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Measuring electricity use

Calculate the electricity used in a typical household and learn about watts, kilowatts and kilowatt-hours.

  • Grade 9
  • Class Discussion
  • 40 mins


Using practical examples of the appliances and objects found in every home, your students perform basic calculations to reveal how much electricity they use.

What you'll need

  • "Measuring electricity use" worksheets for each student


  1. Introduce your students to the terms “watts” and “kilowatts”. These are units used when measuring electrical power.
    • A watt is the rate that an appliance or device uses energy: 1 watt = 1 joule/second
    • The higher the wattage, the more power the device or appliance uses
    • A kilowatt is 1,000 watts
  2. Remind your class that energy is the ability to do work. Electricity is a source of power that can do work for us. To determine how much electrical energy an object uses we have to know two things:
    • The amount of electricity the object uses, called its power rating
    • For how long the object is using electricity.
    • To calculate the amount of work an object does, use the formula: energy = power x time
  3. The amount of energy used over time uses the unit “kilowatt-hours,” which is 1,000 watts used in 1 hour. This is a rate of power: the rate at which work is done.
    • Ask 10 of your students to draw a light bulb and label it 20 W, then stand up with their drawings
    • Imagine that these ten 20 W bulbs are on (working) for five hours
    • How many kilowatt-hours have been used?
    • 10 bulbs x 20 W x 5 hours = 1,000 watt hours = 1 kWh
  4. Have another 10 students draw 20 W bulbs, but vary the amount of time they are “working”.
  5. Hand out the Measuring electricity use” worksheet. As a class, come up with definitions for the following terms:
    • Energy
    • Watt
    • Power
    • Kilowatt
    • Kilowatt-hour
  6. Have your students complete the worksheet individually or in pairs.

Additional Information

You can either collect and assess the Measuring electricity use” worksheet or have the class work through the answers together.

Worksheet Answer Key:

EnergyThe ability to do workenergy = power x time
WattA unit of electrical power. The rate at which an appliance or device uses power1 watt = 1 joule/second
PowerThe rate at which energy is   transformed or the rate at which work is donepower = energy/time
KilowattA measure of the amount of energy used over time that indicates how fast you are using energy1 kW = 1,000 watts
Kilowatt-hourThe amount of energy used when an appliance or device consumes 1 kilowatt of   power for 1 hour100-watt light bulb x 10   hours of use = 1 kWh

Appliances   and productsPower   (watts)Average use (hours per day)Annual energy usage   (kWh)Annual   cost ($ per year)
Vacuum cleaner1,1000.140.2   kWh$3.33
Hair dryer1,2000.25109.5   kWh$9.08
Computer1204.0175.2   kWh$14.52
Microwave9001.0328.5   kWh$27.23
Clothes dryer4,0002.02,920   kWh$242.07
Incandescent light bulb603.065.7 kWh$5.45
Compact fluorescent light bulb143.015.33   kWh$1.27
LED light bulb83.08.76 kWh$0.73
Flat-screen TV2005.0365 kWh$30.26

How is the use of electricity measured?

The watt (W) is a unit of electrical power, which is the rate at which an appliance or device uses energy.

  • 1 watt = 1 joule/second (J/s)
  • The higher the wattage, the more power the device or appliance uses.
    • Example: An 8 W LED bulb or 15 W compact fluorescent light bulb draws less energy than a 60 W incandescent light bulb. Each of these bulbs generate about the same amount of visible light, and the LED and CFL last 5 to 45 times longer than the incandescent

What is a kilowatt?

A kilowatt (kW) is equal to 1,000 watts.

What is energy?

Energy is the ability to do work. Electrical energy use is measured in kilowatt-hours (kWh). Another unit of energy is the joule. For example, in homes we measure the use of natural gas energy in gigajoules.

What is a gigajoule?

A gigajoule (GJ) is equal to 1,000,000,000 (one billion) joules.

How do you convert gigajoules to kWh?

1 GJ equals 277.8 kWh.

Grade 9 Mathematics - Content

  • Two-variable linear relations, using graphing, interpolation, and extrapolation

Grade 9 Mathematics - Curricular Competencies

Understanding and solving
  • Develop, demonstrate, and apply mathematical understanding through play, inquiry, and problem solving
Communicating and representing
  • Represent mathematical ideas in concrete, pictorial, and symbolic forms

Grade 9 Science - Curricular Competencies

Processing and analyzing data and information
  • Seek and analyze patterns, trends, and connections in data, including describing relationships between variables (dependent and independent) and identifying inconsistencies
  • Use knowledge of scientific concepts to draw conclusions that are consistent with evidence

This activity is part of the lesson

Measuring and conserving energy

View Lesson
Lesson Measuring and conserving energy support image

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