At a glance: diesel generator fuel consumption
- Generator fuel consumption depends on the generator, the load and how long you run it. There isn't one litres-per-hour figure that applies to every diesel generator.
- A generator under a lighter load will generally use less fuel than the same generator working closer to its full capacity.
- Generator size matters too. A larger engine and higher output can mean greater fuel use, particularly when more power is being demanded.
- Check the manufacturer's fuel capacity and runtime figures when estimating how much diesel you'll need.
- Don't size a generator much larger than necessary just for extra capacity. Start with the electrical load you actually need to power.
- Petrol and dual-fuel generators are worth comparing for occasional or portable use, particularly if you don't need the heavier-duty capability of diesel.
- Fuel consumption is only part of the decision. Power requirements, runtime, portability and how often you'll use the generator matter too.
Generator fuel consumption can make a noticeable difference to running costs, especially if you’ll be using your generator for long periods. So how much diesel should you expect to use?
There isn’t a single figure that works for every generator. Fuel use changes with the size and efficiency of the engine, the electrical load you’re putting on it and how long it runs.
A generator supplying a relatively light load won’t use fuel in the same way as one working much closer to its rated output. That’s why the specifications for the generator you’re considering are more useful than a generic litres-per-hour estimate.
How do you work out diesel generator fuel consumption?
A useful starting point is to check two figures in the generator specifications:
- Fuel tank capacity: how much diesel the tank holds.
- Runtime at a stated load: how long the generator can run under the test conditions given by the manufacturer.
You can use those figures to get an approximate hourly fuel-use figure:
Fuel tank capacity ÷ runtime = approximate fuel consumption per hour
For example, one of our 6.25 kVA diesel generators has a 16-litre fuel tank and a stated runtime of up to 12 hours at 50% load. Dividing 16 by 12 gives an approximate figure of 1.33 litres per hour under those conditions.
Treat that as a model-specific example, not a rule for all diesel generators. Actual fuel use can change with operating conditions and load.
What affects generator fuel consumption?
The amount of fuel a generator uses isn't fixed. Several things can change it during normal operation.
Electrical load
Load is one of the biggest factors. The more power you're asking the generator to produce, the harder the engine needs to work.
If you’re only using part of the generator’s available capacity, its fuel demand will generally be lower than when it’s working under a much heavier load.
Generator size
A generator needs to be large enough to handle your equipment, including any additional starting demand. But bigger isn't automatically better.
Choosing considerably more capacity than you need can leave you with a generator that's poorly matched to the job. Work out your generator size from the equipment you'll actually run rather than choosing the highest output available.
How long you run it
Even a relatively small difference in hourly fuel consumption becomes more noticeable when a generator runs for long periods.
Think about your likely runtime when budgeting for fuel. Occasional backup power and several hours of regular use will lead to very different overall fuel requirements.
The generator itself
Engine design, output and specification vary between generators, so two diesel models won’t necessarily use fuel at the same rate under similar-looking conditions.
Wherever possible, use the manufacturer’s figures for the specific model you’re considering rather than relying on a generic diesel consumption chart.
Does a generator use less fuel at half load?
Generally, a generator uses less fuel at a lower load than it does when producing its full rated output. However, fuel consumption doesn’t necessarily fall in direct proportion to the electrical load.
That’s why manufacturer specifications often give fuel consumption or runtime at specific load levels such as 25%, 50%, 75% or 100%. When you compare models, make sure the figures have been measured under similar conditions.
The same principle applies to petrol and dual-fuel generators. Fuel consumption will usually increase as the electrical load rises.
How does diesel compare with petrol and dual-fuel?
Fuel consumption alone shouldn’t decide which generator you buy. The right fuel type depends on where, how often and how heavily you expect to use it.
For portable power, home backup, outdoor use and many general-purpose jobs, a petrol generator may be the more practical choice. Our petrol models cover a wide range of outputs, so diesel isn’t automatically necessary simply because you need a capable generator.
A dual-fuel generator gives you another option by allowing you to use petrol or LPG.
Fuel consumption is measured differently between the two fuels. Petrol is typically shown in litres per hour, while LPG may be shown in kilograms per hour, so don’t compare the numbers directly without accounting for the different units. Fuel use will also change depending on the load.
Diesel generators tend to make more sense where your requirements involve sustained loads or heavier-duty use. If that’s how you’ll use your generator, fuel consumption and runtime become particularly important when comparing models.
How can you estimate how much generator fuel you'll need?
Once you have a reliable consumption figure for your generator, you can make a rough estimate based on your expected runtime.
Use this simple calculation:
Estimated hourly consumption × expected running hours = estimated fuel required
If a generator uses approximately 1.3 litres per hour under the conditions you expect to run it in, five hours of use would require roughly 6.5 litres.
Remember that this is an estimate. Your electrical load may change while the generator is running, so real-world fuel use can vary.
How can you reduce generator fuel use?
The aim isn't simply to use as little fuel as possible. It's to run a generator that's properly matched to the job.
A few sensible steps can help:
- Choose a generator with an appropriate output for your electrical load.
- Avoid running equipment you don’t need.
- Keep the generator properly maintained.
- Follow the manufacturer’s operating guidance.
- Plan which equipment needs to run at the same time.
- Compare fuel consumption at realistic load levels when choosing between models.
If your power needs are occasional or relatively light, it’s also worth asking whether diesel is the right fuel type in the first place. Petrol or dual-fuel may be a better fit depending on where and how you’ll use the generator.
What should you check before choosing a generator?
Fuel consumption matters, but it shouldn’t be looked at in isolation.
Check the generator’s rated output, your running and starting power requirements, expected runtime, fuel tank capacity and consumption figures. Then consider practical factors such as portability and how often you’ll use it.
If you’re comparing generators and aren’t sure which option makes sense for your power requirements and expected running time, our chat system can help you narrow down the options and choose a generator that’s right for the job.
