Updated
Guide
Generator Fuel Consumption Per Hour
A 100 kW (125 kVA) diesel generator typically burns about 15.5 litres an hour at half load and 28 litres an hour at full load, according to the widely used Diesel Service & Supply chart; a 20 kW set uses about 3.4 and 6.1 litres at the same loads. Your engine’s fuel data sheet gives the exact figure.
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- Litres per hour at 25 to 100% load
- The load decides the fuel
- A rise in use is a fault sign
- Onsite across Gauteng
How it is calculated
The load decides the fuel, not the kVA on the plate
A generator burns fuel for the power it delivers, not for the size printed on the canopy. On the same chart, a 100 kW set uses about 9.8 litres an hour at quarter load and 28 litres at full load, almost three times as much. That is why we ask for the load as well as the data plate before we talk about running cost.
Light loads are less efficient per unit of power. At quarter load the 100 kW set delivers 25 kW for about 9.8 litres, roughly 0.39 litres per kWh. At full load it delivers 100 kW for about 28 litres, roughly 0.28 litres per kWh. Long runs at light load also cause glazing and wet stacking in diesel engines.
Your engine’s fuel data sheet beats any general chart, and our fuel consumption calculator does the arithmetic for your rating, load and hours. A sudden rise in use on the same load points to injectors, the fuel pump, a blocked air filter or bad fuel. We diagnose that as a fault.
What changes it
What moves fuel consumption
These are the things we look at when a set starts using more fuel than it used to.
The load being carried
The biggest factor. Fuel use rises with the kW the alternator delivers, so the same machine costs different amounts to run on different days.
Running at light load
A diesel carrying very little load runs cooler than designed and uses more fuel per kWh. Long periods at light load also cause glazing and wet stacking, which is an engine problem as well as a cost problem.
Engine condition
Worn injectors, a tired fuel pump and poor compression all raise consumption. We investigate a sudden rise in fuel use as a fault rather than accept it.
Air and exhaust restriction
A blocked air filter or a restricted exhaust makes the engine work harder for the same output. We check both at every service.
Fuel quality
Contaminated or old diesel burns poorly and blocks filters. Our contaminated diesel guide explains how stored standby fuel goes bad.
Governor and speed
An engine that does not hold its set speed is not running at its designed operating point. We measure frequency and speed on the set.
Working it out
How to estimate the figure for your set
1. Read the data plate
Note the kVA, the power factor and the engine model. A 125 kVA plate at 0.8 power factor is a 100 kW set.
2. Find the real load
Read the load on the controller while the set runs. If it shows 50 kW on a 100 kW set, you are at half load.
3. Look up litres per hour
Use your engine’s fuel data at that load. Without it, the typical chart figure for 100 kW at half load is about 15.5 litres an hour.
4. Multiply by your hours
15.5 litres an hour for 10 hours is about 155 litres. Multiply by your price per litre for the cost.
5. Compare with the tank
Track litres used against running hours on the controller. A drift away from the expected figure is the signal worth acting on.
6. Keep the record
Hours and litres recorded at each service make the next comparison meaningful. Our running hours guide explains the meter.
Typical diesel generator fuel consumption in litres per hour
These are typical figures from the widely used Diesel Service & Supply approximate diesel fuel consumption chart, converted from US gallons at 3.785 litres per gallon. The kVA column assumes the typical generator power factor of 0.8. Your engine’s fuel data sheet gives the exact figure for your set.
| Generator rating | 25% load | 50% load | 75% load | 100% load |
|---|---|---|---|---|
| 20 kW (25 kVA) | 2.3 L/h | 3.4 L/h | 4.9 L/h | 6.1 L/h |
| 100 kW (125 kVA) | 9.8 L/h | 15.5 L/h | 22.0 L/h | 28.0 L/h |
| 200 kW (250 kVA) | 17.8 L/h | 29.1 L/h | 41.6 L/h | 54.5 L/h |
| 500 kW (625 kVA) | 41.6 L/h | 70.0 L/h | 99.9 L/h | 135.1 L/h |
Sources: Depco: Estimated diesel fuel consumption chart; Power Gen Enterprises: Approximate diesel fuel consumption chart. The chart describes itself as an approximation.
