Oct. 29 – Electricity Cost and Efficiency

Learning Goals: Understand how to describe the efficiency of a device.

Success Criteria: Be able to calculate the % efficiency of different devices.

I did a few quick examples today and then we spent the rest of the period preparing for the presentations.  Here are the definitions and examples.

Cost of Electricity

In Ontario, the price of electricity varies depending on the time of day.  Here's a summary on this website: http://www.ontarioenergyboard.ca/OEB/Consumers/Electricity/Electricity+Prices

To calculate the total cost, you use on of the following two methods.

Cost = E x Price

Cost = P x t x Price

Example: How much does Ari's fridge cost to run per year?  The enerGuide label reads 772 kWh per year.  The price of electricity is 10.4 cents/kWh.

Given: E = 772 kWh per year

Required: How much is the cost per year?

Analysis: Cost = E x Price 

Solve:

Cost = 772 kWh x 10.4 cents/kWh

        = 8028.8 cents

        = $80.29

Statement: The cost is $80.29 per year.

Example:  Ashley's TV uses 45 W.  How much does it cost her to watch TV for 4 hours during mid-peak time (10.4 cents/kWh).

Given: P = 45 W = 0.045 kW, t = 4 hours

Required: How much is the cost for the four hours?

Analysis: Cost = P x t x Price 

Solve:

Cost = P x t x price

        = 0.045 kW x 4 h x 10.4 cents/kWh

        = 1.872 cents

Statement: The cost for 4 hours of TV is about 2 cents.

Efficiency

How well the energy is used.

All electrical devices require electrical energy.

All of them do some kind of work as output.

Efficency = (energy out ÷ energy in) x 100%

Example: 

An incandescent light bulb uses 100 J of electric energy and produces 2 J of light energy.  Calculate the efficiency.

efficiency = (E out / E in) x 100%

                = (2 J / 100 J) x 100%

                = 2 %

A compact fluorescent bulb uses 20 J of electric energy for the same amount of light.  Calculate its efficiency.

efficiency = (E out / E in) x 100%

                = (2 J / 20 J) x 100%

                = 10 %

We can see that the compact fluorescent is much more efficient.  It is impossible to have 100% efficiency!

You should now be able to do all the questions on this handout: 

• Power and Efficiency Problems

Homework:

• Prepare and Practice your presentations!
• If you're using PowerPoint, send me a copy.
• If you're using Prezi, use the "share" button to send me a link.