In short. A diesel generator burns fuel in proportion to the kW it delivers, not its kVA rating. Modern sets above about 100 kVA consume roughly 0.25 to 0.30 litres per kWh between 75 and 100 percent load, and noticeably more per kWh at light load because fixed losses do not shrink. Estimate from the manufacturer’s consumption at 25, 50, 75 and 100 percent load, then confirm by measuring litres and kWh over the same period.
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Fuel use, runtime, tank size and cost
How this is calculated
Real power is kVA × power factor. Fuel use is estimated as L/h ≈ 0.04 × rated kW + 0.22 × load kW, which follows the usual shape of modern diesel gensets: about 0.26 L per kWh near full load, rising towards 0.38 L per kWh at a quarter load because the engine's fixed losses are spread over less output.
If you enter the full-load figure from your engine datasheet, the curve is scaled to match it. Runtime is usable fuel ÷ L/h. Tank size is L/h × hours, plus the reserve margin, divided by the usable fraction.
These are planning estimates, typically within 10 to 15 percent of a healthy engine. Actual use depends on the engine model, its condition, altitude, ambient temperature and how the load varies. For a figure you can rely on, use the manufacturer's fuel curve or a metered test.
Estimates only, worked out in your browser. For a checked selection, send us your load list and site details or call 098852 01608.
Litres per hour versus litres per kWh
Litres per hour tells you how fast the tank is emptying; litres per kWh tells you how efficiently the set is converting fuel into useful electricity. Both matter, but they answer different questions. Runtime planning and tank sizing use litres per hour at the expected load. Operating cost and comparison between sets use litres per kWh. A set that shows a low litres-per-hour figure may simply be lightly loaded, and lightly loaded diesel engines are the least efficient, so the two numbers should never be compared across sets without stating the load.
Specific consumption (L/kWh) = litres consumed ÷ kWh generated, over the same period
Litres consumed is the fuel actually burned between two readings, corrected for any fuel added or returned, and kWh generated is the energy recorded on the generator’s energy meter between the same two times. Both must refer to the same boundary: if the meter is downstream of a transformer or auxiliaries, the figure describes delivered energy, not alternator output.
Where the manufacturer’s figures come from
An engine data sheet states brake specific fuel consumption in grams per kWh at the flywheel, typically 195 to 225 g/kWh for a modern turbocharged diesel at its best point. To turn that into litres per hour at the alternator terminals, divide electrical kW by alternator efficiency to get engine kW, multiply by the specific consumption to get grams per hour, and divide by the density of diesel, about 0.83 to 0.85 kg per litre. Worked example (illustrative): a set delivering 100 kW electrical through an alternator of 93 percent efficiency needs 107.5 kW at the flywheel; at 215 g/kWh that is 23.1 kg of fuel per hour, or about 27.5 litres per hour at 0.84 kg per litre. The generating-set data sheet usually does this arithmetic already and publishes litres per hour at four load points, which is the figure to use.
Load factor dominates everything else
Fuel burn falls with load, but not in proportion, because friction, pumping losses, the cooling fan and the alternator’s no-load losses continue regardless. The result is that litres per kWh climbs steeply below half load. The table shows the pattern for a 500 kVA, 400 kW set using typical figures; the actual curve for any particular engine must be taken from its data sheet.
| Load | Electrical output | Typical consumption (illustrative) | Resulting L/kWh |
|---|---|---|---|
| 100 percent | 400 kW | 100 to 110 L/h | 0.25 to 0.28 |
| 75 percent | 300 kW | 78 to 85 L/h | 0.26 to 0.28 |
| 50 percent | 200 kW | 55 to 60 L/h | 0.28 to 0.30 |
| 25 percent | 100 kW | 30 to 35 L/h | 0.30 to 0.35 |
| No load | 0 kW | 8 to 12 L/h | Not defined |
Two consequences follow. A set chosen with a large margin for a hypothetical future load will spend its life in the expensive part of the curve, which is a reason to size with care rather than generosity, as explained in generator sizing. And a plant whose demand varies widely through the day is often cheaper to run with two smaller sets sequenced than one large set idling, a point developed in generator loading and spare capacity.
