Paying for Electricity
Electricity, or rather electrical energy 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 average electricity bill for a house in a year would probably be about £500, although this very much depends on how the house is heated.
The electricity board measures the electrical energy that we use not in joules but in units called KILOWATT-HOURS.
One kilowatt hour = 1000 W x 3600 s
= 1000 x 3600
= 3 600 000 J
At the moment this amount of electricity costs about 15p in Britain.
The power is measured in kilowatts and the time in hours.
So a 1 kW electric fire will use 5 kWh in 5 hours but a 100 W light bulb will only use 5 kWh if it is on for 50 hours.
A kilowatt-hour (kWh) is also called a UNIT of electrical energy by the electricity boards.
PROBLEMS
1. What does the electrical energy cost to run:
(a) a 3 kW immersion heater for 20 m
(b) a 200 W light bulb for 24 hours
(c) a 2 kW fire for 4 hours a day for 3 months
2. How long can you use the following for 10p
(a) a 2 kW fire
(b) a 4 kW cooker ring
(c) a 150 W light bulb
(d) a 250 W TV
3. Copy and complete the following table (take the cost of one kW hour as 15p):
Appliance / Power / Time / Cost1 / Kettle / 2.5 kW / 6 min
2 / Iron / 2 hours / 20 p
3 / Washing machine / 250 W / 45 mins
4 / Stereo / 200 W / 10 p
5 / Television / 4 hours / 15 p
6 / Light bulb / 60 W / 4 hours
7 / Light bulb / 3 hours / 9 p
8 / Hair drier / 0.15 kW / 5p
9 / Cleaner / 0.5 kW / 10 p
10 / Freezer / 120 W / 6 hours
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USE OF ELECTRICAL ENERGY
Electric cooker 4 units per day
10 m3 freezer 15 units per week
Electric iron 0.5 units per hour
Washing machine 9 units per week
Unlagged hot water tank 160 units per week
Lagged hot water tank 80 units per week
WHAT CAN YOU DO FOR 10p
Watch TV for 3 hours Drill for 4 hours
Toast 70 slices of bread Mix 60 cakes
Warm a bed for a week
Vacuum for 2 hours
Run an electric clock for 3 months
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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.
You will find that all appliances in the home that give out heat operate at quite high powers and therefore consume a lot of energy.
PROBLEMS
1. Copy and complete the following table:
Current cost of one unit = 15p
Appliance and power / Time / Energy (J) / Cost (p)2kW fan heater / 2 hours
200W light bulb / 15
60W light bulb / 5 hours
3 kW immersion heater / 30
2kW kettle / 5 minutes
150W stereo / 40 minutes
25W motor / 20 minutes
500W cleaner / 7.5
300W hair drier / 15 minutes
250W drill / 22.5
30W radio / 2
120W TV / 2
400W iron / 22.5
7kW shower / 5 minutes
300W mower / 30 minutes
75W electric blanket / 45
2kW cooker ring / 20 minutes
2.5kW oven / 3 hours
750W radiator / 10 hours
20W CD player / 40 minutes
2. Why will the energy used by the fridge in a normal house in a year be much less that the amount that you have worked out?
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