The Cost Of Electricity(GCSE PHYSICS P1)

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).

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).

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

ANSWER:

• 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