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Diesel Generator Fuel Consumption Calculator

Estimate diesel consumption for a generator set from its electrical load and specific fuel consumption (SFC). A visible part-load curve accounts for engines burning more fuel per kWh at light load. Every curve point can be changed, and the result includes daily use, tank runtime and cost.

For estimation and education. Not a substitute for a licensed engineer, the applicable codes (IEC 60364, NEC/NFPA 70, BS 7671, local regulations) or manufacturer data. Read the disclaimer.

Generator and load

kWe
%

10–100 % of rated kW

h

Fuel data

SFC unit
L/kWh

Typical modern diesel: 0.21–0.25 L/kWh

kg/L

Default 0.835

Part-load curve (SFC ÷ SFC at 100 %)

Tank and cost (optional)

Volume unit
L
/L

Any currency; leave empty to skip.

Result

Diesel generator fuel consumption — Engineerly. Estimate for education only; verify against applicable codes and manufacturer data.

Fuel consumption
18.54L/h
Per day (8 h)
148.3L/day
Tank runtime
26.97h
Electrical load
75kW
SFC at this load
0.2472L/kWh
SFC at this load (mass)
206.4g/kWh
Fuel mass rate
15.48kg/h
Part-load SFC multiplier in use
LoadMultiplierBar
25 %1.4
50 %1.12
75 %1.03
75 % ◀1.03
100 %1

Working, with your numbers

  1. 1. Electrical load
    P = rated kW × load %
    P = 100 kW × 75 %
    = 75 kW
  2. 2. Part-load SFC
    SFC(load) = SFC_100% × k(load)
    SFC = 0.24 L/kWh × 1.03
    = 0.2472 L/kWh
  3. 3. Fuel rate
    F = P × SFC
    F = 75 kW × 0.2472 L/kWh
    = 18.54 L/h
  4. 4. Fuel per day
    F_day = F × hours
    F_day = 18.54 L/h × 8 h
    = 148.32 L/day
  5. 5. Tank runtime
    t = V_tank / F
    t = 500 L / 18.54 L/h
    = 26.97 h

Assumptions

  • Steady load for the whole period; no allowance for start-up, idling or load swings.
  • SFC at full load is 0.24 L/kWh, scaled by the part-load multiplier (1.03 at 75 % load).
  • Diesel density 0.835 kg/L (typical at 15 °C; it varies with grade and temperature).
  • Tank volume entered is the usable volume, not the nominal capacity.
Rated power: 100 kW
Load: 75 %
SFC at 100 % load: 0.24 L/kWh
Density: 0.835 kg/L
Hours per day: 8 h
Tank: 500 L
Price: — per L
Curve 25/50/75/100 %: 1.4 / 1.12 / 1.03 / 1

Check before you rely on this

  • A manufacturer's fuel consumption table for your exact engine and rating beats any generic SFC estimate. Use it if you have it.
  • Estimation and education only. Not a substitute for a licensed engineer, the applicable code (IEC 60364, NEC/NFPA 70, BS 7671 or local regulations) or manufacturer data.

How to use this calculator

  1. Enter the set's rating in kWe and the load as a percentage of that rating.
  2. Enter the SFC at full load in L/kWh or g/kWh. If you have the manufacturer's fuel table, use it. Otherwise the default 0.24 L/kWh is a mid-range modern diesel value.
  3. Check the part-load multipliers. Replace them with ratios from the datasheet if available (SFC at each load ÷ SFC at full load).
  4. Add running hours per day, usable tank volume and fuel price for daily use, runtime and cost.

How it works: the formula

P_load = P_rated × load% SFC(load) = SFC_100% × k(load) (k interpolated from the curve) SFC [L/kWh] = SFC [g/kWh] / (ρ × 1000) Fuel rate F = P_load × SFC(load) [L/h] Per day = F × hours Runtime = V_tank / F Cost = F × price

Specific fuel consumption is fuel used per unit of electrical energy produced. It is lowest near rated load and rises at part load, because friction, pumping and alternator losses stay roughly constant while output falls. Multiplying SFC by kW gives litres per hour directly.

Variables and units

SymbolMeaningUnit
P_ratedGenerator prime or continuous electrical ratingkWe
SFCSpecific fuel consumption at the given loadL/kWh or g/kWh
k(load)Part-load multiplier: SFC at load ÷ SFC at 100 %—
ρDiesel density (default 0.835)kg/L
FVolumetric fuel rateL/h

Worked example

A 100 kWe set at 75 % load, full-load SFC 0.24 L/kWh, default curve (k = 1.03 at 75 %), 8 h per day, 500 L usable tank, fuel at 1.20 per litre:

P_load = 100 × 0.75 = 75 kW SFC = 0.24 × 1.03 = 0.2472 L/kWh F = 75 × 0.2472 = 18.54 L/h Per day = 18.54 × 8 = 148.3 L Runtime = 500 / 18.54 = 26.97 h Cost = 18.54 × 1.20 = 22.25 per hour

Using a datasheet value of 200 g/kWh at full load with ρ = 0.835 kg/L: SFC = 200 / 835 = 0.2395 L/kWh. A set at 100 kW then burns 23.95 L/h.

Assumptions

  • Constant load over the period.
  • The default part-load curve (25 %: 1.40, 50 %: 1.12, 75 %: 1.03, 100 %: 1.00) is a generic, deliberately conservative shape, not any maker's data.
  • Below 25 % load the multiplier is extrapolated along the 25–50 % slope, and a warning is shown.

Limitations

  • A manufacturer's fuel consumption table for your exact engine, rating and frequency is always better than a generic estimate.
  • Real loads vary. For a load that changes during the day, work out each period separately and add the results.
  • Fuel density varies with grade and temperature. Volume-based figures shift by a few percent.
  • Engine condition, altitude, ambient temperature and fuel quality all affect consumption.

References

Standards and texts that cover this topic. Use the edition adopted in your jurisdiction.

  • ISO 3046-1, Reciprocating internal combustion engines — Performance — Part 1: declarations of power, fuel and lubricating oil consumptions, and test methods.
  • ISO 8528-1 — generating set ratings (prime, continuous, standby) used to define "rated kW".
  • EN 590 and ASTM D975 — automotive and distillate diesel fuel specifications (density limits for EN 590 diesel are 0.820–0.845 kg/L at 15 °C).

Frequently asked questions

What SFC should I use if I have no datasheet?

Modern turbocharged diesel sets usually fall between 0.21 and 0.25 L/kWh (about 175–210 g/kWh) at 75–100 % load. Small sets sit at the higher end. The default 0.24 L/kWh is a cautious mid-range value. Treat any generic value as approximate.

Why does fuel per kWh increase at low load?

Part of the fuel goes into overcoming the engine's own friction and pumping losses, which barely change with load. At light load those fixed losses are a larger share of the output, so each kWh costs more fuel.

Is running a diesel generator at very low load harmful?

Long periods below roughly 30 % load can cause wet stacking: unburnt fuel and soot building up in the exhaust. Manufacturers usually state a minimum recommended load or require periodic load-bank runs.

How do I convert g/kWh to L/kWh?

Divide by density in g/L. At 0.835 kg/L (835 g/L), 200 g/kWh ÷ 835 = 0.2395 L/kWh. The calculator does this conversion and shows it in the working.

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How these formulas were checked: methodology and testing.