Electric Bills and Math

 

Sometimes it is nice to take a few minutes out of your regularly scheduled pacing to introduce students to the connection between electric bills and energy consumption of appliances and devices. I'm not sure students know how much power is used by the refrigerator or stove.  Yes, it is usually a topic covered in life skills, consumer math, or personal finance but it doesn't hurt to have students take time out of an algebra class to learn this.

I've seen it suggested that students use information on appliances and devices to calculate how much electricity each one consumes to give students an idea of their families monthly electric bill.  To calculate this, students need three pieces of information. They will need the wattage used by the appliance or digital device, the number of hours it is used each day, and the price per kilowatt hour the electric company charges.  

The kilowatt hour cost is easily found on the electric bill or it can be gotten by calling the local electric company.  I know that in many places in Alaska, the cost if about $0.34 per kWh.  The number of hours the item uses electricity will vary depending on what it is.  For instance, the refrigerator is on 24 hours a day while the dishwasher may only be on a couple times a week.  

To find the amount of power it uses, it will be necessary to go one of three places.  First try the back or bottom of the appliance.  You are looking for a small metal tag with the information on it.  If that doesn't work, check for the book that came with it and lastly, look for the information on the internet by searching for the year and model.  The information can usually be found in the technical specs.

Once students have all three pieces of information for all their appliances and electronic devices, they can get a good estimate by multiplying the wattage times the number of hours used in a 24 hour period, before dividing the product by 1000 to convert from watts to kilowatts. The next step is to multiply the result by the price per kWh to get the final amount per day.  The last step is to multiply the daily amount by 30 to get a monthly amount which is a part of the total monthly bill.

If this is done for all appliances and electronic devices used by the family, students can see how much of the total bill each accounts for.  Then take the results of this exercise to create a pie chart or other visualization to see which ones use the most and which use the least.

This exercise can be extended quite easily by having students calculate annual usage based on taking the daily amount and multiplying it by 365 days.  If they do this for each appliance or electronic device, they can use the results to determine how much they spend every year for power.  Another extension would be to get a year's worth of electric bills to see when usage spikes and drops.  They should notice there is an increase in the middle of winter and summer due to the additional heating and cooling demands made by the season.  

Sometimes the yearly amount is more than most students think it is and they are shocked.  It's usually enough to have a nice visit somewhere or even buy that new electronic device they've been wanting.  Let me know what you think, I'd love to hear.  Have a great day.