1. Why We Need a Commercial Unit of Energy
In physics, we normally measure energy in Joules. But in daily life, especially for electricity usage, Joules are too small. Appliances consume thousands or millions of Joules every day.
To make billing easier, electricity companies use a bigger and more practical unit: the kilowatt-hour (kWh).
1.1. Where This Unit Is Used
The kilowatt-hour is used on electricity bills, in household appliances, and in industries to measure energy consumption.
2. What Is a Kilowatt-Hour (kWh)?
A kilowatt-hour is the amount of energy used by a device with a power of 1 kilowatt running for 1 hour.
2.1. Definition
\( 1 \text{ kWh} = 1 \text{ kW} \times 1 \text{ hour} \)
Since 1 kW = 1000 W and 1 hour = 3600 seconds:
\( 1 \text{ kWh} = 1000 \times 3600 \text{ J} = 3.6 \times 10^6 \text{ J} \)
2.2. Why It Is a Convenient Unit
Instead of saying an appliance used 3,600,000 Joules, we simply say 1 kWh — easier to read and calculate.
3. Examples of Energy Consumption
Different devices use different amounts of electrical energy. The kilowatt-hour helps compare them easily.
3.1. Example 1: Electric Iron
If an iron is rated 1000 W and used for 1 hour:
Energy used = 1 kWh
3.2. Example 2: Ceiling Fan
A ceiling fan rated 75 W used for 4 hours:
\( E = 0.075 \text{ kW} \times 4 = 0.3 \text{ kWh} \)
3.3. Example 3: Refrigerator
A 200 W refrigerator running 10 hours a day consumes:
\( 0.2 \text{ kW} \times 10 = 2 \text{ kWh/day} \)
4. Electricity Bills and kWh
Your electricity bill shows the number of units consumed. Here, 1 unit = 1 kWh.
4.1. Reading a Bill
If your bill shows 150 units consumed, that means:
Energy used = 150 kWh
4.2. Cost Calculation
If cost per unit is ₹6:
Total bill = 150 × 6 = ₹900
5. Comparing Joule and kWh
While Joules are used in scientific calculations, kilowatt-hours are much more convenient for everyday energy usage.
5.1. Conversion Recap
\( 1 \text{ kWh} = 3.6 \times 10^6 \text{ J} \)
This means 1 kWh contains as much energy as 3,600,000 Joules.