Generator Fuel Consumption Calculator

How it is calculated

Inputs: Full-load consumption is the fuel the generator burns per hour at 100 % load (litres per hour, from the data plate or manual), load is the average load in percent, and hours is the run time.

fuel per hour = full_lph × (0.25 + 0.75 × load ÷ 100)
total fuel = fuel per hour × hours

The 0.25 term is a built-in assumption that a running engine burns about a quarter of its full-load fuel at idle; above that the burn is assumed to rise in a straight line. Real engines do not follow this exactly, so treat the answer as an estimate. Full-load rate and hours must be above zero. Load may be 0 (pure idle burn, 25 % of the full-load rate) up to 100; a load above 100 or below 0 gives no result. Results are shown to two decimals, labelled in litres.

Common mistakes

• Entering the generator's rated power in kW instead of its full-load fuel use in litres per hour. The first field is a fuel rate, not output.
• Typing the load as a fraction (0.5) instead of a percentage (50), which is read as a load of half a percent and gives almost the idle burn.
• Using the peak load (the moment the biggest appliance starts) rather than the average load over the run. That overstates the fuel.
• Entering the per-hour rate as a per-day or per-tank figure; the field must be litres per hour.
• Expecting half the load to use half the fuel. With the idle allowance, 50 % load gives 62.5 % of the full-load burn, and even 0 % load still burns 25 %.