1.4.1Electricity, Energy and Waves

Calculating the Cost of Electricity — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions

5 min read 6 parts Lesson 22 of 36

0%read

Part 1 of 4Power, energy and units

Power rating

Power rating An appliance's power rating states how many joules it transfers each second, so a 2500 W kettle transfers 2500 J every second.
Power is energy divided by timeP = E ÷ t, and 1 kW = 1000 W = 1000 J/s.

The kilowatt-hour

The kilowatt-hour The kilowatt-hour (kWh) is a unit of energy equal to one kilowatt of power used for one hour.
Energy supplied in kWh = power in kW × time in hours.

Why kWh is used

Why kWh is used A joule is tiny compared with the energy used in a home, so suppliers bill in the much larger kWh unit.

Find the energy used in kWh, either from the meter or from power (kW) × time (h).

Multiply by the unit price: cost = units used (kWh) × cost per unit (p or £ per kWh).

Add the standing charge, which is the daily charge multiplied by the number of days.

Convert pence to pounds by dividing by 100 and round to the nearest penny.

Add VAT, about 20 per cent, if the question asks for it.

Item	Working	Result
Electricity used (284 kWh at 26.47 p)	284 × 26.47 = 7517.48 p	£75.17
Gas used (1366 kWh at 7.00 p)	1366 × 7.00 = 9562 p	£95.62
Electricity standing charge (31 days at 51.38 p)	31 × 51.38 = 1592.78 p	£15.93
Gas standing charge (31 days at 26.16 p)	31 × 26.16 = 810.96 p	£8.11
Total before VAT	75.17 + 95.62 + 15.93 + 8.11	£194.83

Cost calculations

Cost of one appliance

Cost of one appliance Convert watts to kilowatts by dividing by 1000, then multiply by hours and the price per kWh.
A 3 kW heater used for 2 hours uses 6 kWh, which at 28 p per kWh costs £1.68.

Tariffs

Tariffs Different tariffs have different unit prices and standing charges, so compare the total for the same usage.

Units check

Units check If time is given in minutes, divide by 60 before multiplying, because the unit needs hours.

Term	Meaning
Kilowatt-hour	A unit of energy; the energy transferred by a 1 kW device in 1 hour (3 600 000 J).
Power rating	The energy an appliance transfers every second when it is working normally.
Tariff	A pricing plan from an energy supplier.
Standing charge	A fixed daily charge for being connected to the supply.
VAT	A tax added to the bill, around 20 per cent.

Using watts instead of kilowatts in the kWh equation, which gives an answer 1000 times too large.

Leaving out the standing charge or multiplying it by the wrong number of days.

Forgetting to convert pence to pounds at the end.

Entering minutes directly as hours instead of dividing by 60.

Last updated: 20 October 2026