Electricity bills

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Lesson notes

Units of Energy

The **joule (J)** is the standard unit of energy.
One **kilojoule (kJ)** equals **1000 J**.
The **kilowatt hour (kWh)** is used for electricity bills because household energy use is very large in joules.
1 kWh = energy used by a **1 kW** device for **1 hour**.
1 kWh = **3 600 000 J** (since 1 kW = 1000 W and 1 h = 3600 s).

Calculating Energy in kWh

Energy (kWh) = **Power (kW)** × **Time (h)**.
Convert power from watts to kilowatts by dividing by 1000.
Convert time from minutes to hours by dividing by 60.
Example: A 2.2 kW oven used for 1 h transfers **2.2 kWh**.

Calculating Electricity Cost

Total cost (pence) = **Energy (kWh)** × **Price per kWh (pence)**.
As of April 2022, UK price was **28 p per kWh**.
To convert pence to pounds, divide by **100**.
Example: 179 kWh × 28 p = 5012 p = **£50.12**.

Using Meter Readings

Energy used = **present reading** – **previous reading** (in kWh).
Multiply the difference by the price per kWh to get the cost.
Always check units: cost can be in pence or pounds.

A simple circuit used to measure current. Power (W) = voltage (V) × current (A).

Slides

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Practice questions

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1.How many joules are in one kilojoule?
Easy
- A1000 J
- B100 J
- C10 000 J
- D1 000 000 J
2.How many seconds are in one hour?
Easy
- A3600 s
- B60 s
- C1000 s
- D360 s
3.How many joules are in one kilowatt hour?
Medium
- A3 600 000 J
- B3600 J
- C1000 J
- D1 000 000 J
4.An electric oven has a power of 2.2 kW and is used for 1 hour. How much energy does it transfer in kWh?
Medium
- A2.2 kWh
- B22 kWh
- C0.22 kWh
- D220 kWh
5.A television has a power of 0.2 kW and is used for 8 hours. How much energy does it transfer in kWh?
Medium
- A1.6 kWh
- B16 kWh
- C0.16 kWh
- D8.2 kWh
6.A hairdryer has a power of 1.8 kW and is used for 30 minutes. How much energy does it transfer in kWh?
Hard
- A0.9 kWh
- B54 kWh
- C9 kWh
- D0.09 kWh
7.Which equation is used to calculate the cost of an electricity bill?
Easy
- ATotal cost = Energy transferred × price per kWh
- BTotal cost = Energy transferred ÷ price per kWh
- CTotal cost = Power × time
- DTotal cost = Energy transferred + price per kWh
8.A meter reading at the start of the month is 2031 kWh and at the end is 2210 kWh. The price per kWh is 30 p. What is the cost of the monthly bill in pence?
Hard
- A5370 p
- B179 p
- C5012 p
- D537 p

