Commercial Unit of Energy: Kilowatt-hour

Introduction

Energy is used in many forms in our daily life. In homes, schools, offices, shops, and industries, electrical energy is used to run fans, bulbs, refrigerators, computers, motors, heaters, and many other appliances.

The SI unit of energy is the joule (J). However, joule is a very small unit for measuring the large amount of electrical energy consumed in daily life. Therefore, electrical energy is commonly measured in a larger unit called the kilowatt-hour.

Commercial Unit of Energy

The commercial unit of energy is the kilowatt-hour.

It is commonly written as kWh.

Commercial unit of energy = kilowatt-hour (kWh)

In electricity bills, one kilowatt-hour is commonly called one unit of electrical energy.

1 unit of electricity = 1 kWh

Definition of Kilowatt-hour

One kilowatt-hour is the amount of electrical energy consumed by an electrical appliance of power 1 kilowatt when it is used for 1 hour.

\(1\,kWh = 1\,kW \times 1\,h\)

Since,

\(1\,kW = 1000\,W\)

and

\(1\,h = 3600\,s\)

Relation Between kWh and Joule

We know that energy is given by

\(E = P \times t\)

For one kilowatt-hour,

\(1\,kWh = 1000\,W \times 3600\,s\)

\(1\,kWh = 3,600,000\,J\)

Therefore,

\(1\,kWh = 3.6 \times 10^6\,J\)

Thus, one commercial unit of electrical energy is equal to \(3.6 \times 10^6\) joule.

Figure

Commercial Unit of Electrical Energy kWh Electricity Meter Electrical Appliance
\(E=P\times t\)
\(1\,kWh\)
\(=3.6\times10^6J\)
Electricity bill is calculated in units, where 1 unit = 1 kWh.

Explanation

The above figure shows an electricity meter connected to an electrical appliance. The meter records the electrical energy consumed by the appliance.

This energy is not measured in joules on electricity bills because joule is too small for practical use. Instead, the electricity meter records energy in kilowatt-hour.

When an appliance of power 1 kW works for 1 hour, it consumes 1 kWh of electrical energy. This is called one unit of electricity.

Why is kWh Used Instead of Joule?

The joule is a small unit of energy. Electrical appliances consume a large amount of energy over many hours, so expressing this energy in joules gives very large numbers.

For example,

\(1\,kWh = 3,600,000\,J\)

Writing energy consumption in kilowatt-hour is simpler and more convenient for practical and commercial purposes.

Electricity Bill and Unit

Electricity companies charge consumers according to the number of units of electrical energy consumed.

The number of units consumed is calculated by using the formula:

\(\text{Units consumed}=\frac{\text{Power in watt} \times \text{Time in hour}}{1000}\)

or

\(\text{Units consumed}=\text{Power in kW} \times \text{Time in hour}\)

Example 1

A bulb of power 100 W is used for 10 hours. Find the electrical energy consumed in units.

Given,

  • Power = \(100\,W\)
  • Time = \(10\,h\)

Using,

\(\text{Units}=\frac{P \times t}{1000}\)

\(\text{Units}=\frac{100 \times 10}{1000}\)

\(\text{Units}=1\,kWh\)

Therefore, the bulb consumes 1 unit of electrical energy.

Example 2

An electric heater of power 2 kW is used for 3 hours. Find the electrical energy consumed.

Given,

  • Power = \(2\,kW\)
  • Time = \(3\,h\)

Using,

\(\text{Energy consumed}=P \times t\)

\(\text{Energy consumed}=2 \times 3\)

\(\text{Energy consumed}=6\,kWh\)

Therefore, the heater consumes 6 units of electrical energy.

Comparison Between Joule and Kilowatt-hour

Joule Kilowatt-hour
It is the SI unit of energy. It is the commercial unit of energy.
It is a small unit. It is a large unit.
It is used in scientific calculations. It is used in electricity bills.
\(1\,J\) is a small amount of energy. \(1\,kWh = 3.6 \times 10^6\,J\)

Examples of Energy Consumption

  • A 100 W bulb used for 10 hours consumes 1 unit of electrical energy.
  • A 1 kW electric iron used for 1 hour consumes 1 unit of electrical energy.
  • A 2 kW heater used for 2 hours consumes 4 units of electrical energy.
  • A 500 W fan used for 4 hours consumes 2 units of electrical energy.
  • A 1.5 kW air conditioner used for 8 hours consumes 12 units of electrical energy.

Importance of Commercial Unit of Energy

  • It is used to measure electrical energy consumed in homes, schools, shops, offices, and industries.
  • It makes large energy values easy to read and calculate.
  • It is used by electricity boards and power companies for preparing electricity bills.
  • It helps consumers understand their electricity consumption.
  • It helps in comparing the energy consumption of different electrical appliances.

Important Points

  • The SI unit of energy is the joule (J).
  • The commercial unit of energy is the kilowatt-hour (kWh).
  • One kilowatt-hour is commonly called one unit of electricity.
  • One kilowatt-hour is the energy consumed by an appliance of power 1 kW when used for 1 hour.
  • \(1\,kWh = 1\,kW \times 1\,h\).
  • \(1\,kWh = 3.6 \times 10^6\,J\).
  • Electricity bills are calculated on the basis of units consumed.
  • The number of units consumed is given by \(\text{Units}=\frac{P \times t}{1000}\), when power is in watt and time is in hour.
  • Kilowatt-hour is more convenient than joule for measuring electrical energy consumption in daily life.

Conclusion

The commercial unit of energy is the kilowatt-hour (kWh). It is mainly used for measuring electrical energy consumed by electrical appliances. One kilowatt-hour is the energy consumed by an appliance of power 1 kilowatt when it is used for 1 hour. In electricity bills, one kilowatt-hour is called one unit. The relation between kilowatt-hour and joule is \(1\,kWh = 3.6 \times 10^6\,J\). This unit is convenient for practical and commercial measurement of electrical energy.

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