A 3 kW heater runs for 2 hours. How many kilowatt-hours of energy does it use?

6 kWh. Energy in kWh = power in kW × time in hours.

What is a kilowatt-hour a unit of?

energy. It is the energy transferred by 1 kW in 1 hour (3.6 million joules).

A 230 V kettle has a power of 2300 W. What current does it take?

10 A. I = P ÷ V = 2300 ÷ 230.

Which appliance transfers the most energy?

a 3 kW oven on for 1 hour. Energy = power × time: 1 kWh, 0.5 kWh, 3 kWh and 0.5 kWh.

Electrical power and energy · Edexcel IGCSE Physics revision

In plain words

Power is how fast an appliance transfers energy. A 2000 W kettle transfers 2000 joules every second. Multiply the current by the voltage to get the power, and the power by the time to get the energy. Electricity bills use a bigger unit of energy, the kilowatt-hour.

Three things to know

Power = current × potential difference (P = IV), in watts. A 2 kW kettle transfers 2000 joules every second.
Energy transferred = power × time (E = Pt = IVt), in joules.
Electricity is sold by the kilowatt-hour: 1 kWh is the energy a 1 kW appliance uses in 1 hour. Energy (kWh) = power (kW) × time (h).

Worked example

An iron takes 5 A from a 230 V supply. Find its power.

P = I × V.
= 5 × 230 = 1150 W.

Tips and tricks

For the cost of electricity: kilowatts × hours × price per unit. Change watts to kilowatts (÷ 1000) and minutes to hours first.
A kilowatt-hour is a unit of energy, not of power.

6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Electrical power and energy: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

A lamp takes 2 A from a 12 V supply. What is its power?
- 6 W
- 14 W
- 24 W (the answer)
- 144 W
P = IV = 2 × 12.
How much energy does a 100 W appliance transfer in 10 s?
- 10 J
- 110 J
- 1000 J (the answer)
- 10 000 J
E = Pt = 100 × 10.
A 3 kW heater runs for 2 hours. How many kilowatt-hours of energy does it use?
- 1.5 kWh
- 5 kWh
- 6 kWh (the answer)
- 6000 kWh
Energy in kWh = power in kW × time in hours.
What is a kilowatt-hour a unit of?
- power
- energy (the answer)
- current
- time
It is the energy transferred by 1 kW in 1 hour (3.6 million joules).
A 230 V kettle has a power of 2300 W. What current does it take?
- 0.1 A
- 10 A (the answer)
- 23 A
- 2530 A
I = P ÷ V = 2300 ÷ 230.
Which appliance transfers the most energy?
- a 100 W lamp on for 10 hours
- a 2 kW heater on for 15 minutes
- a 3 kW oven on for 1 hour (the answer)
- a 500 W drill on for 1 hour
Energy = power × time: 1 kWh, 0.5 kWh, 3 kWh and 0.5 kWh.

6 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

4 questions, 10 marks. Write your answers on paper, then check them.

Electrical power and energy

Edexcel International GCSE Physics 4PH1 · 10 marks · papermunch.org

Name ______________________________ Date ______________

A heater takes a current of 10 A from a 230 V supply. Calculate its power.[2]
Show answerHide answer
2300 W. P = IV = 10 × 230.
I got this one
Calculate the energy transferred by a 60 W lamp in 5 minutes.[2]
Show answerHide answer
18 000 J. E = Pt = 60 × 300.
I got this one
A 2.0 kW heater is used for 3 hours. Electricity costs 15p per kWh. Calculate the cost.[3]
Show answerHide answer
90p. Energy = 2.0 × 3 = 6.0 kWh; cost = 6.0 × 15.
I got this one
A 12 V motor takes a current of 2.0 A. Calculate the energy it transfers in 1 minute.[3]
Show answerHide answer
1440 J. E = IVt = 2.0 × 12 × 60.
I got this one

Answers: Electrical power and energy

1. 2300 W. P = IV = 10 × 230.
2. 18 000 J. E = Pt = 60 × 300.
3. 90p. Energy = 2.0 × 3 = 6.0 kWh; cost = 6.0 × 15.
4. 1440 J. E = IVt = 2.0 × 12 × 60.