More questions

9.A meter reading at the start of the month is 1500 kWh and at the end is 1750 kWh. The price per kWh is 25 p. What is the cost of the monthly bill in pence?
Hard
- A6250 p
- B250 p
- C43750 p
- D625 p
10.A meter reading at the start of the month is 2031 kWh and at the end is 2210 kWh. The price per kWh is 28 p. What is the cost of the monthly bill in pounds (£)?
Hard
- A£50.12
- B£5.012
- C£5012
- D£0.5012
11.Why are kilowatt hours used for electricity bills instead of joules?
Medium
- ABecause kilowatt hours give smaller numbers for typical household energy use
- BBecause joules are not a unit of energy
- CBecause kilowatt hours are easier to spell
- DBecause electricity is measured in kilowatts, not joules
12.Which of the following is a correct unit for energy?
Easy
- AJoule
- BWatt
- CKilowatt
- DAmpere
13.A household uses a 3 kW electric shower for 15 minutes. How much energy does it transfer in kWh?
Medium
- A0.75 kWh
- B1.25 kWh
- C45 kWh
- D0.45 kWh
14.A 2.5 kW air conditioner runs for 4 hours each day for 30 days. The price per kWh is 28 p. What is the total cost for the month in pounds?
Hard
- A£84.00
- B£8.40
- C£840.00
- D£0.84
15.A meter reading at the start of the month is 4500 kWh and at the end is 4785 kWh. The price per kWh is 28 p. What is the cost of the monthly bill in pounds?
Medium
- A£79.80
- B£7.98
- C£798.00
- D£0.80
16.A 100 W light bulb is left on for 10 hours. How much energy does it transfer in kWh?
Medium
- A1 kWh
- B0.1 kWh
- C10 kWh
- D1000 kWh
17.A 2 kW kettle is used for 3 minutes. How much energy does it transfer in kWh?
Medium
- A0.1 kWh
- B6 kWh
- C0.6 kWh
- D0.01 kWh
18.A household uses the following appliances per day: a 1.5 kW microwave for 0.5 h, a 2 kW heater for 3 h, and five 60 W bulbs for 4 h each. What is the total daily energy use in kWh?
Hard
- A8.7 kWh
- B7.5 kWh
- C9.3 kWh
- D6.9 kWh
19.An electricity bill shows a total cost of £36.00. If the price per kWh is 28 p, how many kWh were used?
Medium
- A128.57 kWh
- B100.8 kWh
- C3600 kWh
- D12.86 kWh
20.A solar panel generates 1.5 kW for 6 hours per day. How many days will it take to generate 90 kWh?
Hard
- A10 days
- B15 days
- C7.5 days
- D20 days
21.Which of the following is equivalent to 1 kWh?
Medium
- A3,600,000 J
- B3,600 J
- C36,000 J
- D360 J
22.An electric heater has a power of 2 kW and is used for 3 hours. How much energy does it transfer in kWh?
Easy
- A6 kWh
- B5 kWh
- C1.5 kWh
- D0.67 kWh
23.A meter reading at the start is 3200 kWh and at the end is 3450 kWh. The price per kWh is 30 p. What is the cost in pounds?
Medium
- A£75.00
- B£7.50
- C£750.00
- D£0.75
24.A 0.5 kW television is used for 4 hours per day, and a 2 kW washing machine is used for 1.5 hours per week. What is the total energy used by both in one week (7 days) in kWh?
Hard
- A17 kWh
- B14 kWh
- C20 kWh
- D11 kWh
25.A student measures the energy transferred by a 2.5 kW heater in 30 minutes using a joulemeter. The joulemeter reading is 4 500 000 J. Which of the following explains why the joulemeter reading is less than the expected 5 400 000 J?
Medium
- AThe heater was not operating at full power for the whole time.
- BThe joulemeter is incorrectly calibrated to measure in kWh.
- CThe time should have been measured in seconds, not minutes.
- DThe power should have been converted to watts before calculating.
26.A household has a 3 kW electric shower and a 2 kW kettle. The shower is used for 10 minutes each day, and the kettle is used for 5 minutes each day. The price per kWh is 28 p. What is the total daily cost of using both appliances?
Hard
- A18.7 p
- B14.0 p
- C4.7 p
- D23.3 p
27.Which of the following is the correct unit for the price of electricity on a household bill?
Easy
- Apence per kilowatt hour
- Bpence per joule
- Cpounds per kilowatt
- Dpence per kilowatt
28.A student is asked to determine the energy transferred by a 1.2 kW microwave in 2 minutes. Which of the following is the correct method to calculate the energy in kWh?
Medium
- AMultiply 1.2 by 2 and divide by 60.
- BMultiply 1.2 by 2 and divide by 3600.
- CMultiply 1.2 by 2 and multiply by 60.
- DMultiply 1.2 by 2 and multiply by 3600.
29.An electricity meter reading at the start of a month is 4500 kWh and at the end is 4785 kWh. The price per kWh is 28 p. What is the cost for the month in pounds?
Medium
- A£79.80
- B£7.98
- C£798
- D£0.80
30.A student investigates how the energy transferred by a fan heater depends on the time it is used. They measure the energy transferred using a joulemeter and record the following data: at 1 minute, 0.1 kWh; at 2 minutes, 0.2 kWh; at 3 minutes, 0.3 kWh; at 4 minutes, 0.4 kWh. What is the power of the fan heater?
Hard
- A6 kW
- B0.1 kW
- C0.6 kW
- D60 kW
31.Which of the following is equal to one kilowatt hour?
Easy
- A3 600 000 J
- B3600 J
- C1000 J
- D1 000 000 J
32.A household uses a 0.5 kW television for 4 hours per day and a 2 kW washing machine for 1.5 hours per week. What is the total energy used by both in one week (7 days)?
Medium
- A17 kWh
- B14.5 kWh
- C9.5 kWh
- D8 kWh
33.A student is given a 1.5 kW electric heater and a stopwatch. They want to measure the energy transferred in 10 minutes. Which of the following pieces of equipment is also needed to measure the energy directly?
Hard
- AA joulemeter
- BAn ammeter
- CA voltmeter
- DA balance
34.A student sets up a circuit with a 2.5 A current and a 12 V supply. They want to calculate the energy transferred in 30 minutes in kWh. Which of the following sequences of calculations is correct?
Hard
- ACalculate power = 2.5 × 12 = 30 W = 0.03 kW, then energy = 0.03 × 0.5 = 0.015 kWh
- BCalculate power = 2.5 × 12 = 30 kW, then energy = 30 × 0.5 = 15 kWh
- CCalculate power = 2.5 × 12 = 30 W = 0.03 kW, then energy = 0.03 × 30 = 0.9 kWh
- DCalculate power = 2.5 × 12 = 300 W = 0.3 kW, then energy = 0.3 × 0.5 = 0.15 kWh
35.Which of the following is a unit of energy?
Easy
- AJoule
- BWatt
- CVolt
- DAmpere
36.One kilowatt hour is equal to how many joules?
Easy
- A3 600 000 J
- B1000 J
- C3600 J
- D1 000 000 J
37.Which of the following is the correct equation to calculate electrical energy transferred in kilowatt hours?
Medium
- AEnergy (kWh) = Power (kW) × Time (h)
- BEnergy (kWh) = Power (W) × Time (s)
- CEnergy (kWh) = Power (kW) ÷ Time (h)
- DEnergy (kWh) = Power (W) × Time (h)
38.A 3 kW electric shower is used for 15 minutes. How much energy does it transfer in kWh?
Medium
- A0.75 kWh
- B0.25 kWh
- C45 kWh
- D1.25 kWh
39.A household uses a 2.5 kW air conditioner for 4 hours each day for 30 days. The price per kWh is 28 p. What is the total cost for the month in pounds?
Medium
- A£84.00
- B£8.40
- C£840.00
- D£0.84
40.A household uses the following appliances per day: a 1.5 kW microwave for 0.5 h, a 2 kW heater for 3 h, and five 60 W bulbs for 4 h each. What is the total daily energy consumption in kWh?
Hard
- A8.7 kWh
- B7.5 kWh
- C9.0 kWh
- D6.5 kWh

