Power and the kilowatt-hour: Class 9 practice
Power = energy ÷ time, measured in watts. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Multiplication of decimals
The idea behind power and the kilowatt-hour
Power = energy ÷ time, measured in watts. Energy used by appliances is billed in kilowatt-hours: kWh = kW × hours. 1 kWh = 3.6 × 10⁶ J.
Step-by-step method
- Convert W to kW (÷ 1000).
- Multiply by hours used.
- Multiply units by the rate for the bill.
Worked example 1 (Basic)
A 1500 W appliance runs for 1 hours. How many kWh does it use?
- 1.5 kW × 1 h
- = 1.5 kWh
- Answer: 1.5 kWh
Worked example 2 (Standard)
Convert 9 kWh to joules.
- × 3.6 × 10⁶
- = 3.24e+7 J
- Answer: 3.24e+7 J
Worked example 3 (Challenge)
A 1500 W heater runs 5 h a day for 24 days at ₹4 per unit. Find the bill.
- Units = 1.5 × 5 × 24 = 180 kWh
- Bill = 180 × 4 = ₹720
- Answer: ₹720
Common mistakes and misconceptions
- Using watts directly
- Forgetting to multiply by number of days
Practice questions
- (Basic) A 200 W appliance runs for 9 hours. How many kWh does it use?
- (Basic) A 200 W appliance runs for 7 hours. How many kWh does it use?
- (Standard) Convert 10.5 kWh to joules.
- (Standard) Convert 0.6 kWh to joules.
- (Challenge) A 100 W heater runs 2 h a day for 27 days at ₹7 per unit. Find the bill.
- (Challenge) A 100 W heater runs 8 h a day for 24 days at ₹8 per unit. Find the bill.
Answer key with full working
- Q1: 0.2 kW × 9 h → = 1.8 kWh → Answer: 1.8 kWh
- Q2: 0.2 kW × 7 h → = 1.4 kWh → Answer: 1.4 kWh
- Q3: × 3.6 × 10⁶ → = 3.78e+7 J → Answer: 3.78e+7 J
- Q4: × 3.6 × 10⁶ → = 2.16e+6 J → Answer: 2.16e+6 J
- Q5: Units = 0.1 × 2 × 27 = 5.4 kWh → Bill = 5.4 × 7 = ₹37.8 → Answer: ₹37.8
- Q6: Units = 0.1 × 8 × 24 = 19.2 kWh → Bill = 19.2 × 8 = ₹153.6 → Answer: ₹153.6
Syllabus note
This skill is listed under "Work and Energy" for Class 9 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is power and the kilowatt-hour in?
"Work and Energy", Class 9 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.