Generator Fuel Consumption Calculator
Enter fuel type, rated kW, load and runtime to estimate fuel use, efficiency and cost per kWh across the load range.
Step-by-step
- Enter valid values to begin.
Fuel use follows a Willans-line model, fuel = a × Prated + b × Pload. The typical diesel pair is 0.030 L/h per rated kW and 0.240 L/h per kW output, which gives about 0.27 L/kWh at full load; the small-set pair 0.0815 and 0.246 is a widely used (HOMER-type) fit for small gensets and gives about 0.33 L/kWh at full load. Gas and LPG use the same shape on an energy basis (about 0.40 kW of fuel per rated kW plus 2.55 kW per kW output, roughly 34% full-load efficiency). All coefficients are typical or approximate values, not a manufacturer curve, and real consumption shifts with engine age, altitude, temperature and fuel quality. Fuel energy is on a higher-heating-value basis: diesel 38.6 MJ/L (10.7 kWh/L), natural gas 38.3 MJ/m³ (10.6 kWh/m³), propane 25.3 MJ/L (7.0 kWh/L). Use the engine data sheet for a firm figure; ISO 8528 and ISO 3046 define how those ratings are stated.
Generator Fuel Consumption: Why Light Loads Waste Fuel
Core Engineering Principles
An engine burns a fair amount of fuel just to turn itself over, overcome friction and run the cooling and the alternator at rated speed, even when no kilowatts come out. Fuel use therefore follows a Willans line: a fixed part tied to the set’s rating plus a part that rises linearly with load. We write it as fuel = a × Prated + b × Pload. For a mid-size diesel, about 0.030 L/h per rated kW and 0.240 L/h per kW produced are typical, and they give 27 L/h at full load on a 100 kW set, or 0.27 L/kWh. A frequently quoted pair of 0.08145 and 0.246 suits small sets but adds up to 0.327 L/kWh, which overstates a large machine.
Because the fixed part is spread over fewer kilowatts, specific consumption rises sharply at light load: 0.36 L/kWh at 25%, 0.30 at 50%. That curve explains why one oversized generator costs more to run than two sensible ones. Light load is also an engine health problem. Below about 30% the combustion chamber runs cool, unburned fuel and soot collect in the exhaust and the cylinder glazes, a condition called wet stacking. We cure it with a load bank, not with more idling.
η = Pload / (Fuel × energy content) • Diesel ≈ 38.6 MJ/L ≈ 10.7 kWh/L
NEC & Standard References
ISO 8528-1 defines the generating set ratings (continuous, prime, limited-time and emergency standby) and the reference conditions. ISO 3046-1 covers reciprocating engine performance, including fuel consumption declarations and the reference air temperature and pressure. NFPA 110 for emergency and standby power systems calls for periodic exercising under load, which is also how you keep a standby set free of wet stacking. Fuel storage rules come from local code. Check the adopted editions and the data sheet before sizing a tank.1. Fuel rate = 0.030 × 100 + 0.240 × 75 = 21.0 L/h.
2. Specific use = 21.0 / 75 = 0.280 L/kWh.
3. Efficiency = 75 / (21.0 × 10.72) = 33.3% on the higher heating value.
4. Cost per kWh = 21.0 × 1.10 / 75 = 0.308.
5. Over 8 h the set makes 600 kWh and burns 168 L, costing 184.80.
6. At 100% load the same set uses 27.0 L/h or 0.270 L/kWh (34.5%); at 25% it uses 9.0 L/h, which is 0.360 L/kWh (25.9%).
- Don’t run a diesel below 30% for long. Wet stacking fouls injectors and exhaust, and a scheduled load-bank run clears it.
- Derate for site conditions. Altitude, ambient heat and a dirty air filter cut output and raise fuel burn.
- Gas sets lose efficiency faster. Spark-ignition natural gas and LPG sets are typically 30–35% efficient at full load and fall off more steeply.
- Fuel degrades. Standby diesel grows microbes and oxidises; test it and keep a reserve above the calculated run fuel.
- Treat these figures as typical. Use the manufacturer’s consumption table at 25, 50, 75 and 100% load for contractual numbers.