Units of Measurement of Energy
Energy is defined as the ability to do work i.e. energy produces work. As per the second law of thermodynamics, whenever the work is done by absorbing the energy (heat) from the reservoir, some heat is always rejected to the sink. The work and heat are also forms of energy; hence the units of measurement of energy, work and heat are same.
A number energy measurement systems exist at present but most commonly used system is SI. Here are the units of measurement of energy, work and heat in various systems:
1) MKS system (Metric system): Calories or cal: This unit is very commonly used to indicate the energy content of the fuel and food, capacity of refrigeration and air-conditioning system, etc. One calorie is the amount of heat required to raise the temperature of one gram of water by one degree Celsius. As such calorie is a very small unit hence larger unit Kilocalories or Kcal is more commonly used. 1Kcal = 1000 cal
2) SI unit system: Joule: This system is most commonly used now. Joule unit has been named after the famous scientist Joule who has made very important contributions in the field of thermodynamics especially in work and energy. Even Joule is a small unit; hence kilo Joule or KJ is used commonly. 1KJ = 1000J
3) British thermal unit: Btu: One Btu is the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit.