Electricity is measured and purchased in units called kilowatt-hours, which are calculated by multiplying the power used by a device in kilowatts by the number of hours it is in use. The cost of electricity depends on the time of day, with peak hours being most expensive and off-peak hours being least expensive. Examples are provided to demonstrate how to calculate the kilowatt-hours and resulting cost for different electrical devices based on their power usage and duration of use.

1. Quantifying Electricity:
Cost of Electricity

2. Cost of Electricity
Electricity is purchased from the power company
in units called kilowatt-hours (kW·h).

3. Cost of Electricity
kW·h are found by multiplying the power used by a load
(in kilowatts) by how long it has been on (in hours).
1 kW·h = (1 kW) (1 h)

4. Cost of Electricity
In Toronto we pay different amounts for electricity
depending on the time of day:
Off-peak hours – least expensive
Mid-peak hours – somewhat expensive
Peak hours – most expensive

5. Try This.
It took 4 hours for a 1200 W oven to cook a chicken.
How many kW·h were consumed?
4.8 kW·h

6. Try This.
How much would it cost to heat-up a frozen dinner in a
800 W microwave for 15 minutes if the cost of
electricity is 7¢/kW·h?
1.4¢

7. Try This.
If the cost of electricity is $0.05/kW·h, what would an
office building pay to keep a 60 W light bulb at its
entrance on all week?
$0.50