This guide covers how to calculate the power and energy used by electrical devices, how electricity bills are calculated, and the essential components of a safe domestic wiring circuit. These are practical skills and important topics for your exam.

1. Electrical Power and Energy

Short Notes & Key Concepts

• Power (P): The rate at which an appliance consumes energy. In simpler terms, it's how "powerful" the device is.

  • Unit: Watt (W).

  • Formula: P = V × I

    • P = Power in Watts (W)

    • V = Voltage in Volts (V)

    • I = Current in Amperes (A)

• Energy (E): The total amount of electricity an appliance consumes over a period of time.

  • Unit: Joules (J).

  • Formula 1: E = P × t

  • Formula 2 (combining both): E = V × I × t

    • E = Energy in Joules (J)

    • P = Power in Watts (W)

    • V = Voltage in Volts (V)

    • I = Current in Amperes (A)

    • t = time in seconds (s)

2. The Kilowatt-Hour (kWh) - The "Unit" of Electricity

Joules are too small for household billing. Instead, we use a larger unit. The electricity meter in your home measures energy in kilowatt-hours (kWh), which is what you see on your bill as a "unit".

• Definition: One kilowatt-hour (1 kWh) is the energy consumed by a 1 kW appliance operating for 1 hour.

• Calculation:

  • Energy (kWh) = Power (kW) × time (h)

• Important Conversions:

  • To convert Watts (W) to kilowatts (kW): kW = W / 1000

  • 1 kWh = 3,600,000 J or 3.6 × 10⁶ J

Example Calculation: How many units (kWh) does a 2000 W electric heater use in 3 hours?

1. Convert Power to kW: 2000 W / 1000 = 2 kW

2. Calculate Energy: Energy = 2 kW × 3 h = 6 kWh (This is 6 "units" on your bill).

3. Energy Efficiency

• Concept: Getting the same job done using less energy. An efficient appliance has a lower power rating (fewer Watts) for the same output.

• Best Example (Common Exam Question): Light Bulbs

  • Filament Bulb: ~60 W (very inefficient, most energy is lost as heat)

  • CFL Bulb: ~11 W (more efficient)

  • LED Bulb: ~6 W (most efficient, lasts the longest)

• Energy Conservation Tips: Switch off unused lights/appliances, use energy-efficient bulbs (LEDs), and iron clothes in one go to avoid reheating the iron.

4. Domestic Electric Circuits (House Wiring)

Safety is the most important aspect of house wiring. You need to know the function of each key component.

• Parallel Connection: All sockets and lights in a house are connected in parallel. This ensures that each appliance gets the full 230 V supply and can be switched on/off independently.

Exam Tips & Tricks

1. Watch Your Units! This is the most common place to lose marks.

   • For E = VIt, time must be in seconds.

   • For Energy = Power × time in kWh, Power must be in kW and time must be in hours.

2. Fuse vs. RCCB: This is a classic question. Remember: Fuses/MCBs protect the appliance. RCCBs protect you.

3. Safety First: In any question about safety, mention the Earth Wire and the RCCB. Also, remember that switches and fuses are placed on the Live Wire so that when they break the circuit, the appliance is completely disconnected from the high voltage.

4. Calculations are Practical: kWh calculation questions are very popular because they relate to real-life electricity bills. Practice them well.