Runtime is never just "tank size divided by engine size." It depends heavily on load. A generator at 25% load can run much longer than the same unit near full output.
For full system planning, use the WattSizing Calculator.
Quick Answer
Use:
Runtime (hours) = Usable fuel (L or gal) / Fuel burn rate at your load
The key is using fuel burn at your actual load percentage, not a generic number.
Step-by-Step Method
- Get tank capacity from specs.
- Use 90% to 95% as usable fuel (real-world reserve and tilt effects).
- Estimate your average load (watts running / generator rated watts).
- Find fuel burn at that load from manufacturer data.
- Divide usable fuel by burn rate.
If you still need load numbers, estimate from Generator Running Watts vs Starting Watts Explained and How to Calculate kWh from Watts and Hours.
Example Runtime Table (Illustrative)
Assume a gasoline generator with:
- Tank: 15 L
- Usable fuel: 13.5 L (90%)
| Load Level | Approx Fuel Burn | Estimated Runtime |
|---|---|---|
| 25% load | 1.2 L/h | 11.3 h |
| 50% load | 1.8 L/h | 7.5 h |
| 75% load | 2.5 L/h | 5.4 h |
| 100% load | 3.2 L/h | 4.2 h |
This is why load management matters as much as tank size.
Worked Example
You have a 3.5 kW generator and expect a 1.4 kW average running load.
- Load fraction:
1.4 / 3.5 = 40% - Manufacturer fuel burn near 40% load:
1.5 L/h - Tank: 12 L, usable at 90% =
10.8 L
Runtime:
10.8 / 1.5 = 7.2 hours
Planning takeaway: if you need 12 hours overnight, either reduce load, refuel, or use a larger/dual-fuel setup.
Practical Runtime Tips
- Keep load around 40% to 70% for practical efficiency and reliability.
- Avoid repeatedly running at maximum output.
- Track real fuel use during one full tank to calibrate your numbers.
- Store fuel safely and rotate stock for emergency readiness.
FAQs
Why does my runtime differ from the box claim?
Marketing runtime is often measured at a light load. Real household loads are usually higher and more variable.
Can I use one fuel burn number for all situations?
No. Fuel burn rises with load, temperature, and engine condition, so use load-specific values.
Should I calculate runtime from starting watts?
No. Runtime is based on average running load over time. Starting watts matter for surge compatibility, not ongoing fuel burn.
Is diesel runtime usually better than gasoline?
Often yes at similar output classes, but actual runtime still depends on engine design, load, and tank size.
CTA
Need a clear runtime and load plan before the next outage? Use the WattSizing Calculator to model your real appliance loads, generator headroom, and daily energy needs.
