Correct Option
The correct option is Kilowatt-hour.
Explanation
Electrical energy is defined as the product of electric power and the time duration for which it is consumed ($E = P \times t$). While the standard SI unit for energy is the Joule (J), it is often too small for expressing large quantities of energy consumed in households and industries. Therefore, a larger derived unit is utilized for commercial purposes.
Detailed Analysis
- Kilowatt-hour (kWh): This is the commercial unit of electrical energy. It represents the energy consumed by an appliance with a power rating of 1 kilowatt (1000 Watts) operating for one hour. In electricity billing, 1 kWh is referred to as one "unit."
Conversion: $1 \text{ kWh} = 1000 \text{ W} \times 3600 \text{ s} = 3.6 \times 10^6 \text{ Joules}$. - Watt-second: This is equivalent to one Joule ($1 \text{ J} = 1 \text{ W} \times 1 \text{ s}$). It is the fundamental SI unit of energy but is impractical for commercial metering due to its small magnitude.
- Joule-second: This unit represents the product of energy and time, which corresponds to the physical quantity known as "Action" (e.g., Planck's constant), not energy consumption.
- Ampere-hour (Ah): This is a unit of electric charge ($Q = I \times t$), commonly used to specify the capacity of batteries, not the total energy consumed.
Key Takeaway: The commercial unit of electrical energy is the Kilowatt-hour (kWh), which is equal to $3.6$ Megajoules ($3.6 \times 10^6 \text{ J}$).