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High School Physics

Commercial unit of energy class 9

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 × 106 J)
=> 1 kWh = 3.6 × 106 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 second

So, 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 × 106 J

Important points about kWh

  • From this discussion, we conclude that 1 kilowatt-hour is equal to 3.6 × 106 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.
See also  Derivation of Gravitational potential energy equations

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

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