The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. . The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg). 4.5 kg of fuel produces 180,000 kJ of heat. Therefore, 1 kg of fuel produces= 180,000/4.5= 40,000 kJ/kg.