In electrical energy calculations, a kilowatt-hour (kWh) is a practical unit used for measuring which physical quantity?

Introduction / Context:
This question focuses on the distinction between power and energy in the context of household electricity usage. Electricity bills often show consumption in kilowatt-hours, so many people encounter this term in daily life. However, learners sometimes confuse kilowatt, which is a unit of power, with kilowatt-hour, which is a unit related to energy. Understanding the difference is essential for correctly interpreting electric bills and solving numerical problems in physics and electrical engineering.

Given Data / Assumptions:

• The unit mentioned is kilowatt-hour, abbreviated as kWh.
• Power is the rate of doing work or using energy, measured in watts or kilowatts.
• Energy is the total work done or total electrical energy consumed, measured in joules in SI units.
• The options include power, electric charge, energy and electric current.

Concept / Approach:
A kilowatt-hour is defined as the amount of energy consumed when a device with a power rating of one kilowatt (1000 watt) operates for one hour. Power is energy per unit time, so multiplying power by time gives energy. In symbols, energy E = P * t. If P is in kilowatt and t is in hour, then E is in kilowatt-hour. Thus, kilowatt-hour is a unit of energy, not of instantaneous power, charge or current. Electric energy companies use kWh because it is a convenient measure for the relatively large amounts of energy consumed in homes and industries.

Step-by-Step Solution:
Step 1: Recall that power P is measured in watt (W) or kilowatt (kW) and represents energy used per second. Step 2: Recall that energy E can be calculated from power and time using E = P * t. Step 3: Consider a device rated at 1 kW running for 1 hour. The total energy used is E = 1 kW * 1 hour = 1 kilowatt-hour. Step 4: Observe that kilowatt-hour therefore represents a quantity of energy, equal in SI units to 1000 watt * 3600 second = 3.6 * 10^6 joule. Step 5: Conclude that kilowatt-hour is a unit of energy, and select the option corresponding to energy.

Verification / Alternative check:
A simple way to confirm is to look at an electricity bill, which shows how many kilowatt-hour have been consumed in a month. Bills charge for energy used, not for power capacity or instantaneous current. Also, physics textbooks explicitly state that 1 kWh is a commercial unit of electric energy. Since both everyday usage and textbook definitions align, we can be confident that kilowatt-hour is indeed a unit of energy.

Why Other Options Are Wrong:
Power is measured in watt or kilowatt, not in kilowatt-hour. Kilowatt-hour includes time and therefore belongs to energy, not power. Electric charge is measured in coulomb in SI units, not in kilowatt-hour; it represents quantity of charge, not energy. Electric current is measured in ampere and refers to the rate of flow of charge, again not represented by kilowatt-hour.

Common Pitfalls:
Many learners mistakenly think that because the word kilowatt appears inside kilowatt-hour, both must be units of power. The key to avoid this confusion is to remember that adding hour converts a rate into a total amount, just as kilometres per hour is a speed while kilometre is a distance. Kilowatt describes how fast energy is used, while kilowatt-hour describes how much energy has been used over a period of time.

Final Answer:
A kilowatt-hour (kWh) is a practical unit of energy, commonly used for measuring electrical energy consumption.