Generator runtime formula
Generator runtime = fuel available ÷ fuel consumption per hour
Actual load percentage = actual load ÷ rated running output × 100
For example, 5 gallons divided by 0.5 gal/hour equals 10 hours. Quantity and rate units must match.
How electrical load affects runtime
A generator's rated power is its rated electrical capability. Actual load is what connected equipment is drawing. A 5kW generator supplying 2kW is at 40% load. Higher load generally increases fuel consumption and reduces runtime on the same fuel supply, but the relationship is generator-specific and not necessarily linear.
In load-curve mode, the calculator uses linear interpolation only between adjacent points you enter. It does not invent a universal consumption curve or silently extrapolate beyond the available data.
Gasoline, diesel, propane and custom fuels
The same runtime formula works with gasoline, diesel, propane or another fuel when quantity and consumption use compatible units: gallons and gal/hour, liters and L/hour, or pounds and lb/hour. Selecting a fuel label never changes the rate behind the scenes.
For propane, a tank described as 20 lb does not necessarily contain exactly 20 lb of usable fuel at the time of calculation. Enter the amount actually available and use a matching manufacturer rate.
Fuel runtime is not a continuous-operation limit
This calculator answers how long the available fuel may last. It cannot determine how long a specific generator can safely run without inspection, cooling or maintenance. Do not apply a universal 8-, 12- or 24-hour limit; consult the owner manual and manufacturer documentation for the exact model.
Worked generator runtime examples
Generator runtime calculator FAQ
How long will a generator run on 5 gallons of gas?
Divide 5 gallons by the generator fuel-consumption rate in gallons per hour. At 0.5 gal/hour, the fuel-limited runtime is 10 hours. Use the rate from the specific generator manual for a better estimate.
How do you calculate generator runtime?
Divide usable fuel available by fuel consumption per hour. Quantity and rate units must match, such as gallons with gal/hour or liters with L/hour.
How does generator load affect runtime?
Higher electrical load generally increases fuel consumption and reduces runtime, but the relationship depends on the generator. Manufacturer consumption data at several load levels gives a better planning estimate.
How long can a generator run continuously?
Fuel runtime does not establish a safe continuous operating limit. Cooling, inspection, oil and maintenance requirements are model-specific, so follow the owner manual and manufacturer documentation.
How long will a generator run on propane?
Divide usable propane quantity by the generator propane-consumption rate using matching units, such as pounds with lb/hour. Tank label capacity may not equal usable fuel, so verify the available amount.
Does a bigger generator use more fuel?
Generator size alone does not determine fuel use. Engine design, fuel, inverter or conventional architecture, actual load, condition and operating environment all affect consumption.
What does 50% generator load mean?
A 50% load means connected equipment is drawing half of the generator rated running output. A 5kW generator supplying 2.5kW is operating at 50% load.
Where can I find my generator fuel-consumption rate?
Check the owner manual, manufacturer specification sheet or official load-versus-consumption chart. Use the value for the correct fuel and the load closest to your planned electrical demand.