Generator Fuel and Runtime Calculator
How long a backup generator runs on a tank, what it burns per hour at your actual load, the fuel and cost for an outage of a given length, and how much fuel to keep on hand.
A generator's fuel use depends on its load, not its rating: at half load it burns well under half its full-load figure, because a running engine has a fixed idle cost.
How the generator fuel and runtime calculator works
A generator's fuel use depends on its load, not its rating: at half load it burns well under half its full-load figure, because a running engine has a fixed idle cost. The page interpolates between the full-load and half-load consumption the maker publishes for the load you will actually draw. Tank size over that rate is the runtime; the outage hours times the rate is the fuel to stock.
Formula: L/h at load = interpolate(half-load L/h, full-load L/h); runtime = tank / L/h; outage fuel = hours × L/h
Worked examples
| Inputs | Runtime on a tank (hours) | Note |
|---|---|---|
| 5 kW on an 8 kW set | 10.53 | about ten hours a tank |
| Lightly loaded | 23.53 | long runtime, poor efficiency |
| A three-day outage | 10.53 | plenty of fuel |
FAQFrequently asked questions
How much fuel does a generator use?
A portable petrol set of 5 to 8 kW burns 2 to 4 liters an hour at full load and roughly half that at half load. Diesel sets are more efficient; the maker's sheet gives both figures.
Why does load matter so much?
Because the engine has a fixed cost to turn over even with nothing connected. Fuel use per kilowatt-hour is worst at light loads, which is also where many engines carbon up.
What load should I plan for?
Add the running watts of what you will actually run at once — fridge, freezer, lights, a few outlets — and allow for motor starting. Fifty to seventy-five per cent of the rating is the sweet spot.
How much fuel should I store?
Enough for the outage you plan for, plus a margin, in approved containers with stabiliser; petrol degrades within months. The outage row gives the quantity.
Is generator power expensive?
Compared with the grid, very — often two to four times the price per kilowatt-hour. It is insurance, not a substitute.
Where these figures come from
- International Energy Agency — Energy Efficiency — appliance consumption and standby power context
- US Energy Information Administration — the federal energy statistics agency
Last checked: September 2026. Appliance power figures are typical ratings; check the label on your own equipment, which is authoritative for it.