CALCULATING THE COST OF ELECTRICITY Electricity costs money. We pay for it when we buy a battery and we pay the electricity board for the electricity that we use from the mains. The electricity board measures the electrical energy that we use not in joules but in units called KILOWATT-HOURS. 1 KWh = 1 Unit of electricity First convert power from W to KW by dividing by 1000. Convert time to hours (h). Then use this formula to calculate the total cost of electricity:
(cost of 1 unit = 10p)
One kilowatt hour (1 UNIT) = 1000 W x 3600 s = 1000 x 3600 = 3 600 000 J In Britain 1 unit of electricity costs about 10p (Rs.14). The cost if calculated in terms of JOULES will give big numbers for the whole month so it is calculated using KW and h not seconds.
Since appliances are usually marked with the power at which they run we can work out how much energy they will use in a certain time. We can do this using the formula: Power = Energy/Time which when rearranged becomes:
[Power in watts, time in seconds, energy in joules] A normal house will use about 15 000 MJ of energy in a year. (1 MJ = 106 J)
By Shafaq Hafeez shafaq@physics.com.pk