Why a healthy set burns more than its data sheet
Measured consumption in the field is often 5 to 15 percent above the catalogue figure, and the gap is diagnostic.
- Blocked air filters and dirty turbocharger compressors starve the engine of air, so the governor adds fuel to hold speed and the exhaust darkens.
- Worn injectors and injection pumps spray poorly, leaving unburnt fuel in the exhaust; injector service is covered under fuel injector and pump service.
- Prolonged running below about 30 percent load causes wet stacking, where unburnt fuel and oil coat the exhaust system, and consumption per kWh rises further.
- High ambient temperature, a hot room and altitude reduce air density and raise consumption per kWh at a given load.
- Fuel quality, including water, microbial growth and old stock, affects combustion and filter life; see fuel cleaning and polishing.
- Governor hunting on a mechanically governed engine wastes fuel on every oscillation.
Power factor does not change fuel consumption for a given kW, because the engine supplies real power only. A poor power factor loads the alternator with more current and may limit the kVA available, but the litres per hour follow the kW.
Measuring consumption properly
The simplest reliable method is a timed test at a steady load: record the energy meter, top the day tank to a mark, run for an hour or more, refill to the same mark through a calibrated measure and record the energy meter again. Temperature changes the volume of diesel by about 0.08 percent per kelvin, so refill at a similar fuel temperature or correct for it. Permanent flow meters on the supply and return lines give continuous data but must be read as the difference between the two, because an engine circulates far more fuel than it burns. Tank level gauges are poor instruments for consumption because tank geometry, return fuel heating and sloshing distort them. Where a plant needs a figure it can defend commercially, a load-bank test at defined load points, described in load-bank testing, produces one.
From consumption to operating cost
Energy cost per kWh is litres per kWh at the operating load multiplied by the delivered price per litre. At 0.28 L/kWh and a diesel price of P rupees per litre, the fuel component is 0.28 × P per unit, before lubricating oil, filters, service and depreciation. The same arithmetic shows why a set running at 25 percent load costs about a quarter more per unit than one at 75 percent, and why fuel, not capital, dominates the cost of ownership for any set that runs more than a few hundred hours a year. The lifecycle cost guide puts fuel alongside the other components.
Common mistakes
- Quoting a single litres-per-hour figure for a kVA rating; consumption depends on the load actually drawn and the figure is meaningless without it.
- Comparing two sets on litres per hour at different loads; the lightly loaded set looks efficient and is the opposite.
- Using the engine’s grams per kWh as if it were electrical output; alternator and fan losses add 7 to 10 percent.
- Reading consumption from a tank gauge over a short period; return-fuel heating and gauge geometry produce errors larger than the quantity being measured.
What to send us
Send the generating-set nameplate photograph, the typical load in kW or a load log, the running hours per month, and any fuel records with the dates and energy-meter readings they relate to. If consumption has risen, add the last service date, a photograph of the exhaust under load and the controller’s event log. Request a service review for an existing set, or send your application details if the question is about a set still to be selected.
Questions engineers ask
How many litres per hour does a 125 kVA generator use?
At full load of 100 kW a modern set uses roughly 26 to 30 litres per hour, at half load about 15 to 17, and around 3 to 4 litres per hour at no load. These are typical ranges; the engine data sheet for the specific set gives the figures to plan with.
Why has my generator’s consumption gone up without any change in load?
The usual causes are a restricted air filter, worn injectors, a leaking or mis-timed injection pump, a slipping turbocharger or coolant running hot. Black smoke points to air or injection, white or blue smoke to timing or oil. A timed consumption test before and after service will confirm the recovery.
Does running at low load damage the engine as well as waste fuel?
Yes. Below about 30 percent load for long periods the cylinders run cool, unburnt fuel and oil collect in the exhaust and turbocharger, and injector nozzles coke. Periodic loading to 70 percent or more on a load bank, or a smaller set for the night load, prevents it.
Is litres per kWh constant across the load range?
No. It is lowest between about 70 and 100 percent load and rises steadily below that, roughly 25 to 40 percent higher at quarter load. Any figure should state the load at which it was measured.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.