What the Generator Fuel Consumption Calculator does
This calculator estimates how much fuel a generating set burns, in litres or gallons per hour, from its rating and the load factor it runs at. It gives the run time from a tank, the cost per hour and the cost per kWh delivered, and plots specific consumption across the whole load range.
It uses the linear fuel curve that engine data sheets and generator models both work in: F = F0 x rating + F1 x output. The first term is fuel burned simply to keep the set turning, and it is the reason a lightly loaded generator is so much worse per kWh than a well-loaded one. A single "litres per kWh" figure cannot show that, which is why this page fits two coefficients rather than one.
How to use it
- Enter the generator's rating in kW, or in kVA with a power factor - most standby sets are rated at 0.8 power factor, so a 125 kVA set is 100 kW.
- Enter the load factor: the average load as a fraction of the rating. If you do not know it, the run-hours meter and the fuel deliveries over a year will tell you more reliably than a guess.
- Add the tank capacity and the usable fraction. Sets are not run to the bottom of the tank, so 85 to 90% usable is the realistic figure.
- Enter the fuel price in whatever currency you use. Nothing is looked up and no exchange rate is applied.
- Open the advanced section and fit the curve to two points from your set's data sheet - the 50% and 100% figures are ideal. That replaces the defaults with your machine's actual behaviour and is by far the biggest improvement available.
- Read the cost-per-kWh column to see what light loading is really costing you.
Reading the results
Litres per hour is the headline operating figure: it decides refuelling intervals and the tank size.
Litres per kWh is the efficiency figure, and it is the one that exposes light loading. It falls steadily as the set is loaded up, because the fixed no-load burn is spread over more output.
Run time from the tank uses the usable volume, not the nominal capacity. A 1,000 litre tank at 90% usable holds 900 litres of run time.
Cost per kWh is what the generated electricity actually costs, before maintenance, before the capital cost of the set and before the operator. It is usually several times a mains tariff, which is why generators are for when the mains is absent rather than expensive.
Worked example: a 100 kW standby set at 75% load
With the default diesel coefficients F0 = 0.0246 L/h per kW rated and F1 = 0.2455 L/kWh, consumption is 0.0246 x 100 + 0.2455 x 75 = 2.46 + 18.41 = 20.87 litres an hour, or 5.51 US gallons an hour.
The set delivers 75 kW, so specific consumption is 20.87 / 75 = 0.278 litres per kWh. At 1.20 a litre that is 25.05 per hour of running and 0.334 per kWh delivered.
A 500 litre tank at 90% usable gives 450 litres, which at 20.87 L/h is 21.6 hours - so the set needs refuelling roughly once a day if it runs continuously.
Now drop the load to 25%. Consumption falls to 2.46 + 0.2455 x 25 = 8.60 litres an hour, which sounds like a saving. But it is only delivering 25 kW, so specific consumption rises to 8.60 / 25 = 0.344 litres per kWh - 24% worse per unit of electricity. The 2.46 L/h intercept has not changed at all; it is simply being spread over a third as much output.
That is the case for right-sizing a set or for load banking a standby machine during its monthly test, and it is invisible if you only look at litres per hour. Running below about 30% load for long periods also causes wet stacking in a diesel engine, so the cost is mechanical as well as financial.
Formulas and scoring rules
- Linear fuel curve
F = F0 x P_rated + F1 x P_outF in litres per hour. F0 is the no-load intercept per kW of rating, F1 the marginal slope per kWh delivered.- Specific consumption
L/kWh = F / P_outRises sharply at low load because F0 is fixed.- Fitting the curve to two points
F1 = (Fb - Fa) / ((Lb - La) x P_rated); F0 = (Fa - F1 x P_rated x La) / P_ratedLa and Lb are the two load fractions, Fa and Fb the quoted consumption at each.- Run time
hours = tank litres x usable% / 100 / F- Cost per hour
price per litre x F- Cost per kWh
cost per hour / P_out- Gallons
1 US gallon = 3.785412 L; 1 imperial gallon = 4.546090 L- Electrical efficiency
P_out / (F x energy per litre)Diesel is taken as about 10 kWh per litre.
How this differs from the fuel cost and generator size calculators
A2Z already has a Fuel Cost Calculator, which prices a vehicle journey from distance and fuel economy, and a Generator Size Calculator, which picks a generator rating from the load you need to run. Neither can answer the question in between: what does the set you already have cost to run?
