The Cost Of Electricity(GCSE PHYSICS P1)

These revision cards will sum up everything you need to know for the P1 exam about electricity! I hope it help!:D

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Converting kWh to Joules.

1kWh = 3,600,600J (Joules).

The way to turn kWh into Joules is as follows:

  • E = P x T or,   

    Energy(JOULES) = Power(WATTS) x Time(SECONDS).

(You must convert to the units before using the formula, eg.. change 1 minute to 60 seconds, then add that to the formula).

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Calculating the Cost of Electricity.

There are 2 very important formula's for calculating the cost of electricity, here they are:

  • Number of units used(kWh) = Power(kW) x Time(Hours).

(This looks the same as the formula on the first page at a glance, but they are different! Make sure you don't mix them up).

  • Cost = Number of units x price per unit.

(We get the number of units by using the first equation. You may need to use both).

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An elecricity supplier charges 7p per unit. Find the cost of leaving a 60W lightbulb on for 30 minutes.


  • Number of units(kWh) = power(kW) x time (hours)

Number of units = 0.06kW(because there are 1000 kW in 1 W) x 0.5 hours

0.06kW x 0.5 hours = 0.03 UNITS

  • Cost = Number of units(kWh) x Price per unit.

Cost = 0.03 x 7p = 0.21p

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