The correct option is (c) 2 and 3 only.
Explanation
Electrical energy is the product of power and time. While the SI unit of energy is the Joule (J), it is often too small for practical commercial applications. Therefore, a larger unit known as the kilowatt-hour (kWh) is utilized for measuring electrical energy consumption in households and industries.
Statement-wise Analysis
- Statement 1 is Incorrect: The kilowatt (kW) is a unit of power, which represents the rate at which energy is consumed or generated. The commercial unit of energy is the kilowatt-hour (kWh). Energy is calculated as Power × Time ($E = P \times t$).
- Statement 2 is Correct: The Joule is defined as one watt-second ($1 \text{ J} = 1 \text{ W} \times 1 \text{ s}$). Since commercial energy consumption involves large quantities of energy over hours or days, expressing it in Joules would result in inconveniently large numbers. For example, $1 \text{ kWh} = 3.6 \times 10^6 \text{ Joules}$. The kilowatt-hour provides a more manageable figure for recording and billing purposes.
- Statement 3 is Correct: In commercial electricity metering and billing, the term "one unit" specifically refers to one kilowatt-hour (1 kWh). This represents the energy consumed by an appliance rated at 1000 watts (1 kW) operating for one hour.
Key Takeaway: Power is measured in kilowatts (kW), while Energy is measured in kilowatt-hours (kWh). The commercial "unit" of electricity is equal to 1 kWh, which is equivalent to $3.6 \times 10^6$ Joules.