Calculating Energy Efficiency

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  • 0:04 What is Energy Efficiency?
  • 0:35 The Formula
  • 1:27 Calculating
  • 2:41 Example of Energy Efficiency
  • 3:47 Lesson Summary
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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

In science class, energy efficiency is not so much about using an LED light versus an incandescent or fluorescent light. No, it's about how much energy you get from something versus the amount of energy you put in.

What Is Energy Efficiency?

You have probably heard the words 'energy efficiency' in connection with using energy efficient appliances for financial and environmental benefit. In the sciences, though, energy efficiency gets a bit more technical.

Energy efficiency is how much energy is produced in a process versus the amount of energy given. For example, a light bulb is given a certain amount of electricity. Energy efficiency is about how much of that electricity gets turned into usable light. It's about comparing the output of something versus the input.

The Formula

Now imagine that you are the owner of a local art gallery. You display all your pieces of art from 8 am to 8 pm. You are interested in energy efficiency because you actually spend quite a bit of money on powering all the lights in your gallery.

You want to know just how much of the electricity you pay for actually gets turned into usable light by your light bulbs. To help you figure this out, you use the following formula for finding energy efficiency:


  • The Greek letter eta, the one that looks almost like an 'n', represents your energy efficiency.
  • W represents the amount of work or energy in units of Joules. It's multiplied by 100 to turn it into a percentage.


Go ahead and use this formula to see just how well your light bulbs are doing their job. Your light bulbs are currently using 4,500 Joules of energy every minute. And every minute, they emit 99 Joules of light energy. So, we get:


Wow! Your light bulbs are using a lot of energy but not producing much light. So, where is the rest of the energy going? Remember, the law of conservation of energy says that energy is neither lost or created in any closed system, just changed.

So, since only 2.2% of the electricity you are giving your light bulbs is turning into light, where is the other 97.8% of the electricity going? Have you ever touched a light bulb that's on? Burned your hand didn't it? Yep, that energy turns into heat, lots of heat.

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