Generator Run Time Calculator

Fuel consumption follows the load, not the label. A generator running at forty percent of its rating burns a little over half of what it burns flat out, so dividing tank capacity by the full-load figure — which is what every other calculator does — gives a run time roughly half of the real one. This calculator applies the part-load curve, converts between fuels by energy content rather than volume, and flags the two load extremes that damage engines: running so lightly that the cylinders glaze, and running so close to the rating that nothing is left for a motor start.

Run time: 11.8 hours 8 gal of gasoline at 3,000 W, which is 43% of the generator's rating. Consumption at part load is 57% of the full-load rate, not 100%.
Fuel rate at this load: 0.68 gal/hr Against 1.19 gal/hr at full load. Estimated from the generator rating — if your manual publishes a figure, use that instead. An engine at part load still burns fuel to overcome its own friction and pumping losses, which is why consumption does not fall to zero as load does.
If you divided the tank by the full-load rate: 6.7 hours The arithmetic every other calculator does. It understates run time by 5.0 hours here — 75% — because almost nobody runs a standby generator at its full rating.
Load fraction: 43% In the range where the engine is reasonably efficient and has headroom for a motor starting surge. Half to three quarters is the comfortable band.
Fuel for a 3 day outage: 49 gal Running continuously. Cycling the generator — eight hours on, running the fridge and freezer down, then off — typically cuts this by half or more, and is how most people actually get through a long outage.
Refuelling interval: 11.8 hr Portable generators must be shut down and allowed to cool before refuelling. A tank that runs out at three in the morning is a design decision made earlier, and it is worth checking against the hours you can realistically attend to it.
Energy delivered: 35 kWh Over the full tank. At utility rates that is a few dollars of electricity; from a generator it costs many times more, which is the honest reason to run only what matters.
What this does not model: Altitude and temperature Engines lose roughly 3% of output per 1,000 feet of elevation and more in high ambient temperatures, so a generator at altitude carries less load and burns fuel less efficiently. Motor starting surges also draw far more than running load for a second or two — size those separately.

Fuel rate against load — the line does not pass through zero

Consumption is not proportional to load

An engine burns fuel to do two things: produce useful output, and overcome its own internal losses. The second part is roughly constant regardless of what the generator is powering, because the engine is turning at the same speed either way.

That gives the familiar shape in published fuel tables. A generator rated at seven kilowatts might list one and two tenths gallons an hour at full load and three quarters of a gallon at half load. Half the output, but well over half the fuel — around sixty percent.

Extrapolated down, an engine at no load still burns roughly a quarter of its full-load rate. That is the intercept of the line, and it is why generators are wasteful when lightly loaded and why the honest way to save fuel is a smaller generator rather than a lighter load on a big one.

For run time estimates the practical effect runs the other way and is welcome. Most standby use sits between a quarter and half load, where the tank lasts considerably longer than the full-load figure suggests.

Fuels are not interchangeable by volume

A gallon of gasoline carries about a hundred and fourteen thousand BTU. A gallon of propane carries around ninety-one and a half thousand — roughly eighty percent as much. A gallon of diesel carries about a hundred and thirty thousand.

So a twenty gallon propane cylinder does not run a generator as long as twenty gallons of gasoline, and the difference is not small. Sizing a propane supply by copying gasoline figures leaves you short by a fifth.

Propane wins anyway for standby use, for a reason that has nothing to do with energy density: it does not go stale. Gasoline degrades in months and gums up carburettors, which is why so many generators that were fine when stored will not start when needed. Propane stored in a sealed cylinder is good for decades.

Natural gas is different again. It arrives continuously so run time is governed by maintenance intervals rather than fuel, but its lower energy density means most generators are derated by around ten percent when running on it compared with propane.

Both ends of the load range cause damage

Running a generator far below a quarter of its rating for long periods causes real harm. Cylinder bores glaze because combustion temperatures never get high enough to seat the rings properly, and diesels wet-stack, accumulating unburnt fuel and soot in the exhaust that eventually needs cleaning out.

This is a common outcome of buying a large generator for peace of mind and then running a fridge, a few lights and a modem on it. The machine is oversized for the way it is used, and the engine suffers for it.

The opposite end is less obviously harmful but more likely to strand you. A generator loaded to ninety percent has nothing left when the well pump or the fridge compressor tries to start, and motor starting surges are several times running current for a second or two. The generator trips, and the outage becomes a manual load-shedding exercise in the dark.

Half to three quarters of the rating is the band worth designing for. It gives reasonable fuel efficiency, keeps combustion temperatures where the engine wants them, and leaves headroom for the surges.

What this is based on

Published fuel consumption tables at 50% and 100% load for portable and standby generators
Fuel energy content — gasoline 114,000 BTU/gal, propane 91,500 BTU/gal, diesel 129,500 BTU/gal, natural gas 1,030 BTU/ft³

An estimate based on a typical part-load consumption curve. Actual consumption varies with engine design, altitude, ambient temperature, and load power factor. Where the manufacturer publishes a consumption figure, use it. Never refuel a running or hot generator, and never operate one indoors or in an attached garage.

Frequently asked questions

How long will my generator run on a tank?

Longer than the full-load figure suggests, because consumption follows load. At half load a generator burns around sixty percent of its full-load rate, so a tank rated for eight hours flat out will typically give twelve or more at a realistic standby load.

Does a generator use less fuel with a smaller load?

Yes, but not proportionally. An engine at no load still burns roughly a quarter of its full-load rate to overcome its own friction and pumping losses. Halving the load saves considerably less than half the fuel.

Is propane or gasoline better for a generator?

Propane carries about twenty percent less energy per gallon, so the same volume runs shorter. It wins for standby anyway because it does not degrade — stale gasoline and gummed carburettors are the most common reason a stored generator will not start when it is needed.

Can I run a generator at very low load?

Not for long periods without consequences. Below about a quarter load, small engines glaze their cylinder bores and diesels accumulate unburnt fuel in the exhaust. If your real load is that small, a smaller generator is both cheaper to buy and healthier to run.

How much fuel do I need for a three day outage?

Running continuously, three times the daily consumption at your actual load — which this calculates. In practice most people cycle the generator, running it eight hours a day to keep the fridge and freezer cold, which cuts the requirement by half or more.