How the fuel curve is calculated
Engine fuel use is well approximated by a straight line with an offset — the Willans line: consumption = idle burn + slope × load. That offset is why the curve "isn't linear" in the way people expect: litres per hour rises linearly, but litres per kilowatt-hour rises steeply as load falls. If your datasheet lists both full-load and half-load consumption, the calculator fits the line exactly through both points; with only the full-load figure it assumes the idle burn is 25% of it, which matches the widely used default curve from the generator modelling literature.
Example: a 5 kW unit burning 2.0 L/h at full load and 1.3 L/h at half load has an idle burn of 0.6 L/h. At 25% load it uses 0.95 L/h — only half the full-load rate, but each kilowatt-hour costs 0.76 L against 0.40 L at full load, nearly twice as much.
What the model leaves out
The fit assumes steady load: frequent compressor starts and surging loads burn extra. Altitude and heat derate output and shift consumption. And for diesels there is a mechanical reason beyond economics to avoid long low-load running — wet stacking — covered in the FAQ. To pick the generator size itself, start with the Generator Sizing calculator; to compare fuel cost per kWh against grid power, the Electricity Cost calculator gives the other side.
Methodology
The calculator UI, the load-range table and the displayed formula all call the same calculation function, and the fitted intercept and slope are shown rather than hidden.
- Linear fuel curve with idle offset (Willans line) — Skarstein & Uhlen (1989), the default generator fuel model used in NREL's HOMER energy software
- Full- and part-load consumption figures — Generator manufacturer datasheets — always prefer your model's own numbers
- Wet stacking risk below ~30% load on diesels — Diesel generator manufacturer maintenance guidance
Assumptions last reviewed on 27 August 2026. If a source disagrees with your battery or equipment datasheet, trust the datasheet.
Frequently asked questions
Why does fuel per kWh get worse at low load?
An engine burns a fixed amount of fuel just to keep itself spinning. At 25% load that idle burn is spread over a quarter of the kilowatt-hours, so each one costs more fuel.
Where do the datasheet figures come from?
Generator spec sheets list fuel consumption at rated (100%) load, and often at 50% or 75% too. With two points the calculator fits the curve exactly; with one it assumes a typical shape.
What is wet stacking?
Diesel engines run for long periods at low load never reach full operating temperature, so unburned fuel and soot accumulate in the exhaust system. Keep diesels loaded to roughly 30% or more, or give them regular runs near full load.