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.

**"Measuring electricity use"****worksheets**for each student

- 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

- 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. 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

- 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.

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 |

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

**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.

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

- Develop, demonstrate, and apply mathematical understanding through play, inquiry, and problem solving

- Represent mathematical ideas in concrete, pictorial, and symbolic forms

- 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