Units of Energy

The capacity of an agent to do work is known as its energy. The most important forms of energy are mechanical energy, electrical energy and thermal energy. Different units have been assigned to various forms of energy. However, it must be realized that since mechanical, electrical and thermal energies are interchangeable, it is possible to assign the same unit to them.


The unit of mechanical energy is newton-metre or joule on the M.K.S. or SI system.

Defined as:

The work done on a body is one newton-metre (or joule) if a force of one Newton moves it through a distance of one metre i.e.

Mechanical energy in joules = Force in Newton  –  distance in metres.


The unit of electrical energy is wattsec or joule and is defined as follows:

One watt-second (or joule) energy is transferred between two points if a p.d. of 1 volt exists between them and 1 ampere current passes between them for 1 second.

Electrical energy in watt-sec (or joules) = voltage in volts – current in amperes – time in seconds

Joule or watt-sec is a very small unit of electrical energy for practical purposes. In practice, for the measurement of electrical energy, bigger units viz., watt-hour and kilowatt hour are used.

1 watt-hour = 1 watt × 1 hr = 1 watt × 3600 sec = 3600 watt-sec

1 kilowatt hour (kWh) = 1 kW × 1 hr = 1000 watt × 3600 sec = 36 x 105 watt-sec.


Heat is a form of energy which produces the sensation of warmth. The unit of heat is calorie, British thermal unit (B.Th.U.)  and centigrade heat units (C.H.U.) on the various systems.


It is the amount of heat required to raise the temperature of 1 gm of water through 1ºC.

1 calorie = 1 gm of water × 1ºC

B.Th.U. It is the amount of heat required to raise the temperature of 1 lb of water through 1ºF

1 B.Th.U. = 1 lb × 1ºF

C.H.U. It is the amount of heat required to raise the temperature of 1 lb of water through 1ºC

1 C.H.U. = 1 lb × 1ºC





1 kWh = 1 kW × 1 hr = 1000 watts × 3600 seconds = 36 × 105 watt-sec. or Joules

1 kWh = 36 × 105 Joules

It is clear that electrical energy can be expressed in Joules instead of kWh.


1 calorie = 4·18 Joules

1 C.H.U. = 1 lb × 1ºC = 453·6 gm × 1ºC

= 453·6 calories = 453·6 × 4·18 Joules = 1896 Joules

1C.H.U. = 1896 Joules

1 B.Th.U. = 1 lb × 1ºF = 453·6 gm × 5/9 ºC

= 252 calories = 252 × 4·18 Joules = 1053 Joules

1 B.Th.U. = 1053 Joules


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