In general, energy is defined as the ability to perform work. According to the first law of thermodynamics, the total sum of all energy forms in a closed system is constant. It is also referred to as the principal of energy conservation. In order to discuss quantitatively and comparatively various energy alternatives, it is necessary to bring them into a common footing in terms of measurement units.
The basic and physical unit of energy is Joule (J) which is based on the classical definition of work as the multiplication of force by distance. Hence, one Joule is equivalent to the multiplication of one Newton (N) of force by 1 m distance, and this definition gives J = N m. The joule is named after the 19th century scientist, James Prescott Joule who demonstrated by experiments the equivalence of heat and work. Unfortunately, the Joule is far too small a unit to be convenient for describing different resources of world energy supplies. It is, therefore, necessary to define its greater versions as mega-Joule (MJ) as 106 joule and giga-Joule (GJ) equivalent to 109 joule and tera-Joule (TJ) as 1012 J.
Another difficulty in practice with Joule is that oil producers measure the output of a well by barrels and coal producers by tons. Such different units require unification of the energy units by a common base. For instance, the coal equivalent ton (cet) is a basic unit which has been adopted by United Nations. A commonly used value for the cet is 38.6 X 106 kJ. Likewise, it is also possible to define oil equivalent ton (oet) which is equal to 51 X 106kJ.
In general, electrical energy is expressed, in terms of kilowatt hours (kW h). It is, therefore, necessary to know the energy conversion factors between different energy units .