How it's estimated
Fuel rates at full load are typical values: about 0.5 L/kWh for petrol, 0.33 L/kWh for diesel and 0.4 kg/kWh for propane. The (0.25 + 0.75 × load) term reflects the fact that a generator still burns about a quarter of its full-load fuel just turning over. Inverter generators in eco mode do better than this at light loads; open-frame builders' generators often do worse.
Worked example
A 3 kW petrol generator running a 1 kW load: 3 × 0.5 × (0.25 + 0.25) = 0.75 L/h. A 10 L tank lasts about 13 hours. Check the manufacturer's quoted run time at 25% and 50% load for your model.
Charging batteries with a generator
Battery charging is a good use of generator time because the charger can load the generator well. A bigger charger finishing in 2 hours burns less fuel than a small charger running for 6.
Frequently asked questions
How much fuel does a 2000W generator use per hour?
Typically 0.3–1.0 L (0.1–0.25 US gal) per hour, depending on load. Most quote 8–10 hours on about 4 L at 25% load in eco mode.
Does a generator use less fuel at lower load?
Yes, but not proportionally. At quarter load it uses roughly 40–50% of its full-load fuel.