Generator Fuel Consumption: How to Estimate Diesel Burn per Hour
Estimate diesel use from load and specific fuel consumption. A planning method for tank sizing and running cost, with the caveats that matter at part load.
Fuel burn rises with load, and a diesel set is most efficient near 75 to 85 percent load. Estimate it from the load in kW times the specific fuel consumption, about 0.25 to 0.30 litres per kWh at full load. That gives a tank size and a running cost before you order, using the same load figure from the sizing method. For the rest of the order, the buyer's process guide walks from enquiry to commissioning.
The basic relationship
Fuel use in litres per hour equals the power output in kW multiplied by the specific fuel consumption in litres per kWh. The specific consumption is not constant: it is highest at light load, falls to its best around 75 to 85 percent load, then rises slightly at full load. So a set run at 30 percent load burns more per kWh than the same set run at 80 percent, which is why undersizing and then loitering at part load is expensive in fuel as well as in capital.
| Load level | Litres per kWh (approx.) |
|---|---|
| 25% load | 0.40 - 0.45 |
| 50% load | 0.30 - 0.35 |
| 75% load | 0.27 - 0.30 |
| 100% load | 0.25 - 0.30 |
Worked example
A 500 kW prime set running at 75 percent load delivers about 375 kW. At an indicative 0.28 litres per kWh, that is roughly 105 litres per hour. Run it 10 hours a day and the daily draw is about 1,050 litres, which sets the base-tank size and the refuelling interval. Compare this against the derated output: at a hot, high site the same set delivers fewer kW, so the load percentage rises and the per-kWh burn improves, but total available power is lower.
These are planning figures for tank sizing and cost estimates, not nameplate data. The engine manufacturer's fuel map on the datasheet is the authority, and it varies by engine family, turbocharger and after-treatment. Always confirm before you commit a fuel budget.
What changes the number
- Load level. The single biggest lever; running near 75 to 85 percent load minimises fuel per kWh.
- Engine family and rating. Larger, turbocharged prime-rated engines are typically more fuel-efficient per kWh than small standby units.
- Site conditions. Altitude and heat reduce output, which raises the load percentage and can shift the operating point on the fuel map.
- Maintenance. Clean filters, correct injection timing and good cooling keep consumption at the datasheet figure rather than above it.
Sizing the fuel tank
Turn the hourly burn into autonomy: the run hours you need between refuels, times the litres per hour, plus a reserve. Remote and unattended sites, such as a telecom tower, want long autonomy so a single fuel delivery covers weeks, not days. A bunded base tank also contains spills and keeps water out of the fuel.
Questions buyers ask about fuel
Frequently asked questions
How accurate is a fuel estimate before I buy?
Good enough for tank sizing and a cost range, not for a contract. The indicative specific consumption gives a defensible planning figure; the engine datasheet fuel map gives the number you should budget against.
Why does part-load burn more per kWh?
Because the set still runs its own parasitic loads and friction at low output, so each useful kWh carries a larger fixed overhead. Efficiency peaks around 75 to 85 percent load.
Should I size the set small to save fuel?
Only if the load is genuinely steady and known. A set sized too small runs at high load and loses the margin you need for motor starts and growth. Right-sized near 75 to 80 percent load is the fuel sweet spot.
Does altitude change fuel consumption?
Altitude cuts power, so for a given load you run at a higher load percentage, which usually improves the per-kWh figure even as total available power falls. Net fuel for the same useful work stays close to the sea-level estimate.
How do I stop fuel going bad in a remote tank?
Keep the tank sealed and bunded, fill it rather than leaving it partly empty, and plan a fuel-quality check on the service interval. Long idle with air space invites condensation and microbial growth.