Unit 4.7 – Commercial Unit of Energy

1. Usage in Electricity Bills:
   • The electricity consumed by households is measured in kilowatt-hours (kWh).
   • Electricity meters record the total energy consumption in kWh, which is then used to calculate the bill.
2. Cost of Energy:
   • The cost of electricity is often expressed as price per kilowatt-hour (e.g., ₹5 per kWh).
   • Total cost = Energy consumed (in kWh) × Cost per kWh.
3. Examples of Energy Consumption:
   • A 100 W bulb operating for 10 hours consumes $100\text{W}\times 10\text{hours}=1000\text{Wh}=1\text{kWh}$.
   • A 2 kW air conditioner running for 5 hours consumes $2\text{kW}\times 5\text{hours}=10\text{kWh}$.

Quick Review, Exam Tips, Tricks & Traps

1. The commercial unit of energy is the kilowatt-hour (kWh).
2. $1\text{kWh}=3.6\times {10}^6\text{J}$.
3. Energy consumption in households is measured in kWh, and electricity bills are calculated based on this unit.

1. Always convert power into kilowatts (kW) and time into hours before calculating energy in kWh.
2. Use the formula:
   $$\text{Energy (kWh)}=\text{Power (kW)}\times \text{Time (hours)}$$
3. For questions involving cost, multiply the energy consumed (in kWh) by the cost per kWh.

1. Students often forget to convert power into kilowatts or time into hours, leading to incorrect answers.
2. Misinterpreting the relationship between power, time, and energy. For example, doubling the power while keeping time constant doubles the energy consumed.

1. Use proportional reasoning:
   • If power doubles, energy consumed doubles (for the same time).
   • If time doubles, energy consumed doubles (for the same power).
2. In MCQs, eliminate options where energy decreases with increasing power or time—it’s impossible unless the device is turned off.

Additional Content: Practical Applications of kWh

• A typical household consumes between 200–500 kWh of electricity per month, depending on usage patterns.
• Appliances like refrigerators, air conditioners, and water heaters consume significant amounts of energy.

• Factories and industries use megawatt-hours (MWh) to measure large-scale energy consumption.
• For example, a factory consuming 1 MW of power for 10 hours uses $10\text{MWh}=10,000\text{kWh}$.

• Solar panels and wind turbines generate electricity, and their output is often measured in kWh.
• For instance, a solar panel system might generate 5 kWh of energy per day under ideal conditions.

• The energy consumed in kWh directly impacts carbon emissions.
• Reducing energy consumption by using energy-efficient appliances helps lower electricity bills and environmental impact.