Use a real reference
Enter the manufacturer’s fuel rate or derive it from a published “hours per tank” figure at a stated load.
Fuel + load planning
Generator Fuel Runtime Calculator
Estimate gasoline, diesel or propane runtime from usable fuel, generator load and a manufacturer fuel-rate reference—while checking running watts and startup surge.
Load-dependent fuel curveSurge validationFuel reserveUncertainty range
Why generator runtime changes with load
A generator burns some fuel simply to keep the engine running, then burns more as electrical load rises. That means fuel use is not perfectly proportional to watts. This calculator anchors the estimate to a real fuel-consumption figure from your generator manual and adjusts it with an editable no-load fuel fraction.
Check both watt ratings
Continuous watts must fit the running rating, while motor starting demand must fit the surge rating.
Keep a reserve
Do not plan around using every drop. A reserve covers measurement error and changing loads.
The load-adjustment formula
Target fuel rate = reference fuel rate × [base fraction + (1 − base fraction) × target load] ÷ [base fraction + (1 − base fraction) × reference load]
Runtime is usable fuel divided by the adjusted fuel rate. The result also includes a ±15% consumption range because temperature, altitude, maintenance, fuel quality and transient loads affect real operation.
Worked example
| Input | Value |
|---|---|
| Generator rating | 3,500 running W / 4,375 surge W |
| Connected load | 1,750 W (50%) |
| Fuel available | 3.5 gal with 10% reserve |
| Known fuel rate | 0.35 gal/h at 50% load |
| Estimated result | 9.0 hours; planning range 7.8–10.6 hours |
How to find the reference fuel rate
- Find a runtime claim in the generator manual or specification sheet.
- Confirm the tank quantity and the load percentage used for that claim.
- Divide usable tank quantity by published runtime to get fuel units per hour.
- Enter that rate and its reference-load percentage above.
- Validate the estimate with a monitored test before depending on it.
Important generator safety
Carbon monoxide can kill. Never operate a fuel-burning generator indoors, in a garage, basement, shed or near doors, windows or vents. Follow the generator manufacturer’s placement, grounding, refueling and maintenance instructions. Allow equipment to cool before refueling.
Common questions
How do I calculate generator runtime from a tank specification?
Divide the usable fuel quantity by fuel consumption per hour. If the consumption figure is quoted at a different load, adjust it before calculating runtime.
Why not use one universal gallons-per-hour number?
Engine size, generator design, fuel type and load all change consumption. A model-specific reference is more reliable than a generic average.
Does a generator use half as much fuel at half load?
Usually not. The engine still consumes fuel while unloaded, so half electrical load often uses more than half the full-load fuel rate.
What is the difference between running and surge watts?
Running watts are sustained output. Surge watts are short-duration capacity for motor and compressor startup. Both must cover the connected equipment.
Read the full method in how much fuel does a generator use? Related tools: power station runtime calculator, home office backup calculator, and appliance wattage library.