That needs three things those tools do not have. Litres per hour from a rating and a load factor rather than from a distance. Run time from a tank capacity. And cost per kWh delivered, which requires knowing both the fuel burned and the electricity produced. This page does those three, plus the load-range curve that shows how all of them move as the loading changes - the figure that usually decides whether the set is the right size.
Where the coefficients come from, and why yours are better
The defaults here are the published coefficients for a diesel generator in HOMER's generator model: an intercept of 0.0246 litres per hour per kW of rating and a slope of 0.2455 litres per kWh. They are a reasonable starting point for a typical set and they are wrong for yours, possibly by 15% either way.
Every generator data sheet quotes consumption at 25, 50, 75 and 100% load. Put any two of those into the fitting fields and the page solves for your set's actual intercept and slope. The straight line through those points is a good fit between about 25% and 100% load; below 25% the real curve bends upward and the fit understates consumption.
The petrol and LPG figures on this page are the diesel curve scaled by energy content per litre, not manufacturer data, and they are labelled as such. Treat them as an order of magnitude.
Limitations: what the result does not prove
- The default coefficients are a published generic model, not your set's data. Fit the curve to two data-sheet points for a figure you can rely on.
- The linear model is good between about 25% and 100% load. Below 25% the real curve rises more steeply than the straight line predicts.
- Ambient temperature, altitude, fuel temperature and density, engine age, condition and load type all move real consumption, and none of them is modelled.
- Fuel prices are entered by you. Nothing here looks up a price, a rate or any live data, and no currency conversion is applied.
- The petrol and LPG coefficients are scaled from the diesel curve by energy content rather than measured, and should be treated as indicative only.
- It does not size a generator - use the Generator Size Calculator for that - and it says nothing about emissions, noise, maintenance intervals or the cost of the set itself.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- ISO 8528-1 - Reciprocating internal combustion engine driven generating sets - checked 19 Sep 2026
- HOMER Energy - Generator fuel curve model
- ISO 3046-1 - Reciprocating internal combustion engines: performance declarations and fuel consumption
Frequently asked questions
How much diesel does a 100 kW generator use per hour?
About 21 litres an hour at 75% load, or 5.5 US gallons, using the generic linear model. At 50% load it drops to about 14.7 litres an hour and at 25% to about 8.6. Your set's data sheet will differ from those figures by perhaps 15% in either direction, which is why the page lets you fit the curve to its actual numbers.
Why is a lightly loaded generator less efficient?
Because part of the fuel burn does not depend on the load at all. A 100 kW set burns about 2.5 litres an hour just turning over, whether it is producing 25 kW or 75 kW. Spread over 75 kW that overhead is small; spread over 25 kW it is three times as significant per unit of electricity, which is why specific consumption rises so sharply at light load.
How long will my generator run on a full tank?
Divide the usable fuel by the consumption per hour. A 500 litre tank at 90% usable holds 450 litres, and at 21 litres an hour that is about 21.5 hours. Never use the nominal capacity: the pickup sits above the tank floor and the last tenth is not reliably available.
What load factor should I assume?
For a standby set that runs only during outages, 0.5 to 0.8 is typical. For a prime-power site set, 0.6 to 0.75. If you have run hours and fuel deliveries over a year, work it backwards from those instead - it is far more reliable than any assumption, and it often reveals that the set is much larger than the load needs.
What does generated electricity cost per kWh?
At 0.278 litres per kWh and 1.20 a litre, the fuel alone is about 0.33 per kWh - typically several times a mains tariff. That is fuel only: maintenance, oil, filters, the operator and the capital cost of the set are all on top, and a full cost per kWh is often 0.45 to 0.60 in the same units.
Is it bad to run a generator at low load?
For a diesel engine, yes, if it is sustained. Below about 30% load the cylinders do not reach their proper operating temperature, fuel does not burn completely, and unburned fuel and carbon accumulate in the exhaust - wet stacking. It fouls injectors and turbochargers and eventually needs a load-bank exercise or an engine strip to clear.
How do I convert litres per hour to gallons per hour?
Divide by 3.785412 for US gallons or by 4.546090 for imperial gallons. The two differ by 20%, so it is worth being explicit about which one a data sheet is quoting - both columns are shown here for exactly that reason.
Does the tool use live fuel prices?
No. You enter the price and the tool does the arithmetic in whatever currency that price is in. No rate is fetched, no exchange rate is applied, and nothing you type leaves the page - the whole calculation runs in your browser.
Last reviewed by the A2Z.Tools team against the sources listed above.
