How Much Fuel Does a Generator Use? Petrol & Diesel Running Costs Explained

Fuel consumption is an important consideration when choosing a generator, particularly if you expect to run it regularly or for long periods.

However, there is no single figure that applies to every generator.

The amount of fuel a generator uses depends on several factors, including its engine size, rated output, the electrical load being placed on it and whether it uses petrol or diesel.

Understanding these factors can help you choose a generator that provides the power you need without unnecessarily increasing your running costs.

What Affects Generator Fuel Consumption?

Several things influence how much fuel a generator will use.

The biggest factors include:

• Generator size
• Engine capacity
• Petrol or diesel fuel
• Electrical load
• Engine efficiency
• Inverter or conventional design
• Maintenance condition
• Running time
• Ambient conditions

Generally, a larger generator will use more fuel than a smaller generator.

However, the amount of electricity being demanded from the generator is also extremely important.

A generator running at 25% of its capacity will normally use considerably less fuel than the same generator operating close to full load.

Does a Generator Use the Same Amount of Fuel All the Time?

No.

Generator fuel consumption normally changes depending on the load being placed on the engine.

For example, a generator supplying a few lights and small appliances will usually consume less fuel than when powering heavy machinery, heating equipment or multiple power tools.

This is why manufacturer fuel-consumption figures are often quoted at specific loads, such as:

• 25% load
• 50% load
• 75% load
• 100% load

When comparing generators, make sure you are comparing consumption figures at similar load levels.

How Much Petrol Does a Generator Use?

Petrol generator fuel consumption varies significantly depending on the generator's power output and engine design.

A small portable generator used for camping, leisure or occasional backup power may consume relatively little fuel.

Larger petrol generators capable of supplying several kilowatts will naturally use considerably more.

Rather than looking only at the size of the fuel tank, check the manufacturer's estimated runtime at a stated load.

For example, if a generator has a 10-litre fuel tank and is rated to run for approximately 8 hours at 50% load, its average consumption under those conditions would be around:

10 litres ÷ 8 hours = approximately 1.25 litres per hour

This is only an example. Actual fuel consumption depends on the individual generator and operating conditions.

If you are looking for this type of generator, browse our range of petrol generators.

How Much Diesel Does a Generator Use?

Diesel generators are commonly chosen for commercial, industrial and longer-duration applications.

One reason is that diesel engines can offer good fuel efficiency, particularly when operating for extended periods.

As with petrol generators, diesel consumption depends heavily on generator size and load.

A large three-phase diesel generator supplying commercial equipment will use substantially more fuel per hour than a small portable diesel generator.

When comparing diesel generators, look at the manufacturer's fuel-consumption figures at the expected working load rather than simply comparing engine size.

You can browse our range of diesel generators for commercial, industrial and backup power applications.

Petrol vs Diesel Generator Running Costs

Choosing between petrol and diesel is not simply a question of which fuel costs less per litre.

You should also consider:

• purchase price
• expected running hours
• fuel consumption
• servicing
• maintenance
• intended application
• generator size
• availability of fuel

Petrol generators are often attractive for portable, occasional and lighter-duty applications.

Diesel generators are commonly selected for heavier commercial use, longer runtimes and applications where the generator may operate regularly.

You can also read our Petrol vs Diesel Generators guide in the Power Generator UK Knowledge Centre.

Does Generator Size Affect Fuel Consumption?

Yes.

Choosing a generator that is significantly larger than required can result in unnecessary purchase and running costs.

However, choosing a generator that is too small can also cause problems.

If the generator regularly operates very close to maximum capacity, the engine may be working harder and you will have little spare capacity for startup loads or additional equipment.

The aim is to choose a generator with enough capacity to comfortably handle your normal load while allowing reasonable headroom.

Our What Size Generator Do I Need? guide explains generator sizing in more detail.

Why Generator Load Matters

Generator load is the proportion of the generator's available output that is currently being used.

For example, if you have a 6kW generator supplying a 3kW electrical load, it is operating at approximately 50% of its rated output.

The same generator supplying close to 6kW would be operating near full load.

As load increases, fuel consumption will generally increase as the engine needs to produce more power.

This means the cheapest generator to buy is not necessarily the cheapest generator to operate.

Correct sizing is important.

Are Inverter Generators More Fuel Efficient?

Inverter generators can be particularly efficient at lower loads because many models can automatically adjust engine speed according to the amount of power being demanded.

A conventional generator may need to maintain a more constant engine speed in order to produce the required frequency.

An inverter generator can often reduce engine speed when demand is low.

This may provide several benefits:

• reduced fuel consumption
• lower noise
• reduced engine wear
• longer runtime from a tank of fuel

This is one reason inverter generators are particularly popular for camping, caravans, leisure applications, mobile businesses and backup power.

Browse our range of inverter generators, or read our Inverter vs Conventional Generators guide for a more detailed comparison.

What About Silent Generators?

Noise can be just as important as fuel consumption, particularly for residential, commercial or event applications.

Silent generators are designed with enclosed canopies and noise-reduction features to help lower operating noise.

They can be particularly useful where a generator needs to run for extended periods in environments where noise levels need to be kept under control.

Fuel efficiency still varies by model, engine size and load, so always compare runtime and consumption figures alongside the stated noise level.

Are Open-Frame Generators More Economical?

Open-frame generators are often chosen for construction, trade and industrial environments because of their simple, robust design.

They may offer excellent value where noise is less of a concern.

As with any generator, fuel consumption depends mainly on engine size and the load being applied.

An open-frame generator running efficiently at a sensible load can be a very practical choice for site work and trade applications.

Three-Phase Generator Fuel Consumption

Three-phase generators are typically used for commercial and industrial equipment requiring three-phase power.