Revision notes

## Electricity Bills - **Energy units**: Joule (J), kilojoule (kJ = 1000 J), kilowatt hour (kWh = 3,600,000 J). kWh is used for bills because it keeps numbers manageable (average UK home uses ~3000 kWh/year). - **Energy formula**: Energy (kWh) = Power (kW) × Time (h). - **Cost formula**: Total cost (pence) = Energy (kWh) × Price per kWh (pence). - **Example**: Meter readings 2031 → 2210 kWh, difference = 179 kWh. Cost = 179 × 28 = 5012 p = £50.12. - **Conversion**: p ÷ 100 → £.

Kilowatt hour (kWh): Unit of energy equal to 3,600,000 J, used for electricity bills.
Energy (kWh): Power (kW) × time (h).
Total cost (pence): Energy (kWh) × price per kWh (pence).

Flashcards

What is a kilojoule (kJ)?

One kilojoule is equal to one thousand joules.

What is a kilowatt hour (kWh)?

One kilowatt hour is the energy transferred by a device with a power of 1 kW used for 1 hour.

How many joules are in one kilowatt hour?

One kilowatt hour equals 3,600,000 J (since 1 kW = 1000 W and 1 hour = 3600 s, so 1000 W × 3600 s = 3,600,000 J).

Why are kilowatt hours used for electricity bills instead of joules?

Because electrical devices in an average UK household transfer over 10 billion joules per year; using kWh makes the numbers more manageable.

How many kWh does the average UK household transfer per year?

3000 kWh.

What is the formula to calculate energy transferred in kWh?

Energy (kWh) = Power (kW) × time used (h).

What was the price per kWh in the UK as of April 2022?

28 pence per kWh.

What is the formula to calculate total cost of electricity?

Total cost (pence) = Energy transferred (kWh) × price per kWh (pence).

How do you convert pence to pounds?

Divide the number of pence by 100.

How many seconds are in one hour?

3600 seconds.

An electric oven has a power of 2.2 kW and is used for 1 hour. How much energy does it use in kWh?

Energy = 2.2 kW × 1 h = 2.2 kWh.

A television has a power of 0.2 kW and is used for 8 hours. How much energy does it use in kWh?

Energy = 0.2 kW × 8 h = 1.6 kWh.

A hairdryer has a power of 1.8 kW and is used for 30 minutes. How much energy does it use in kWh?

30 minutes = 0.5 h, so energy = 1.8 kW × 0.5 h = 0.9 kWh.

How do you calculate the number of kWh used from meter readings?

Subtract the previous reading from the current reading: kWh used = current reading - previous reading.

If the previous meter reading is 2031 and the current reading is 2210, how many kWh were used?

2210 - 2031 = 179 kWh.

If 179 kWh are used at 28 p/kWh, what is the cost in pence?

179 × 28 = 5012 pence.

Convert 5012 pence to pounds.

5012 ÷ 100 = £50.12.

If the previous meter reading is 3450 and the current reading is 3620, how many kWh were used?

3620 - 3450 = 170 kWh.

If 170 kWh are used at 30 p/kWh, what is the cost in pence?

170 × 30 = 5100 pence.

If the previous meter reading is 5000 and the current reading is 5200, how many kWh were used?

5200 - 5000 = 200 kWh.

If 200 kWh are used at 25 p/kWh, what is the cost in pence?

200 × 25 = 5000 pence.

If 200 kWh are used at 28 p/kWh, what is the cost in pounds?

200 × 28 = 5600 pence; 5600 ÷ 100 = £56.00.

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