Unit of Energy

What is the Unit of Energy?

In physics, we encounter various quantities having varying attributes, and describing them becomes difficult as we talk about the terms like energy, time, work, or any physical quantity that needs to have a standard measure to distinguish. In physics, the units determine the standard measure of these quantities. Such as if I say my aunt is 36 kg and she lives 1200 km from my city. Here, kg (for weight) and km (for distance)  are the units to describe these physical quantities. Other examples are Kelvin for temperature; etc.

According to the international system (SI), the unit of energy is Joule which is denoted by J.


On this page, we will learn about the following:

  • Unit of energy

  • The SI unit of energy

  • CGS unit of energy

  • The SI unit of potential energy

  • MKS unit of energy

  • The unit of energy in the MKS system

  • CGS unit of power

  • CGS unit of work

  • The SI unit of electrical energy

The SI Unit of Energy

The SI unit of energy is Joule (the basic energy unit of the metric system) symbolized as J is named in honor of an English physicist James Prescott Joule (1818–1889) for his experiments on the mechanical equivalent of heat.


The unit of energy is the same as that of work (As work and energy are the two sides of the same coin).

CGS Unit of Energy

The CGS stands for centimeter-gram-second.

The CGS unit of energy is erg equivalent to 1 g cm^2 s^ - 2.

Ergs are a lot smaller than Joules, equivalent to 10 ^ - 7 J.

The SI Unit of Potential Energy

The potential energy is the energy stored in the matter. Therefore, the SI unit of potential energy is the same as the SI unit of all energies given by,


1 J =  1 kg m^2 s^ - 2 where the potential energy due to height is described as,

  P.E. =  mgh whose MKS unit is also kg m^2 s^ - 2 = 1 Joule.

MKS Unit of Energy

The MKS stands for meter-kilogram-second.

1 Joule is equivalent to one Newton-meter (N-m) where 1 J is the amount of work done by a newton of force to a distance of 1 meter.


Where 1 J = kg (m/s)^2 = 1 Watt-second = 1 kg m^2 s^- 2

So MKS unit of energy is kg m^2 s^- 2

A force does the work to accelerate a body of mass 1 kg at the rate of 1 ms^ - 2.

CGS Unit of Power

Power is defined as the rate of doing work  given by,

Power = Work / time

The CGS unit of power is erg per second. 

CGS Unit of Work

The CGS unit of work is Erg where 1 erg = 1 gm cm square/ second square.

The word erg is derived from the Greek word ergon (ἔργον) which means 'work' or 'task'.

One erg is also defined as the work done by a force of 1 dyne on a one-centimeter distance.

The SI Unit of Electrical Energy

The movement of charged particles inside the wire produces electricity. The electrical energy is the form of energy resulting from the flow of electrical charge is called electrical energy.


The SI unit of electrical energy is Joule symbolized as J.

1 Joule is equal to the energy transferred (or work done) when applying a force of one newton through a distance of one meter (1 newton-meter or N-m), 

It is also defined as the work done in passing an electric current of one ampere through a resistance of one ohm for one second. 

FAQ (Frequently Asked Questions)

1. What is energy? List the different forms of energy.

In physics, energy is a quantity that must be transferred on to the body to perform the work or any task as energy is defined via work.

There are two main forms of energy, namely, Potential energy (P.E), and Kinetic energy (K.E.). The energy is further classified into two types, namely renewable and non-renewable energy. Other forms of energy are:

  • Radiant energy

  • Sound energy

  • Elastic energy

  • Light energy

  • Motion energy

  • Gravitation energy

  • Nuclear energy

  • Electromagnetic energy

  • Mechanical energy

  • Wind energy

  • Biomass energy

  • Tidal energy

  • Geothermal energy

  • Hydroelectric energy

  • Wave energy

  • Hydrogen energy

  • Thermal energy

2. What do you mean by the law of conservation of energy?

The law of conservation of energy states that the energy can never be created or destroyed but can be transformed into another form.

For example, when a box slides through a slop, at rest it has potential energy stored in itself when comes into motion that energy gets changed into the kinetic energy. The friction acts in the opposite direction. Therefore, the kinetic energy from the box gets transformed into the thermal energy heats the box and the slope.


3. How is power related to work and energy?

Work is done by energy and the rate at which any task is done is called the power. Let’s take an example, suppose you’re driving a car. The air friction is trying to slow down your car, the engine is powerful enough to accelerate the car. So here the work done is equal to the force applied by an engine to a distance. Now the rate at which the engine does this work is called the power.

4. Why do physical quantities need units?

The units are important because, without units, we can never express physical laws precisely just from qualitative reasoning. Such as I am 8 but if I say I am 8 years old so years old is the unit to define the quantity age.