In electrical engineering, the standard SI derived unit used to measure electric power is the:

Introduction / Context:
Electric power is the rate at which electrical energy is transferred or converted per unit time. It is an important quantity when describing the performance of electrical devices such as bulbs, motors, and heaters. Each physical quantity has an associated SI unit, and knowing the correct unit for power helps in interpreting ratings like 60 W lamps or 2 kW heaters. This question asks you to identify the correct SI derived unit used specifically for electric power.

Given Data / Assumptions:

• The focus is on electric power, not on energy, voltage, or angle.
• Options include steradian, watt, joule, and volt.
• We assume the standard relation P = V * I for power, where P is power, V is voltage, and I is current.
• We also recall the relationship between power and energy: energy equals power multiplied by time.

Concept / Approach:
In the SI system, power is measured in watts, symbol W. One watt is defined as one joule of energy transferred per second, W = J/s. Electric power is often calculated as P = V * I, the product of potential difference and current. Joule is the SI unit of energy, not power, and volt is the SI unit of electric potential difference. Steradian is the SI unit of solid angle. Therefore, among the given choices, only watt correctly represents the unit of electric power.

Step-by-Step Solution:
Step 1: Recall that power is defined as energy transferred per unit time, mathematically P = E / t. Step 2: In SI units, energy E is measured in joules (J) and time t in seconds (s), so power P is measured in joules per second, J/s. Step 3: The derived SI unit representing one joule per second is called the watt, symbol W. Step 4: In electric circuits, power is also expressed as P = V * I, and the result is still measured in watts. Step 5: Therefore, the correct answer is watt, not joule, volt, or steradian.

Verification / Alternative check:
Look at common electrical appliance ratings. A typical light bulb might be rated 60 W, a microwave oven 800 W, and a room heater 2000 W or 2 kW. Energy bills often show consumption in kilowatt hours (kWh), which is power in kilowatts multiplied by time in hours. This reinforces the idea that watt is the unit of power while energy is often measured in joules or kilowatt hours. Volt appears in voltage ratings such as 230 V mains supply, and joule appears in mechanical and electrical energy calculations, confirming their different roles.

Why Other Options Are Wrong:
Steradian: This is the SI unit of solid angle used in geometry and radiometry, not related to electric power. Joule: Joule is the SI unit of energy, not the rate at which energy is used or produced. Volt: Volt is the SI unit of electric potential difference, appearing in formulas for power but not itself a unit of power.

Common Pitfalls:
A common mistake is to confuse energy and power, using joule and watt interchangeably. Remember that energy tells you how much work is done, while power tells you how fast that work is done. Another confusion arises from kilowatt hour appearing on electricity bills; students may think it is a unit of power because it contains watt, but in fact it is a unit of energy. Keeping the definitions P = E / t and W = J/s in mind will help you choose the correct unit in exam questions.

Final Answer:
The standard SI derived unit for measuring electric power is the watt (symbol W).