Quick answer: There is no single figure. A 10 kW standby generator burns 0.97 gal/h of propane at half load and 1.48 gal/h at full load, and the manufacturer publishes both. Doubling the load raises the burn by 1.5 to 1.8 times, not twice. Portable generators publish no consumption figure at all.
Generator fuel consumption per hour is quoted everywhere as one number, and one number cannot answer it. The same machine burns far less at half load than it does flat out, so the figure means nothing until somebody tells you the load. Our generator sizing guide works out what load you actually have; this page turns that load into gallons per hour.
The load is the missing half of the question
A generator is an engine with a governor that feeds it in proportion to what you draw. Any page handing you a flat “0.75 gallons an hour” has quietly picked a load for you and not said which. Standby manufacturers publish it properly, in two columns: Generac’s Guardian 10/14/18 kW spec sheet gives propane consumption at half load and at full load for every model.
| Model | Propane, 1/2 load | Propane, full load | Full ÷ half |
|---|---|---|---|
| 10 kW | 0.97 gal/h | 1.48 gal/h | 1.53× |
| 14 kW | 1.81 gal/h | 3.07 gal/h | 1.70× |
| 18 kW | 1.70 gal/h | 3.02 gal/h | 1.78× |
That last column is the point. Going from half load to full costs 1.5 to 1.8 times the fuel, not twice. An engine pays a fixed cost in friction and pumping whether you draw anything or not, so the fuel curve does not start at zero. Running a big generator lightly loaded wastes fuel per kilowatt-hour delivered.
Portable generators do not publish this at all
Move to a portable machine and generator fuel consumption per hour disappears from the paperwork. You get a run time and a tank size, and are expected to divide. Champion publishes 11 hours at 1/2 load from a 7.7 gallon tank for one 6,250 W dual fuel model, which works out at 0.70 gal/h — that division is ours, not theirs.
Some manuals do not give you even that. The Westinghouse WGen6000 owner manual lists a 6.6 gallon tank and 6,000 running watts across two pages of specifications, and publishes neither a run time nor a consumption figure. The run times quoted for that machine elsewhere come from marketing pages.
And where one portable runs on both fuels, the two columns are not comparable. That Champion is rated 6,250 W on gasoline and 5,600 W on propane, so its 11 hours and 6.5 hours are measured at different loads: half load is 3,125 W in one column and 2,800 W in the other.
A gallon of propane is not a gallon of gasoline
The other reason they do not line up is chemistry. The Department of Energy’s fuel properties comparison puts gasoline at 120,388 to 124,340 Btu per gallon, diesel at 138,490 and propane at 91,420. A gallon of propane carries about three quarters of the energy of a gallon of gasoline, so a propane machine that empties its tank sooner is not necessarily less efficient: it is carrying less fuel in the same volume. A 20 lb barbecue cylinder holds about 4.7 gallons at 4.24 lb per gallon, barely more than half that 7.7 gallon tank.
Cost to run it
| Case | Burn | Per hour, USD | Per hour, GBP |
|---|---|---|---|
| 6,250 W portable, gasoline, 1/2 load | 0.70 gal/h | $2.85 | £4.36 |
| 10 kW standby, propane, 1/2 load | 0.97 gal/h | $2.59 | n/a |
| 10 kW standby, propane, full load | 1.48 gal/h | $3.96 | n/a |
| 14 kW standby, propane, full load | 3.07 gal/h | $8.21 | n/a |
Two warnings about the propane rows. That $2.674 is the latest published US residential figure, but it dates from March: the EIA collects heating fuel prices weekly only from October through March. And it is bulk propane delivered to a tank, not the cylinder exchange cage most portable owners actually use, which is dearer per gallon.
Diesel is an outlier on price rather than on burn. At $5.599 a gallon that week it cost 38 per cent more than gasoline while carrying 11 to 15 per cent more energy.
Run time is not the same as hours of power
A published run time is how long a tank lasts, not how long the lights stay on. Refuelling is not a pit stop you take while the machine works: the Westinghouse manual states that it is “not safe to add gasoline to the fuel tank… while the engine is running or the engine and muffler are hot”, because fuel meeting a hot surface can ignite.
Every tank therefore ends in a shutdown and a cooling wait. Over a multi-day outage an 11 hour run time becomes roughly two stops a day, power off at each one. If uninterrupted is what you need, that argues for a battery in the loop rather than a bigger tank.
How we read this
Every figure above is either printed by the manufacturer at a stated load or derived from a published run time and tank size, labelled as ours where that happened. Prices carry their dates because an undated cost per hour is worthless. No generator was tested and none is recommended. Find your real load in the appliance wattage chart, remember that starting watts size the generator while running watts feed it, then read the column that matches.
Fuel and exhaust, before anything else
The generator runs outside. Its manual puts it plainly: never operate it in an enclosed area, only outside and away from windows, doors and vents, and at least 15 feet from any building or combustible material. Never in a garage, even with the door open. Carbon monoxide from a portable generator kills quickly and without warning, so fit a battery-operated CO alarm indoors.
Stop the engine and let it cool before refuelling, and store fuel in approved containers away from the machine. Never run a generator into a wall socket to feed the house: that is backfeeding and it can kill a utility worker. Connecting one to a building is a job for a qualified electrician working to local code, which is the manufacturer’s own instruction and not just ours.
Frequently asked questions
How much gas does a generator use per hour?
For a mid-size portable at half load, around 0.7 gallons an hour, derived from a published 11 hour run time on a 7.7 gallon tank. Expect appreciably more at full load.
Does running at half load use half the fuel?
No. On the published standby figures, half load uses 56 to 66 per cent of the full load burn, not 50. The engine pays a fixed cost before it delivers a single watt.
How long will a generator run on 5 gallons of gas?
At 0.70 gal/h, about seven hours at half load on a 6,000 W class machine, assuming a steady load.
Is propane cheaper to run than gasoline?
Per gallon yes, but a gallon of propane carries about three quarters of the energy and dual fuel machines derate on it — 6,250 W falls to 5,600 W above. Compare cost per hour at the same load, never run time against run time.
Why does mine use more fuel than the spec says?
Usually because your load is higher than the specification’s, or it swings. A well pump cycling or a fridge compressor cutting in pushes the engine up its fuel curve. Altitude, temperature and a dirty air filter move it too.
The practical version is short. Work out the load first, read generator fuel consumption per hour at that load rather than a headline figure, and treat any number offered without a load as one somebody invented a load for. If you are sizing rather than budgeting, start from the appliances you want to keep running and come back to the fuel column.
This article is general information, not an installation guide and not a substitute for a licensed electrician. Electrical work on a service panel, a transfer switch or a utility connection is regulated in most jurisdictions and normally requires a permit and an inspection. Fuel-burning equipment produces carbon monoxide and must never be run indoors, in a garage or near an opening into the home.
