**The commercial unit (or trade unit) of energy is kilowatt-hour which is written in short form as kWh.** Kilowatt-hour is usually used as a commercial unit of electrical energy.

**joule unit is not used as a commercial unit of electrical energy – Reason**

Kilowatt-hour is usually used as a commercial unit of electrical energy.

The SI unit of electrical energy is Joule.

A joule is the amount of electrical energy consumed when an appliance of 1 watt power is used for one second.

Actually, joule represents a very small quantity of energy and, therefore, it is inconvenient to use where a large quantity of energy is involved.

So, for commercial purposes, we use a bigger unit of electrical energy which is called “kilowatt-hour”.

**kilowatt-hour definition**

One kilowatt-hour is the amount of electrical energy consumed when an electrical appliance having a power rating of 1 kilowatt is used for 1 hour.

Since a kilowatt means 1000 watts, we can also say that one kilowatt-hour is the amount of electrical energy consumed when an electrical appliance of 1000 watts is used for 1 hour.

**State the relationship between the commercial unit of energy and joule class 9**

1 kilowatt-hour = 36,00,000 joules (or 3.6 × 10^{6}J)

=> 1 kWh =3.6 × 10^{6}J

Kilowatt-hour is usually used as a commercial unit of electrical energy. joule is the SI unit of electrical energy.

**Derive the relationship between the commercial unit of energy and si unit of energy**

Let’s establish (or derive) the

Relation Between Kilowatt-Hour and Joule.1 kilowatt-hour is the amount of energy consumed at the rate of 1 kilowatt for 1 hour.

That is, 1 kilowatt-hour = 1 kilowatt for 1 hour

or 1 kilowatt-hour = 1000 watts for 1 hour …(1)

But : 1 watt = 1 joule/1 secondSo, equation (1) can be rewritten as :

1 kilowatt-hour = 1000 joules/seconds for 1 hour

And, 1 hour = 60 × 60 seconds

So, 1 kilowatt-hour = 1000 joules/seconds × 60 × 60 seconds

or 1 kilowatt-hour = 36,00,000 joules

=>1 kilowatt-hour = 3.6 × 10^{6}J

**Important points about kWh **

- From this discussion, we conclude that 1 kilowatt-hour is equal to 3.6 × 10
^{6}joules of electrical energy. It should be noted that watt or kilowatt is a unit of electrical power but kilowatt-hour is a unit of electrical energy. - The electrical energy used in homes, shops, and industries is measured in kilowatt-hours (kWh). The electricity meter installed in our home records the electrical energy consumed by us in kilowatt-hours.
- 1 kilowatt-hour (or 1 kWh) of electrical energy is commonly known as
**‘1 unit’**of electricity. - Our electricity bill shows the electrical energy consumed by our household in a month in ‘kilowatt-hours’ or ‘units’ of electricity.

**Relevant & useful post**: How to calculate kWh (sample calculation shown)