, outside living areas, away from fuel-burning appliances. Never refuel a hot or running generator; the CPSC and NFPA both advise letting it cool first, because fuel spilled on hot engine parts can ignite. Gasoline goes stale; a fuel stabilizer and rotation help. Read storing generator fuel for details.