Generator Runtime: What Actually Determines It
Three factors control how long your generator runs: fuel tank capacity, load percentage (how much of rated wattage you're actually using), and engine efficiency. Understanding these lets you plan exactly how much fuel to stock.
Fuel Consumption by Generator Size
| Generator Size | At 25% Load | At 50% Load | At 75% Load |
|---|---|---|---|
| 3,500W | ~0.9 L/hr | ~1.3 L/hr | ~1.8 L/hr |
| 6,500W | ~1.5 L/hr | ~2.4 L/hr | ~3.2 L/hr |
| 10,000W | ~2.5 L/hr | ~3.8 L/hr | ~5.1 L/hr |
| 13,000W | ~3.2 L/hr | ~4.9 L/hr | ~6.5 L/hr |
Figures approximate — vary by engine make, altitude and temperature
Working Out Your Runtime
Runtime (hours) = Tank capacity (litres) ÷ Consumption (L/hr)
Example: A 26-litre tank on a 10,000W generator at 50% load: 26 ÷ 3.8 = 6.8 hours per tank.
Propane Runtime
Propane has lower energy density than petrol. Expect roughly 10–15% more consumption (litres equivalent) to produce the same output. However, propane stores indefinitely — ideal for emergency preparedness when you can stockpile large tanks.
How to Extend Runtime
- Run at 50–60% load rather than 75–90% — efficiency improves significantly
- Switch off loads when not needed (don't run AC overnight if it's cool)
- Add a larger auxiliary fuel tank if available for your model
- Use an inverter generator — they throttle down at low load, saving 20–40% fuel
FAQ
Can I leave a generator running overnight?
Yes, if it's correctly positioned outdoors with CO detector inside. Some generators have a fuel cutoff when the tank empties, so running overnight on a smaller tank means a cold restart in the morning. Plan fuel accordingly.
What is the most fuel-efficient generator type?
Inverter generators are 20–40% more fuel-efficient than conventional open-frame generators at partial load. For long outages, the fuel savings can justify the higher purchase price.
