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The electricity companies do not measure
energy consumption in joules but rather in kilowatt hours.
The electricity meter in your house
counts 'kWh' energy units and the electricity supply company then
charges about 8p for each of these units. (The cost varies depending
on which 'tariff' you are billed at and the amount of electrical energy
the household uses - ask at home to see a bill and look at how it
is made up!)
Back in 1990 each time a 1kW appliance is switched
on for 1 hour the meter registers 1 kWh and your household owes the
Electric Company 7.72p (nowadays it is more like 20p per kWh... costs have risen dramatically!).
It therefore follows that
a 500W (or 0.5kW) appliance switched on for 30 minutes (or 0.5h) would
cost 2p as only 0.25kWh is used (and the company round up the tiny fractions of a penny owed!).
The formula for calculating
the number of kilowatt-hours is:
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Energy
E
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=
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Power
P
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x
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Time
t
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(kWh)
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(kW)
Remember the power
must be in kilowatts (thousands of watts - in all other physics
calculations it must be in watts)
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(h)
Remember the time must
be in hours (in all other physics calculations it must be
in seconds)
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NB
Don't use 'T' for time the upper case letter T is used for temperature, lower case
't' stands for time!
Once you have the number of kWh calculating
the cost is simple. Multiply the number of 'kWh units' by the cost
of one.
Follow these
steps in calculations:
- Extract
the information from the question an put it in the top right hand corner
- remember to extract the units too!
- Write out the
general equetion in the centre of the page - put the unit for each part
in brackets underneath.
- Compare the
units you should have with those you have been given and where necessary
make a conversion
- Put the (correct
unit) numerical values into the formula
- Calculate the
answer.
- Think carefully
about the unit the answer should be expressed in.
Common mistakes
made in answering questions:
- Not
reading the question carefully.... extracting the information as suggested
above helps you to think carefully about what the questions says!
- Not changing
the values given into the values required in the calculation. You need
to know what units the equation requires and know how to change from
one type to another.
- Forgetting that
Units cost pennies not pounds!.... therefore giving answers that are
100 times too much!
Now try the questions below and then click on the answer
icon to check your solution.
| Q1. |
A cooker has
a power-rating of 3kW. If a unit of electricity costs 20p, how
much does it cost to run the appliance for 3 hours? |
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| Q2. |
A spindrier
has a power-rating of 2.5kW. If a unit of electricity costs 24p,
how much does it cost to run the appliance for 30 minutes? |
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| Q3. |
A washing machine
has a power-rating of 2500W. If a unit of electricity costs 24p,
how much does it cost to run the appliance for 90 minutes? |
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| Q4. |
A vacuum cleaner
has a power-rating of 1.2kW. If a unit of electricity costs 20p,
how much does it cost to run the appliance for a year if it is
switched on for 1 hour each day? |
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| Q5. |
A hairdrier
has a power-rating of 750W. If a unit of electricity costs 21p,
how much does it cost to run the appliance for a year if it is
switched on for 10 minutes each day? |
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To try some further
(more challenging) questions: click here
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The kilowatt hour (kWh)
