Kilowatt-hour
Explanation
In physics, it is fundamental to distinguish between Power and Energy. Power is defined as the rate at which work is done or energy is transferred, while Energy is the quantitative property that must be transferred to an object to perform work. The mathematical relationship is:
Energy = Power × Time
Option Analysis:
- Watt: Incorrect. The Watt (W) is the SI derived unit of Power, defined as one joule per second ($1 \text{ W} = 1 \text{ J/s}$). It measures the rate of energy consumption, not the total energy itself.
- Kilowatt: Incorrect. The Kilowatt (kW) is simply a multiple of the Watt ($1 \text{ kW} = 1000 \text{ W}$). It remains a unit of Power.
- Kilowatt-hour Kilowatt-hour: Correct. The Kilowatt-hour (kWh) is a composite unit derived by multiplying power (Kilowatt) by time (hour). Therefore, it is a unit of Energy. It is commonly used as a billing unit for electrical energy.
Conversion: $1 \text{ kWh} = 1000 \text{ J/s} \times 3600 \text{ s} = 3.6 \times 10^6 \text{ Joules}$. - Horsepower: Incorrect. Horsepower (hp) is a unit of Power used primarily in mechanical engineering. One mechanical horsepower is approximately equivalent to 746 Watts.
Key Takeaway:
Any unit of Power (e.g., Watt, Kilowatt, Horsepower) multiplied by a unit of Time (e.g., hour, second) results in a unit of Energy (e.g., Kilowatt-hour, Watt-second/Joule).