Work out diesel generator efficiency instantly with clear inputs, formula shown and shareable results.
Generator efficiency is electrical output divided by fuel energy input. Fuel mass per hour times net calorific value gives the input in kilowatts. A well-loaded modern diesel generator reaches 38-43% at full load, falling steeply below about 50% load, which is why oversized sets are expensive to run.
Thermal efficiency
Input (kW) = fuel (kg/h) x CV (MJ/kg) x 1000 / 3600; efficiency = output / input
Specific fuel consumption
SFC (g/kWh) = fuel (kg/h) x 1000 / output (kW)
195-215 g/kWh at full load for a modern medium-speed diesel. Above 240 g/kWh indicates part-load operation, poor maintenance or a fuel quality problem.
Friction and auxiliary losses are nearly constant while output falls, so their share of fuel rises. Below 30% load, wet stacking and cylinder glazing also become a maintenance risk.