 Energy

# 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

# 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.

## What you'll need

• "Measuring electricity use" worksheets for each student

### Instructions

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:

 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

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

### Measuring and conserving energy

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