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Electrical Energy



Know that larger amounts of energy can also be measured in units of "kilowatt-hours" or kWh.

Know that gas and electricity companies measure energy usage in these units.

Be able to calculate the energy usage in kilowatt hours



Task 1 - Recap

Remind the person next to you:

- the definition of a Watt

- the units used to measure energy

- the units used to measure power



Task 2

Write a title "Using electrical energy" in your exercise book.  Copy the three sentences below, and the formula, into your book.


The amounts of energy we use in our homes are far too large to measure in Joules.  The energy suppliers which provide your gas and electricity use units called kilowatt-hours (kWh).  A one kilowatt appliance switched on for one hour will use one kilowatt-hour of energy.



The formula is the same as before, but the units are different:

P = E t

P = Power (measured in kW)

t = time (measured in h)

E = energy (measured in kWh)




Task 3

Try to calculate the energy used by each of the following, showing full working for each calculation.  Your teacher will show you how to do the first one.


1. A 3 kW oven used for 2 hours.

2. A 3 kW water heater used for 2.5 hours.

3. A 1.8 kW electric heater used for 3 hours.

4. A 1.2kW vacuum cleaner, used for 30 minutes*.

5. A 7 kW shower used for 15 minutes*.

6. A 100 W* light bulb used for 5 hours.

7. A 1300 W cement mixer used for 30 minutes*.


*Think carefully about the units for these questions!



Task 4

Electricity costs around 14p per unit.  We can work out the total cost of using an appliance if we know how much energy has been used, using the formula:


Cost = units used cost per unit


Use the formula to work out the cost of using each of the appliances from the questions in task 3.




Complete this sheet of calculations to find the amounts of energy used and the cost of the electricity in each case.