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What is Commercial Unit of Energy and its Explanation

Grade 9
Aug 22, 2022

Key Concepts

  • Commercial unit
  • Standard mechanical meters
  • Dial meters
  • Digital meters


You must have observed that you receive an electric bill every month from the service provider of your area. It contains the energy consumed by you for that month along with the cost that you need to pay.  

In this session we are going to learn about energy consumption, its unit and the method to calculate this cost that you are required to pay. 


Commercial unit of energy (kWh or “unit”) 

In your previous session you learnt about energy consumption. You also learnt how to mathematically calculate it if the power rating (W) and the time for which the appliance was on (s) is given to you.  


P= EtP= Et


ÞE = P × t 


E is the energy consumed (its SI unit is Joule), 

P is the power rating (its SI unit is Watt) and 

t is  the time taken (its SI unit  is second). 


The unit of the measurement taken was Joule (J) or watt-seconds (Ws). This is the unit if the power consumption is less, in case the power consumption is more that is in the case of commercial uses in the house or industries, the unit that is taken is kWh or “units.” 

For the small unit of electrical energy  

1.0 joule = 1.0 watt × 1.0 second 

For larger unit of electrical energy,  

1.0 kilowatt-hour = 1.0 kilowatt × 1.0 hour 


1.0 kWh = 1.0 kW × 1.0 h 

kWh is the commercial unit of energy. It is also known as “unit.”  

Relation between 1 kWh and Joules: 

One kilowatt-hour is equivalent to the energy of 1,000 joules used for 3,600 seconds or 3.6 million Joules. 

1 kWh = 1 kW × 1 h 

= 1000 W × (60 × 60) s 

= 1000 W × 3600 s 

= 3600000 Ws 

= 3.0 × 106

Electric meters: 

To measure this electrical consumption there are electric meters. These meters measure the electrical consumption in kWh or in “units.”  

Different kinds of meters are there to note the readings of the power electrical consumption. Some of them are: 

  1. Standard mechanical meters 
  1. Dial meters 
  1. Digital meters 

Digital meter based on the payment mode can further be classified into two types. 

  1. Prepayment meter in which you pay a fixed amount every month.  
  1. The standard type in which you consume electricity and receive a bill equivalent to the amount consumed at the end of the month or a quarter.  

Standard mechanical meters  

  1. They are quite common. 
  1. They have a mechanical display to show the readings. 
  1. You must only take into account the five black numbers from left to right which are displayed ignoring any numbers in red. 
Standard mechanical meters 

Dial meters 

Dial meters 
  1. They look like traditional clocks and are a little complicated to read.  
  1. You would notice that in the dial meters, the dials are moving in opposite directions. 
  1. To read the numbers, you should read the numbers on the dials (again left to right); ignore the red dial or the dials in which there are no figures mentioned.  
  1. While an accurate measurement is not possible, dial meters are good in offering a rough estimate of the electricity consumed over a period. 

Digital meters 

 Digital meters 
  1. Digital meters are the easiest to read.  
  1. They give an accurate measure.  
  1. Use the first five figures on the display while ignoring the last figure that begins with 0.1 to note the reading.  

Questions and answers 

Question 1: How much kWh of energy is present in 1 Joule? 


It is known that 

1kWh = 3.6 × 106 Joules 

Using the Unitary method, 

1 Joule =


× 106 (kWh) 

1 Joule = 2.77 × 10-7 kWh 

Question 2: An Electric Iron of 2000 W is used for 1 Hour every day. Find the Electrical Energy Consumed by the Iron in a week in “units”? 


1 week = 7 day 

1 hour every day = 7 Hours 

Power consumed in 1 hour = 2000 W 

Energy = Power × Time 

Energy = 2000 W × 7 h 

Energy = 2 kW × 7 h 

Energy = 14 kWh 

Hence, The Electrical energy consumed by the electric iron in a week is 14 units. 


  • For the smaller magnitude of electrical energy, energy consumption is measured in Joule.
  • For the higher magnitude of electrical energy, energy consumption is measured in kWh.
    • 1 kWh = 3.6 x10 J
  • Electric metre used to record the energy consumption.
  • Different electric meters are used such as:
  1. Standard mechanical meters
  2. Dial meters
  3. Digital meters


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