Diesel Generator Fuel Consumption Guide

Why a generator burns more fuel as electrical load increases.

A diesel generator does not use the same amount of fuel at every load. As the connected electrical load increases, the alternator must supply more electrical power and current. The engine must then produce more mechanical power to drive the alternator, which requires more fuel.

1. Generator load drives fuel demand

At a light electrical load the engine is required to produce less power. As more equipment is connected and current demand increases, the generator engine must work harder to maintain the required voltage and frequency. Fuel consumption therefore rises as generator load rises.

2. What 25%, 50%, 75% and 100% load mean

Fuel-consumption figures are commonly shown at several load points so expected diesel use can be compared at different operating conditions. A 100 kVA generator at 25% load is carrying approximately one quarter of its rated load, while at 100% it is operating at its full applicable rating. GenTech generator specification pages show 25%, 50%, 75% and 100% fuel-consumption figures where confirmed data is available.

3. Fuel consumption is not a fixed litres-per-hour figure

The litres per hour shown for a generator should always be read together with the load percentage. Quoting only one consumption number can be misleading because a generator supplying a small load will not normally consume the same amount of fuel as the same set operating near full load.

4. Correct generator sizing matters

Selecting the correct generator is important for more than maximum capacity. The expected running load, starting loads and operating duty should be considered so the generator is appropriately matched to the application. An excessively oversized generator may spend much of its operating time at a low percentage of its available capacity.

5. Fuel-tank capacity and expected run time

Fuel-tank capacity and fuel consumption can be used together when planning approximate operating duration between refuelling. Actual run time will depend on the real site load and operating conditions, so calculations should use the expected load profile rather than assuming the generator will remain at one load continuously.

6. Why actual fuel use can differ

Published or specified consumption figures provide a useful planning reference, but actual diesel use can vary with engine model, generator condition, ambient conditions, altitude, load changes and the type of equipment being supplied.

7. Compare consumption at the load you expect to run

When comparing generator options, look at the consumption figure closest to the site's expected operating load. GenTech SA can assist with generator sizing and selection where the load profile is not yet known.