Activity: Comparing a compact fluorescent lamp and an ordinary incandescent lamp  Print page

Key Learning Points:
1.   Energy and power
2.   Conversion of electric energy into other forms of energy
3.   Domestic energy efficiency
4.   Searching for information
5.   Communication
6.   Making informed decisions


Fig. 1   This 20 W compact florescent lamp can produce a brightness similar to a 100 W incandescent lamp.

Do you know how much you pay for your electricity bill? Would you like to save energy consumption in your home? Compact florescent lamps may help you to save energy and money.

In the first part of this activity, you will

  • calculate the cost of using a compact fluorescent lamp for a period of time
  • compare the cost with that of an incandescent lamp (ordinary light bulb) of similar brightness
  • estimate the payback period of a compact fluorescent lamp

In the second part of the activity, you will

  • judge whether the information in an advertisement is reliable
  • search for some information about compact florescent lamps
  • discuss whether all the light bulbs in Hong Kong should be replaced with compact fluorescent lamps.
  ................ [top]

Part I

Video of the experiment  Watch videoWatch videoDownload video: 8.4mb


100 W incandescent lamp (light bulb), compact fluorescent lamp (CFL) that can produce the same brightness, lamp stand, AC wattmeter, domestic power supply.


Fig. 2   The experimental setup.
  1. Install the CFL on the lamp stand. Connect it to the wattmeter and the mains supply as shown in Fig. 2.
  2. Turn on the lamp. Wait for five minutes and then record the reading of the wattmeter. Also note the brightness of the light bulb.
  3. Repeat step 1 and 2 with the light bulb.
  4. Record your observations on the worksheet. Were the power consumptions in step 2 and 3 the same?
  5. Compare the total cost of using an ordinary light bulb with that of using a CFL, for a period of time equal to the lifetime of a CFL (this information can be found on the packing of a CFL, e.g. Fig 3), assuming that the cost of electricity is HK$ 0.9 per kWh.
  6. Fig. 3   The description on the packing of a CFL.
    Although the cost of a CFL is much higher than a light bulb, it will ultimately help you to save energy and money. Explain why this is so and calculate the payback period of the CFL, that is, the amount of time a CFL must be used until its cost equals that of the ordinary light bulbs.


  1. How is power related to energy consumption and the time of use?
  2. What is kWh? How is it related to the joule (J)?
  3. Why should we wait for five minutes before taking the reading of the wattmeter?
  4. How much more do you have to pay initially for a CFL than a light bulb? Why can a CFL help you save more energy and money in the long run?
  5. Present the information found in Part I as two straight line graphs, showing the average cost against time of using a CFL and light bulbs respectively. What does the intersection of the graphs tell you?
  ................ [top]

Part II

Do you believe this?

Fig. 4   An advertisement on compact fluorescent lamps.

The advertisement (Fig. 4) claims that when six incandescent lamps of 100 W are replaced with six compact fluorescent lamps of 20 W, up to $999 will be saved a year if the lights are switched on for six hours per day. Is the information in this advertisement accurate? Use your knowledge of physics to decide and present your analysis to the class.

(Since only the cost of electricity is considered in this advertisement, you may ignore the cost of buying the compact fluorescent lamps and their lifetimes)

Do you think that all the incandescent lamps in Hong Kong should be replaced with compact fluorescent lamps? Give reasons to support your view. Besides the cost and energy efficiency, consider other factors also such as the potential pollution to the environment that may be brought about by the disposal of mercury-loaded compact fluorescent lamps.

Find some other information about CFLs in magazines and on the web. List as many advantages and disadvantages of using compact fluorescent lamps as you can. Discuss the information with your classmates and try to draw a conclusion. The following references may be a good starting point for your research.

  1. EMSD websites on Hong Kong Energy Efficiency Labelling Scheme for Compact Fluorescent Lamps
  2. US Environmental Protection Agency - Energy Star
  3. Brethren Press - Youth CLF Project
  4. CHOICE, Volumes 285 (7/00), 292 (2/01), 308 (6/02), Consumer Council, Hong Kong