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 10p 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 10p):
USE OF ELECTRICAL
ENERGY
Electric cooker 4 units per day
10 m
3 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
Toast 70
slices of bread
Warm a bed for a week
Vacuum for 2 hours
Run an
electric clock for 3 months
Drill for 4 hours
Mix 60 cakes
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:
In this table take the cost of one unit to be 8p
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?
