The exact generator fuel table is always the best source. When it is unavailable, this calculator provides a planning estimate from rated output, average load and a generalized fuel curve.
Choose between an automatic estimate and an exact hourly rate
Which calculation method should you use?
| Method | Inputs | Best use |
|---|---|---|
| Estimate from generator size and load | Rated kW, average load percentage, fuel and operating time | Early fuel planning and comparing load scenarios |
| Manufacturer or measured rate | Hourly fuel use at the relevant operating load | Fuel purchasing, tank runtime and refuelling schedules |
The automatic mode is deliberately transparent. It does not pretend that every 7.5 kW gasoline generator or every 22 kW standby generator consumes the same amount. The result includes a planning range, editable adjustment and a warning to verify the exact model.
Fuel use does not fall in direct proportion to electrical load
A generator still uses fuel to keep the engine turning, operate cooling systems and cover mechanical and electrical losses. At 25% load it normally consumes more than 25% of its full-load fuel rate. This is why an oversized generator can be expensive during long light-load operation.
What to expect as load changes
| Average load | Typical planning implication | Check |
|---|---|---|
| Below 25% | Poorer fuel use per delivered kWh is common | Smaller or inverter generator, load scheduling and manufacturer minimum-load guidance |
| Around 50% | Common comparison point in portable-generator runtime claims | Tank size and whether the rating applies to gasoline, diesel or LPG |
| 75–85% | Often a productive operating range when conditions are suitable | Continuous rating, ambient temperature and starting-load reserve |
| Above 85% | Little remaining output headroom | Motor starts, altitude derating, cooling and maximum continuous rating |
US customary and metric modes cover the four target markets
The English calculator opens in US customary mode with US gallons and pounds. Switch to metric for litres and kilograms, which better matches common fuel purchasing in Canada, the United Kingdom and Australia. Power remains in kW because generator ratings use it across all four markets.
Gasoline and petrol are the same calculator fuel option
Gasoline is the usual US and Canadian term; petrol is standard wording in the UK and Australia. The option includes both terms so the calculation works naturally for portable generators in every target market. Diesel results use the same volume units but a separate fuel curve.
Propane and LPG may be sold by weight or liquid volume
A US portable-generator cylinder is often described as 20 lb, while comparable LPG cylinders are commonly described in kilograms elsewhere. Enter the fuel-content weight, excluding the metal cylinder. For a bulk tank, switch to liquid volume and enter litres or US gallons.
What should be entered for a standard US cylinder labelled for 20 lb of propane?
Answer: Enter 20 lb when that is the stated propane capacity. Do not enter the combined filled-cylinder weight shown on a scale.
Explanation: The calculator converts the propane content to an estimated liquid volume. Including tare weight would substantially overstate the available fuel and runtime.
Cylinder runtime is not controlled by fuel quantity alone. Cold weather and high withdrawal rates can reduce vaporization, so a cylinder may fail to support the generator's demand while liquid propane remains inside.
Natural gas billing differs across countries and utilities
Supported natural gas price units
| Billing unit | Where it may appear | Calculator treatment |
|---|---|---|
| m³ | Canada and many metric markets | Direct volume conversion |
| CCF or 100 ft³ | Some North American gas bills | Cubic metres are converted to hundreds of cubic feet |
| Therm | United States gas tariffs | Volume is converted using the entered energy content |
| kWh | United Kingdom gas bills | Uses editable kWh per m³ |
| MJ | Australian gas tariffs | Converts gas energy from kWh to megajoules |
Gas composition and billing correction factors vary. Use the calorific value or energy conversion shown on your bill when cost accuracy matters.
Tank runtime is a fuel-quantity result, not a refuelling instruction
- Calculate or enter the hourly fuel rate.
- Reduce the entered tank or cylinder quantity by the usable-fuel percentage.
- Divide usable fuel by hourly consumption.
- Compare the result with planned operating time and schedule a safe shutdown before refuelling.
Fuel price gives operating cost, not the complete cost of backup power
- Fuel cost per hour and for the selected operating period are included when a price is entered.
- Generator purchase, rental, depreciation, delivery and transfer equipment are not included.
- Oil, filters, spark plugs, servicing and repairs are separate.
- Cylinder exchange fees, deposits and minimum refill charges are separate.
- Fixed utility charges are not added to natural gas usage cost.
Use the detailed table before buying fuel
Automatic mode compares estimated consumption at 25%, 50%, 75% and 100% load for the same generator. Manual mode instead shows the fuel and cost for 1, 8, 12 and 24 hours. These comparisons make an incorrect load assumption easier to notice.
Generator safety is part of every fuel plan
Operate a portable generator outdoors only, well away from doors, windows and vents. Never run it in a garage, basement, shed or enclosed porch. Allow the unit to stop and cool before refuelling, and use approved transfer equipment to prevent backfeeding the utility supply.
Technical references for checking the estimate
The generalized curves were checked against published examples from portable gasoline and dual-fuel generators, residential gaseous standby units and industrial diesel generator sets. Product data varies materially, which is why the manual-rate mode and planning range remain central to the calculator.
Generac dual-fuel portable generator specification: gasoline and propane runtime at 50% loadCaterpillar diesel generator data: fuel consumption at 25%, 50%, 75% and 100% loadCalculate the generator output needed for your appliances before estimating fuel useRead how HomDera handles assumptions, editable inputs, rounding and calculator limitations.