Generator Fuel Consumption & Runtime
Estimate generator fuel consumption and runtime from rated wattage, load percentage, fuel type, and tank size.
Covers gasoline, propane, and diesel generators.
Generator runtime depends on the fuel type, the actual load (not the rated maximum), and the tank size. A 5,000-watt gasoline generator at 50% load typically burns 0.5-0.6 gallons per hour, giving about 8-10 hours on a 5-gallon tank.
Fuel consumption at load
Generators are much less efficient at partial load than the nameplate suggests. The published fuel consumption rate is usually measured at full rated load. Real-world usage at 25-50% load may actually be more efficient per kWh produced, but the generator still consumes meaningful fuel to maintain engine speed and idle losses.
Empirical consumption rates (approximate, at stated load level):
- Gasoline: 0.20 gallons per hour per kW of actual load
- Diesel: 0.15 gallons per hour per kW of actual load (more efficient fuel)
- Propane: 0.25 gallons per hour per kW of actual load (lower energy density per gallon)
Actual kW = rated_kW x (load_percent / 100)
Fuel per hour = actual_kW x fuel_rate_constant
Runtime (hours) = tank_size / fuel_per_hour
Propane note
Propane tanks are sold by weight in pounds. One pound of propane = 0.236 gallons of liquid propane. A standard 20 lb grill tank holds about 4.7 gallons. A 100 lb tank holds about 23.6 gallons. Enter the gallon equivalent in the tank size field.
Altitude effects
At 5,000 feet elevation, naturally aspirated gasoline engines produce about 10% less power. Some generator manufacturers derate output by 3.5% per 1,000 feet above 1,000 feet. Turbocharged diesel generators are less affected.
Load management
To stretch runtime: prioritize critical loads (refrigerator, medical equipment, a few lights) and avoid high-draw appliances (electric dryer, 240V AC unit). A 5kW generator can run a 1,500W fridge, 500W of lighting, a TV, and phone chargers on about 2.5 kW total.