A generator’s real spec is not watts — it is days. How many days will the fuel you have actually stored keep the fridge cold and the lights on? That number hides in three places: the tank, the burn rate at the load you really run, and how many hours a day you run it. This calculator puts them together. Our fuel storage guidance lives in the Preserve the Power hub; this is the math underneath it.
Run your fuel
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The burn rates above are the honest middle of what these classes drink. A small inverter sipping at light load is one of the most fuel-efficient machines ever sold — around a tenth of a gallon an hour. Load it near its limit and the same machine triples that. This is why the oldest advice in generator ownership is also the truest: buy the smallest generator that covers your real loads, and run it in shifts — a few hours to chill the fridge and charge the batteries, then off. Runtime is a habit before it is a spec.
One storage fact belongs next to every number on this page: gasoline goes stale. Untreated fuel is trustworthy for a few months; treated with a stabilizer like STA-BIL, it keeps about a year — and propane, for those with a dual-fuel machine, keeps indefinitely. The calculator counts gallons. The calendar quietly spends them, which is why rotation — burn the stored fuel in the mower each season and replace it — is part of the plan and not a footnote.
Worked example: the starting numbers — a small inverter generator with a 1-gallon tank burning 0.19 gallons an hour at a moderate load, run 6 hours a day with 10 gallons stored — last about 8.8 days. One tank runs about 5.3 hours, and each day of running drinks about 1.14 gallons.
Questions people ask
How much gas should I store for a generator? Work backward from days, not gallons. At the moderate pace above — six hours a day at half load — a small inverter drinks a little over a gallon a day, so a week is about 8 gallons. A large portable at the same pace wants three times that. Your number is on the screen: change the hours and the load until it looks like your house.
Does a bigger generator use more fuel? At the same load, yes — a big engine turning a small load is wasting fuel to spin itself. The gap is widest at light loads, exactly where most outage hours are spent. This is the quiet argument for inverter generators and for owning a small one alongside any large one.
Gasoline or propane for storage? Propane keeps forever, burns cleaner, and never gums a carburetor — but carries less energy per gallon, so the same tank runs fewer hours, and big propane storage costs more to set up. Gasoline is denser and everywhere, but it is on a one-year clock even when stabilized. Dual-fuel machines exist precisely so you do not have to choose forever. For the tank-by-tank math, run our Propane Runtime Calculator.
The generators behind the classes
All three are the machines from our portable generator comparison:
Honda EU2200i
The small-inverter benchmark: sips fuel at light loads, and the quietest name in the class.
Champion 4250 Dual Fuel Inverter
The mid-size pick: runs on gasoline or propane — and propane is the fuel that never goes stale.
WEN 56203i
The budget inverter: the 2,000-watt class at the lowest buy-in of the three.
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