In short. Fuel use rises with load but not in proportion: lightly loaded engines burn more fuel per kWh. Estimate litres per hour from the manufacturer’s consumption table at your loading, then multiply by running hours. Check against site records, because real consumption depends on engine condition, fuel and loading pattern.
Basic estimate
Diesel density is roughly 830 to 850 g/L. A typical specific fuel consumption for a modern diesel generator at good load is in the region of 200 to 230 g per kWh of electrical output, but the manufacturer’s data for the model governs.
Worked example (illustrative)
A set delivers 300 kW at an assumed 215 g/kWh. Fuel mass = 300 × 215 = 64,500 g/h. Dividing by 840 g/L gives about 77 L/h. Over 8 hours, about 614 litres. At a fuel price of, say, 90 per litre, the fuel cost is about 55,300 for the shift, or about 23 per kWh. Prices and consumption figures here are examples only.
Load matters
| Load (% of rating) | Fuel use (% of full-load rate) | Efficiency |
|---|---|---|
| 25 | Roughly 35 to 40 | Poor |
| 50 | Roughly 55 to 60 | Fair |
| 75 | Roughly 78 to 82 | Good |
| 100 | 100 | Best or near best |
The percentages are typical orders of magnitude. Use the manufacturer’s table at 25, 50, 75 and 100 per cent loading for the engine concerned.
Factors that change consumption
- Engine condition, injector wear and air filter state.
- Fuel quality and temperature.
- Ambient temperature and altitude.
- Cooling fan and auxiliary loads.
- Very light loading, with wet-stacking effects.
- Rating basis: standby, prime or continuous.
Using it for planning
- Fuel tank sizing and runtime. See fuel tank and runtime.
- Rental cost estimates. See rental planning.
- Operating cost comparison between options.
- Hybrid savings. See DG, UPS and BESS hybrid.
Checking against reality
- Record fuel added and running hours over a period.
- Record average load from the meter or controller.
- Compute litres per kWh and compare with the estimate.
- Investigate large deviations, such as leaks or theft.
See fuel consumption.
Estimating fuel use
Fuel (litres/h) ≈ load (kW) × specific fuel consumption (g/kWh) ÷ (density 0.84 kg/L × 1000)
A common rule is about 0.25 to 0.30 litres per kWh at good load, higher at light load. Always check the maker’s fuel curve for the engine.
Worked example (illustrative)
A set carries 400 kW with specific consumption 210 g/kWh: mass per hour = 84 kg; volume = 84 ÷ 0.84 = 100 litres per hour. Over 10 hours = 1,000 litres.
| Load | Approximate litres/kWh (typical diesel) |
|---|---|
| 25 percent | 0.34 or higher |
| 50 percent | 0.28 |
| 75 percent | 0.26 |
| 100 percent | 0.26 |
Indicative only; use the engine data sheet.
Why light load costs more per kWh
At light load the engine’s fixed losses are a larger share of fuel burned, so efficiency falls. Running two lightly loaded sets burns noticeably more than one well-loaded set.
Cost estimate
Multiply litres by the current diesel price, which changes often; use the latest price you pay. Include idle and warm-up fuel in the estimate.
See fuel tank and runtime guide.
Common mistakes
- Using full-load consumption for part-load operation.
- Ignoring the idle and warm-up consumption.
- Forgetting the gap between standby and prime rating bases.
What to send us
Send the generator model, the typical load and running hours. Send your application details.
Questions engineers ask
How many litres per hour does a 500 kVA generator use?
It depends on load and engine. As an order of magnitude, a set near 400 kW output may use around 90 to 110 litres per hour at high load. Use the manufacturer’s data.
Why does light loading waste fuel?
Fuel per kWh rises because the engine is less efficient at low load.
Is the catalogue figure reliable?
It is a good start for a new engine in good condition. Real figures vary with wear, fuel and conditions.
How can I reduce fuel use?
Match generator size to load, dispatch units well, maintain the engine and consider hybrid storage.
Figures and prices are illustrative. Use manufacturer data and current fuel prices.