Worked example: a 125 kVA set is 100 kW at 0.8. At half load it uses about 15.5 litres an hour, so a 10-hour run needs about 155 litres. Multiply by the price you pay per litre for the cost. For ratings between the rows, use our generator fuel consumption calculator.
Common Questions
Generator fuel consumption: questions
How many litres per hour does a generator use?
Typically about 15.5 litres an hour for a 100 kW (125 kVA) diesel set at half load and 28 litres at full load, on the Diesel Service & Supply approximate chart. A 20 kW set uses about 3.4 litres at half load. Your engine’s fuel data sheet gives the exact figure.
How much diesel does a 500 kW generator use?
About 70 litres an hour at half load and 135 litres an hour at full load, according to the same approximate chart. At a power factor of 0.8, that set is rated 625 kVA.
Does a bigger generator always use more fuel?
Not for the same load, but a large set carrying a small load uses more fuel per kWh, and long periods at light load harm a diesel. We match the set to the load, as our generator sizes guide explains.
Why is my generator suddenly using more fuel?
On an unchanged load, a rise usually points to injectors, the fuel pump, compression, a blocked air filter, a restricted exhaust or poor fuel. We diagnose the cause on the set.
Is running a diesel generator at low load a problem?
Yes, over time. A diesel held at very light load runs cooler than designed, which leads to glazing and wet stacking. A load bank test exercises the set on a proper load.
How do I work out my running cost?
Multiply litres per hour by your running hours and the price you pay per litre. For example, 15.5 litres an hour for 10 hours is 155 litres. Our fuel consumption calculator does this for your rating and load.
Does servicing reduce fuel consumption?
A service restores the things that raise consumption when they are neglected, such as air filtration, fuel filtration and engine condition. It does not change the fuel a given load needs.
Still unsure what the fault is? Call or WhatsApp 082 057 4605 for onsite diagnosis.
Related Pages
Load, sizing and the systems that affect fuel
- Fuel Consumption Calculator Litres per hour and cost at your load
- kVA to kW Reading the rating correctly
- Generator Sizes Matching the set to the load
- Load Bank Testing Output under a controlled load
- Running Hours Hours against litres over time
- Fuel System Repairs Filters, pumps and delivery
- Injector Service Injectors that affect burn
- Black Smoke Burning fuel badly under load
- Generator Servicing Air, fuel and oil on schedule
- Generator Guides The rest of the practical guides
Service Areas
Onsite visits across Gauteng
We come to your generator anywhere in Gauteng, 24/7. Choose your region for local suburbs, call-out details and the faults we see most there.
What to send before we attend
These details help us plan the visit and bring the right parts. Send what you can on WhatsApp; if you can’t find the model number, we still book the visit.
- Location
- The suburb or site, so the visit can be booked in the right area.
- Make and model
- The generator brand and model, if they are known.
- kVA
- The size printed on the data plate.
- Fuel type
- Diesel, petrol, LPG, natural gas, or dual-fuel.
- Controller model
- The module name on the panel, when it is visible.
- Fault or alarm
- What the set does, and any text on the screen.
- Photos
- The generator, the data plate, and the controller or alarm. Send these on WhatsApp.
Book a Visit
Send the data plate and the load
A photo of the data plate and the controller reading is enough to start. We use the engine model and the load to work out your fuel figure.
- Call
- 082 057 4605
- Message us on WhatsApp
What happens next
- We reply with questions about the generator, or to book a visit.
- A technician comes to you. For a breakdown we arrive within 1 hour.
- The fault is diagnosed on site and you get a written quote. No work starts until you accept it.
Charlie, lead technician: Degree in Mechanical Engineering, Degree in Electrical Engineering, Diesel Plant Fitter. Generator specialists since 2000.