Calorific value of fuels
Heat energy is measured in units of joules or calories (1calorie = 4.18 joules). The heat generated by fuels when they burn in joules or calories measures quality of fuels. All fuels do not burn efficiently. Thus there are fuels that produce more heat than the others are. This can be distinguished in terms of number of joules or calories that they generate on burning.
The amount of energy generated when 1 unit mass of fuel is burnt completely is known as the calorific value of the fuel. The word calorific is used, not “joulific” because of the use of the word calorific has been in use for a very long time. When 1 gram of charcoal is burnt, it produces 33 kilo joules. Thus the calorific value of charcoal is 33kJ/g. Sometimes instead of calorific value, another term kilowatt per kilogram (KWh/kg) is used.