• 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
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
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
E ÷ t
Power (measured in kW)
t = time (measured
E = energy (measured
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!
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
this sheet of calculations to find the
amounts of energy used and the cost of the electricity in each case.