Because these machines are often larger and designed to supply substantial loads, their fuel consumption can be higher than smaller portable units.

However, the same basic rule applies: fuel use depends on the amount of power being drawn from the generator.

A correctly sized three-phase generator operating within a sensible load range can be considerably more efficient than an oversized unit running lightly loaded.

How Can I Reduce Generator Fuel Consumption?

There are several practical ways to improve generator efficiency.

Choose the Correct Size

Avoid buying a generator that is unnecessarily large for the intended application.

Calculate your expected electrical load before choosing a generator.

Avoid Unnecessary Loads

Switch off equipment that is not being used.

Every additional electrical load requires the generator to produce more power.

Maintain the Generator Properly

A poorly maintained engine may not operate as efficiently as one that is correctly serviced.

Follow the manufacturer's recommendations for:

• engine oil
• air filters
• spark plugs where applicable
• fuel filters
• servicing intervals

Use Fresh, Suitable Fuel

Always use the fuel type recommended by the manufacturer.

Old or contaminated fuel can cause poor running and starting problems.

Consider an Inverter Generator

If your power requirement frequently varies, an inverter generator may offer better efficiency at lighter loads.

How Long Will a Generator Run on One Tank?

Generator runtime depends on:

• fuel tank capacity
• fuel consumption
• electrical load
• engine efficiency

A generator may have a large fuel tank but still have a shorter runtime if it has a powerful engine operating under heavy load.

This is why tank capacity should not be considered in isolation.

When comparing products, check the stated runtime and the load at which that runtime was measured.

For example:

12-hour runtime at 25% load

is very different from:

12-hour runtime at 75% load

Always check the conditions behind the manufacturer's figures.

Does Running a Generator at Full Load Use More Fuel?

Yes.

A generator operating close to its maximum rated output will generally consume more fuel than the same generator operating under a lighter load.

Regularly operating at the generator's absolute maximum may also leave little capacity for temporary surges in demand.

Some electrical equipment requires significantly more power during startup than during normal running.

Motors, compressors, pumps and certain power tools are common examples.

This startup demand should be considered when selecting generator capacity.

What Is the Most Economical Generator for Home Backup?

The best generator for home backup depends on what you need to power.

If the generator is only required to operate:

• lights
• internet equipment
• refrigerators
• chargers
• televisions
• small appliances

a relatively modest generator may be sufficient.

If you intend to operate larger equipment such as pumps, heating systems or multiple high-power appliances, considerably more generator capacity may be required.

Our Best Generators for Home Backup UK guide covers this in more detail.

What About Generators for Site Work?

Construction and site generators often need to power equipment with demanding startup loads.

Typical examples include:

• power tools
• compressors
• mixers
• pumps
• site lighting
• cutting equipment

Fuel consumption is therefore only one part of choosing a site generator.

Reliability, output, sockets, engine design, portability and durability should all be considered.

For demanding trade and site applications, our open-frame generator range may be particularly suitable.

See our Best Generators for Site Work guide for more information.

Generator Fuel Consumption and Long Running Times

If a generator will be used for many hours each day, fuel economy becomes increasingly important.

A relatively small difference in litres consumed per hour can become significant over weeks or months of use.

For regular commercial operation, it can therefore be worth comparing:

• fuel consumption at 50% load
• fuel consumption at 75% load
• tank capacity
• expected runtime
• servicing requirements
• fuel type
• purchase price

The cheapest generator initially may not always provide the lowest overall operating cost.

Can a Portable Power Station Be Cheaper to Run?

For smaller electrical loads, a traditional fuel-powered generator may not always be necessary.

Portable power stations store electricity in a battery rather than generating it with a petrol or diesel engine.

They can be useful for:

• charging phones and laptops
• powering small appliances
• camping
• mobile work
• backup power for low-demand equipment

They produce no exhaust gases during use and do not require petrol or diesel while operating.

However, they are not a direct replacement for a generator in every application, particularly where high power or extended runtime is required.

Don't Ignore Generator Quality

Fuel economy should not be the only consideration.

A generator also needs to provide reliable and stable power for the equipment it is supplying.

Important factors include:

• rated output
• maximum output
• voltage regulation
• inverter technology where applicable
• engine quality
• warranty
• spare parts availability
• after-sales support

Buying an appropriately sized, well-supported generator is often more important than simply choosing the model with the lowest claimed fuel consumption.

Excel Power Generators

We stock a growing range of Excel Power generators, covering portable petrol and inverter models for domestic, leisure, trade and backup applications.

Excel Power can be a strong choice where value, portability and straightforward operation are important.

As always, compare the rated output, runtime and expected load to make sure the generator is suitable for the intended application.

Pramac Generators

For professional, commercial and industrial applications, we also supply a wide range of Pramac generators.

Pramac produces petrol, diesel and inverter generator solutions across a broad range of outputs.

These generators can be particularly suitable where reliability, professional build quality and longer-term use are key considerations.

Choosing the Right Generator

Before purchasing a generator, consider:

• what equipment you need to power
• total running wattage
• startup wattage
• expected operating time
• petrol or diesel
• portability requirements
• noise levels
• fuel tank capacity
• expected fuel consumption
• whether clean inverter power is required

Power Generator UK supplies generators for a wide range of domestic, commercial, leisure and industrial applications.

Browse our ranges of petrol generators, diesel generators, inverter generators, silent generators, open-frame generators, three-phase generators and portable power stations.

We also stock ranges from brands including Excel Power and Pramac.

Need Help Choosing a Generator?

If you are unsure which generator is most suitable for your application, the Power Generator UK team can help.

Tell us what equipment you need to power and we can help you work out the appropriate generator capacity and type.

Browse the full range at power-generator.uk or contact our team for advice.