Energy

Measuring electricity use

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

Grade

Duration

40 mins

Type

Class discussion

Overview

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

BC Hydro

Instructions

What you'll need

"Measuring electricity use" worksheet for each student
"How do we measure electricity" video

Introduce your students to the terms “watts” and “kilowatts” by watching the "How do we measure electricity" video. 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.
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.
The amount of energy used over time uses the unit “kilowatt-hours,” which is 1,000 watts used in 1 hour.
- 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.
Have another 10 students draw 20 W bulbs, but vary the amount of time they are “working”.
Hand out the “Measuring electricity use” worksheet. As a class, come up with definitions for the following terms:
- Energy
- Watt
- Power
- Kilowatt
- Kilowatt-hour
Have your students complete the worksheet individually or in pairs.

Modify or extend this activity

Extensions

Use the "Home energy detectives" activity to calculate electricity usage with real time data and explore ways to conserve electricity at home.

Curriculum Fit

Grade 9 Mathematics

Content

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

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

Teaching Notes

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.

Assessment

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

Worksheet answer key:

Term	Definition	Formula
Energy	The ability to do work	energy = power x time
Watt	A unit of electrical power. The rate at which an appliance or device uses power	1 watt = 1 joule/second
Power	The rate at which energy is transformed or the rate at which work is done	power = energy/time
Kilowatt	A measure of the amount of energy used over time that indicates how fast you are using energy	1 kW = 1,000 watts
Kilowatt-hour	The amount of energy used when an appliance or device consumes 1 kilowatt of power for 1 hour	100-watt light bulb x 10 hours of use = 1 kWh

Appliances and products	Power (watts)	Average use (hours per day)	Annual energy usage (kWh)	Annual cost ($ per year)
Vacuum cleaner	1,100	0.1	40.2 kWh	$3.33
Hair dryer	1,200	0.25	109.5 kWh	$9.08
Computer	120	4.0	175.2 kWh	$14.52
Microwave	900	1.0	328.5 kWh	$27.23
Clothes dryer	4,000	2.0	2,920 kWh	$242.07
Incandescent light bulb	60	3.0	65.7 kWh	$5.45
Compact fluorescent light bulb	14	3.0	15.33 kWh	$1.27
LED light bulb	8	3.0	8.76 kWh	$0.73
Flat-screen TV	200	5.0	365 kWh	$30.26