How to Calculate Generator Fuel Consumption

As a rough rule, a diesel generator’s fuel consumption is estimated at approximately 0.28 litres/kWh at full load: hourly consumption ≈ active power (kW) × 0.28. At part load, consumption does not fall in proportion to load; at half load, slightly more than half of full-load consumption is used. These values are for preliminary calculations; the exact consumption should always be read from the engine data sheet.

Basic formula

1. If the generator rating is given in kVA, convert it to active power: kW = kVA × 0.8
2. Hourly consumption at full load: L/h ≈ kW × 0.28

Example: 250 kVA → 250 × 0.8 = 200 kW → 200 × 0.28 ≈ 56 L/h (at full load).

0.28 L/kWh is a rounded value representing the full-load specific fuel consumption of modern diesel generators. Engine make and model, turbocharger and fuel system, speed (50/60 Hz), ambient temperature, altitude and maintenance condition move this value up or down.

Consumption at part load

Generators rarely run at full load. Even when running with no load, the engine burns fuel to overcome friction and auxiliary equipment losses. This is why consumption does not fall in proportion to load. The typical ratios used in our fuel consumption calculator are:

Load	Ratio to full-load consumption (approximate)	L/h for a 200 kW set
25%	35%	≈ 19.6
50%	55%	≈ 30.8
75%	75%	≈ 42
100%	100%	≈ 56

Note when reading the table: at 25% load the set produces 50 kW and burns approximately 19.6 litres; that is approximately 0.39 litres per kWh. At full load it is approximately 0.28 litres per kWh. At low load, every litre produces less electricity.

Tank size and runtime

How long the generator can run during an outage is found as follows:

Runtime (hours) = usable fuel (L) ÷ hourly consumption (L/h)

Example: 200 kW set running at an average of 75% load (≈ 42 L/h), 500-litre tank:

500 ÷ 42 ≈ 12 hours

When sizing the tank:

• Use the usable volume. Sediment at the bottom of the tank and fuel below the suction pipe cannot be used; running the tank completely dry can let air and dirt into the fuel system.
• Use the real load profile. Assuming full load is on the safe side, but can lead to the cost of an unnecessarily large tank.
• Plan refuelling. During long outages, how quickly fuel can be supplied determines the autonomy required. For critical facilities such as hospitals, data centres and telecoms, the target duration is usually defined in the project specification.
• Remember fuel ageing. Water accumulation and microbial growth can occur in diesel that is stored for a long time. An oversized tank that is rarely emptied can lead to fuel quality problems.
• Regulations: Fire and environmental regulations apply to fuel storage above certain volumes; they should be checked at the design stage.

Factors affecting consumption

• Load level: Running continuously at very low load is inefficient and can also lead to unburnt fuel build-up (wet stacking).
• Maintenance: A dirty air filter, a faulty injector or incorrect adjustment increases consumption.
• Ambient conditions: High temperature and altitude affect engine efficiency.
• Frequency: The same engine generally consumes more fuel at 60 Hz (1,800 rpm) than at 50 Hz (1,500 rpm).
• Fuel quality: Off-specification or water-contaminated fuel impairs combustion.

A quick cost example

A 100 kVA (80 kW) set running 20 hours a month at an average of 50% load:

• Full-load consumption ≈ 80 × 0.28 = 22.4 L/h
• At 50% load ≈ 22.4 × 0.55 ≈ 12.3 L/h
• Monthly ≈ 12.3 × 20 ≈ 246 litres

Multiply this quantity by the current price per litre to estimate the monthly fuel cost.

If the data sheet shows g/kWh

Engine data sheets often give consumption in g/kWh (specific fuel consumption). To convert it to litres, the density of diesel fuel is used; this is in the range of approximately 0.83–0.85 kg/L:

L/h ≈ (g/kWh × kW) ÷ 1,000 ÷ 0.84

One point needs attention here: the kW in the engine data sheet is the mechanical power at the engine shaft. The electrical power produced by the generator is slightly lower because of alternator losses. If the set data sheet gives L/h directly, that is the most practical source.

Why are the figures approximate?

General rules reflect the average behaviour of different engines. Two sets of the same power can have noticeably different consumption because of different engine makes and technology. For purchasing decisions, tank sizing or operating budgets, the 25%, 50%, 75% and 100% load consumption values in the engine manufacturer’s data sheet should be used.

Run the calculation now

Use the fuel consumption calculator to enter your generator’s power and tank volume and see consumption and runtime at different loads. If you do not know the generator power you need, start with the power calculator; model-specific consumption values are given in the technical tables on the product pages.