Lesson 10: Electricity bills
What you need to know:
1. Work is the amount of energy transferred to a component.
2. Work can be calculated from the power and the amount of time the component is
3. How electricity companies calculate the amount of energy we use.
The news is currently full of stories of rising electricity costs, but how do the
energy companies calculate how much electricity each house is using?
The amount of electricity used by the appliances in each house depends on:
1. The amount of power each appliance uses. This is called the power
2. The time that each appliance is on for.
Using the power rating of an appliance and the amount of time the appliance is
on, we can calculate the energy transferred (work) to the appliances in your
house. However, on electricity bills, instead of measuring the work in joules,
energy companies use Kilowatt-hours (KWh) as the unit of energy. This is
because each house uses millions of joules of energy and it would be confusing
to put this on the electricity bill.
So the amount of energy each appliance uses in your house is:
Energy transferred (KWh) = Power rating (KW) × Time (hours)
On electricity bills the electrical energy used by appliances in your home is given
in ‘units’. One unit is equal to 1 KWh.
Energy transferred (KWh) = Number of units used
In your exam you may be asked to calculate the energy transferred to an
appliance in KWh.
The cost of electricity can be calculated if we know the number of units that a
house has used, and the cost of each unit:
Cost of electricity = Number of units used × Cost per unit
So, for example, how much will it cost to have an appliance with a power rating
of 2KW on for 1.5 hours, if each unit costs 10p?
Number of units used = 2KW × 1.5 hours
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Number of units used = 3 units (KWh)llllllllllllllll
Cost = 3 units × 10p
Cost = 30p llllllllllllllllllllllll
In your exam you may be asked to calculate the cost of electricity for an
Not all the energy that is transferred to an appliance actually goes into doing
what we want it to. It is not usefully used. For example, in a light bulb, only
around 15% of the energy is actually used to produce light! This means that
85% of the energy is wasted. The greater the amount of energy that is usefully
used (transferred), the more efficient we can say the appliance is. Efficiency can
be calculated with the following formulae:
For example, what is the efficiency of a light bulb which uses 200 J of electrical
energy to radiate 30 J of energy as light?
Efficiency = 15%
In your exam you may be asked to calculate the efficiency of a variety of
1. The power used by a household is dependent on the power used by each appliance,
and the time each appliance is one for.
2. Energy transferred (KWh) = Power rating (KW) × Time (hours).
3. Energy used in your home is measured in Kilowatt-hours, as this is easier to write
than the amount of joules your house uses.
4. On your electricity bill Kilowatt-hours are called units.
5. The efficiency of an appliance tells us how much of the energy supplied to the
appliance is actually used in the way we want.
1. A woman receives a £200 million electricity bill:
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