As a thumb rule, if a diesel generator is consuming 100 liter per hour fuel. The heat of 30 liter is used for actual power production i.e. generate electricity, 30 liter goes to radiator water cooling, 5 liter is wasted through engine body heat waste and lube oil cooler and about 35 liter goes to exhaust as heat. Hence, you will observe that a major portion is going in exhaust, we recover about 20 liter equivalent heat of this and use it for producing hot water or steam at low pressure.