Electric power and energy consumption: Class 10 practice
Power P = VI = I²R = V²/R, in watts. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Ohm's law
The idea behind electric power and energy consumption
Power P = VI = I²R = V²/R, in watts. Energy used = power × time; electricity bills count energy in kilowatt-hours (1 kWh = 1 'unit' = 3.6 × 10⁶ J).
Step-by-step method
- Find power in watts.
- Convert to kW (÷1000) and time to hours.
- Energy (kWh) = kW × hours.
- Cost = units × rate.
Worked example 1 (Basic)
An appliance draws 2 A from a 220 V supply. Find its power.
- P = VI = 220 × 2
- P = 440 W
- Answer: 440 W
Worked example 2 (Standard)
A 500 W appliance runs 7 hours a day for 30 days. How many units of electricity does it use?
- 500 W = 0.5 kW
- Energy = 0.5 × 7 × 30 = 105 kWh
- Answer: 105 units (kWh)
Worked example 3 (Challenge)
A 500 W heater runs 3 hours daily for 30 days at ₹7 per unit. Find the cost.
- Energy = 0.5 × 3 × 30 = 45 kWh
- Cost = 45 × ₹7 = ₹315
- Answer: ₹315
Common mistakes and misconceptions
- Multiplying watts by hours and calling it kWh
- Forgetting the number of days
Practice questions
- (Basic) An appliance draws 0.5 A from a 220 V supply. Find its power.
- (Basic) An appliance draws 0.5 A from a 12 V supply. Find its power.
- (Standard) A 200 W appliance runs 2 hours a day for 30 days. How many units of electricity does it use?
- (Standard) A 100 W appliance runs 8 hours a day for 30 days. How many units of electricity does it use?
- (Challenge) A 2000 W heater runs 4 hours daily for 30 days at ₹5 per unit. Find the cost.
- (Challenge) A 500 W heater runs 5 hours daily for 30 days at ₹8 per unit. Find the cost.
Answer key with full working
- Q1: P = VI = 220 × 0.5 → P = 110 W → Answer: 110 W
- Q2: P = VI = 12 × 0.5 → P = 6 W → Answer: 6 W
- Q3: 200 W = 0.2 kW → Energy = 0.2 × 2 × 30 = 12 kWh → Answer: 12 units (kWh)
- Q4: 100 W = 0.1 kW → Energy = 0.1 × 8 × 30 = 24 kWh → Answer: 24 units (kWh)
- Q5: Energy = 2 × 4 × 30 = 240 kWh → Cost = 240 × ₹5 = ₹1200 → Answer: ₹1200
- Q6: Energy = 0.5 × 5 × 30 = 75 kWh → Cost = 75 × ₹8 = ₹600 → Answer: ₹600
Syllabus note
This skill is listed under "Electricity" for Class 10 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
- Electricity chapter notes (Class 10)
- Ohm's law and resistors in series and parallel
- Resistivity and resistance of a wire
- Heating effect of current (Joule's law)
- Mixed series–parallel circuits
- Power rating, current and fuse choice
- Lens formula and magnification
- Mirror formula and magnification
- Refractive index and speed of light
- Power of a lens and lens combinations
- All Class 10 practice
Frequently asked questions
Which chapter is electric power and energy consumption in?
"Electricity", Class 10 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.