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The Discovery Channel’s Mythbusters Test the Effects of Turning off the Lights

Posted by on Wednesday, December 13, 2006 in Energy, News.

In an episode of the Discovery Channel’s hit popular science show, Mythbusters, the cast tests if turning the lights off every time you leave the room really conserves energy. Details about the experiment and its results are included below…

This myth is featured in Episode 69: Lights On or Off?, which was originally aired on December 13, 2006.

Myth Title: Lights On or Off?

Myth Description: Do you really save money turning off the lights when leaving the room even for a brief moment? And, does turning on and off the lights often reduce the longevity of the bulbs?

Hypothesis: The “start up” burst of energy consumes more energy than leaving the lights on continually

Procedure/Experimental Design: The MythBusters test common types of lightbulbs: incandescent, fluorescent, metal halide, and even an LED

Results: Even with the fluorescent bulb having the largest start-up surge, needing to be off for 23 seconds before it is more energy efficient to leave it off, it is still a small amount of time before it is more energy efficient to leave them off.

Conclusion: It is better to turn the lights off when leaving the room even for a brief instant.

Busted or Not Busted: Busted

Additional Information:

Mythbusters talked with Mark Reisfelt, manager of the Independent Electric Supply where they purchased their light bulbs. He felt that it was best to turn the lights off. To test the myth, they needed to measure energy usage during startup, maintenance (steady state), and shutdown. For steady state energy consumption, they turned on several different types of bulbs for 60 minutes and measured their consumption using a Kill A Watt:

  • Incandescent: 90 Watts
  • Compact Fluorescent: 10 Watts
  • Halogen: 70 Watts
  • Metal Halide: 60 Watts
  • LED: 1 Watts
  • Fluorescent: 10 Watts

For startup energy consumption, Grant hooked up an inductive current loop to a computer and measured the amount of energy used when the turned on the bulbs. With an inductive current loop, you run a wire through the center, which induces a current in the loop. This current is then measured by a digital sampling oscilloscope. Based on the amount of energy consumed turning on the bulb, they were able calculated how long the bulb would have to be turned off in order to make it worth the energy savings, i.e. “It’s best to turn off the bulb if you are leaving the room for”:

  • Incandescent: 0.36 seconds
  • CFL: 0.015 seconds
  • Halogen: .51 seconds
  • LED: 1.28 seconds
  • Fluorescent: 23.3 seconds

In other words, its almost always best to turn the bulb off. Even the 23 seconds for the fluorescent lights isn’t very long, and the rest of the times are pretty much blinks of an eye.

Bulb Longevity

They tested one final element of this myth: frequently turning lights on and off decreases their life span, thus leading to greater costs. Grant setup a timer and relay to turn the bulbs on and off repeatedly every 2 minutes. After six weeks, only the LED bulb was still working. Based on this test, they extrapolated that it would take five years of ordinary usage to cause the bulbs to burn out.

Busted or Not Busted: Busted

Additional Links:

Annotated Mythbusters

The Complete Mythbusters Show Guide

Buy Mythbusters Episodes and Gear


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