A typical portable gasoline generator burns 0.5 to 0.75 gallons an hour at half load, so a standard 5-gallon gas can lasts roughly 7 to 10 hours. Run it flat out and that window shrinks to 4-6 hours; run it barely loaded and you burn more fuel per kilowatt-hour delivered, not less, for reasons that come down to how a small engine works.
Fuel burn scales with load, but not in a straight line, and not proportionally. A generator’s engine has to keep spinning at a near-constant speed — usually around 3,600 RPM for a conventional unit, or a governed range for an inverter model — whether it is powering a single lamp or a house full of appliances. Some of every gallon goes to just keeping the engine turning over, before any of it reaches your outlets. That fixed overhead is why light loads are the least efficient use of a gallon of gas, covered in detail below.
Fuel use by generator size and load
The table below uses typical consumption figures published in manufacturer spec sheets across the common portable sizes — we have not run these engines on a dyno ourselves, so treat them as representative rather than exact for any one model. Load is the share of the generator’s rated wattage actually being drawn; see the appliance wattage chart to work out what your own gear draws, and what a 3,500-watt generator will run for a sizing walkthrough.
| Generator size | 25% load (gal/h) | 50% load (gal/h) | 100% load (gal/h) |
|---|---|---|---|
| 2,000 W inverter | 0.15 | 0.25 | 0.45 |
| 3,500 W | 0.25 | 0.40 | 0.75 |
| 5,000 W | 0.35 | 0.55 | 1.00 |
| 7,500 W | 0.50 | 0.75 | 1.30 |
| 10,000 W | 0.65 | 1.00 | 1.70 |
Read across a row and the shrinking gap between 25% and 100% load looks like efficiency. It is the opposite. What matters is not gallons per hour but gallons per kilowatt-hour actually delivered, and that number gets worse, not better, the lighter the generator is loaded.
Why running at 25% load wastes fuel
Take the 3,500 W line from the table. At full load it delivers 3.5 kW while burning 0.75 gal/h — that is 4.7 kWh per gallon. At 25% load it delivers 0.875 kW while burning 0.25 gal/h, which works out to 3.5 kWh per gallon. Loading the same engine to a quarter of its rating makes each gallon roughly 25% less productive, because the fixed cost of just spinning the engine is now spread over far less delivered power.
The practical upshot: oversizing a generator for the load you actually run — a fridge and a few lights, say — burns more gas over the course of an outage than sizing correctly, or running two loads together instead of cycling the generator on and off for one small appliance at a time. It is also why manufacturers quote their best runtime figures at 25% or 50% load for inverter generators specifically, and worse figures for a conventional generator with no load-sensing at all.
What a gallon of gas actually costs you, per kWh
At $3.50 a gallon and the 5,000 W generator’s 50%-load figure of 0.55 gal/h delivering 2.5 kW, one gallon buys 4.5 kWh (2.5 kWh ÷ 0.55 gal). That gallon costs $3.50, so the delivered electricity works out to about $0.77 per kWh. Push the same generator to 25% load and the math gets worse: 1.25 kW at 0.35 gal/h is 3.6 kWh per gallon, or about $0.98 per kWh.
Compare that with a US grid average around $0.17 per kWh, and gasoline backup power costs four to six times more than the electricity it is replacing — before counting the generator’s purchase price, oil changes, or the noise. It is the same math used to compare running costs across battery power stations on the power station price index: a generator’s fuel bill is a real, calculable cost per kWh, not a rounding error, and it is worth knowing before you buy a bigger unit than you need.
Ethanol, stabilizer, and fuel that goes bad
Most pump gasoline in the US is E10, 10% ethanol by volume, and ethanol is the main reason generators fail to start after sitting unused. Ethanol absorbs moisture from the air, and over a few months that water settles out and can corrode carburetor jets or gum up fuel lines — the most common reason a generator that ran fine last year won’t start this year. Two fixes, used together: add a fuel stabilizer rated for ethanol blends at every fill, and either run the tank dry before long storage or run the engine for a few minutes on stabilized fuel so the carburetor itself is protected, not just the tank. See generator maintenance for the full storage and startup checklist, including oil changes and spark plug intervals.
Ethanol-free gasoline (sometimes sold as “rec fuel” at marinas and some stations) avoids the moisture problem entirely and is worth the premium for a generator that sits idle most of the year. It is not required, but a stabilizer is, if the alternative is fuel sitting in the tank for more than 30 days.
Refueling safely during an outage
Never refuel a running or hot generator. Gasoline vapor ignites easily on a hot muffler or exhaust manifold, and this is one of the more common ways people are injured during storm cleanup. Shut the engine off, wait at least two minutes for it to cool, then refuel outdoors, away from doors, windows, and any air intake — carbon monoxide from a generator running near a house is a separate and more common killer than fuel fires, and the reason a generator belongs outdoors and at a distance, never in a garage with the door open.
Store spare fuel in approved, labeled containers, away from the generator itself and out of direct sun, and keep no more on hand than you would use in the outage windows your area actually sees — a stockpiled 15 or 20 gallons of gasoline is itself a fire-code and safety concern most homes do not need. If outages in your area regularly run past a few days, it is worth working out what a solar setup would cost to cover the same load, or looking at battery power stations that need no fuel and no refueling trips in the dark at all.
How this page makes money. Some links here are affiliate links and we earn a commission if you buy through them — currently BLUETTI, ALLPOWERS, Mango Power, Renogy and MoonCool. Every other brand on this page is here because leaving it out would make the comparison useless to you, and we are paid nothing either way. Prices are each maker’s own list price at the time of writing and change often.