Commercial Unit of Energy
A unit of work is a joule. The symbol is J.
We spend energy to do work. Hence, the amount of work done is the same as the energy utilized. So the unit of energy is the same as that of work.
Example: An engine with a power of 250 W does work for 10 s. What is the work done?
$P=\frac{W}{t}\phantom{\rule{0ex}{0ex}}\mathrm{Hence},W\mathit{}\mathit{=}\mathit{}Pt\mathit{}\mathit{}\mathit{=}250\times 10\mathit{=}\mathit{}\mathit{2500}\mathit{}\mathrm{J}$
Here, the unit of energy is the joule. This unit is too small for some purposes. In such situations, we use the kilowatthour (kWh), which is a large unit.
1 kWh is the amount of energy utilised by a gadget of power 1 kW in one hour.
$P=\frac{W}{t}=\mathrm{J}/\mathrm{s}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}1\mathrm{kW}=1000\mathrm{W}=1000\mathrm{J}/\mathrm{s}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}1\mathrm{kW}=1000\times 3600=3600000\mathrm{J}=3.6\times {10}^{6}\mathrm{J}$
The commercial unit of electrical energy is the kilowatt hour. It is popularly known as a unit.
Example: How much time is required for a bulb of 100 W to consume 1 unit of electricity?
$P=100\mathrm{W}=100/1000\mathrm{kW}=0.1\mathrm{kW}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}Energy\mathit{}\mathit{=}\mathit{}power\mathit{\times}time\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}Time\mathit{}=\mathit{}\frac{energy}{power}=\frac{1\mathrm{kWh}}{0.1\mathrm{kW}}=10\mathrm{hour}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}$
Summary

Work is done only when a force acts on the body and the body displaces in the direction of the force.
ie., W = Fs

SI unit of work is Joule (J)

If the body moves in the direction of the applied force, then the work done is taken as positive.

If the displacement of the body is in a direction opposite to that of the applied force, then the work done is considered negative.

An object having the capability to do work is said to possess energy. Energy has the same unit as work.

The energy possessed by a body due to its motion is called kinetic energy.

The kinetic energy possessed by an object of mass, m, and moving with a uniform velocity, v is
${E}_{k}=\frac{1}{2}m{v}^{2}$

Energy possessed by an object due to its change in position or height is called potential energy.

A body of mass m being raised to a height ℎ, E_{p }= mgh

The law of conservation of energy states that energy can neither be created nor be destroyed.It can only be converted into another form.

The work done in unit time is otherwise named the rate of doing work or power.

The energy used in one hour at the rate of 1kW is called 1 kWh.