Does it really save electricity to turn off the lights when you know you are coming back shortly? An age old question. Here is the proof!
by The Engineer on April 16, 2009 · 7 comments
in DIY,frugal,save money
Warning: My inner geek may bore you, LOL. I’ll try not to be overly technical.
As many of you know I am an engineer by trade, my disciplines are in both mechanical and electronics. I spend most of my days knee deep designing, developing and troubleshooting electrical systems, specifically in the lighting industry. One thing that I have seen many people talk about (and have had many emails about this myself) is whether it really saves electricity to turn off the lights when you leave the room, especially if you know you are coming back in a minute or even a couple minutes. Read: many people talk about it and speculate… but how many of them can prove it?
When I am cleaning house, I bounce around room to room like a chicken with my head cut off trying to pick things up, sometimes I leave the lights on merely for convenience… but still one has to wonder about the old wives tale that it uses less electricity to leave the lights on.
The train of thought that most people have is that when a light bulb (incandescent, florescent, CFL, neon, LED) is turned on, it draws more current when first powered up.
Is this true? Yes and no.
Allow me to explain. Some light sources do indeed draw more current when first powered on, this is technically known as inrush current. Incandescents are probably the worst for this. From experimenting, most modern CFL’s, florescents with electronic ballasts, neon and LED lighting does not have an inrush current (at least not measurable). Most modern CFL’s and electronic ballasts (florescent and neon) have a bit of a soft start involved, meaning in order to illuminate and complete the circuit it must ignite the gas into plasma. This is not an instantaneous process, so it is inherently slower starting.
So even in the worst case scenario, using incandescent lamps, we know that there is indeed an inrush current involved. But does this inrush current make it cheaper to leave the lights on for a minute while you grab the phone?
NO!!!!
Here’s proof: I took a 60W incandescent light bulb and monitored the current on my oscilloscope (an instrument that takes measurements vs time), here is the results. Notice the large spike of current when first powered on? This is the inrush current. Nominally this inrush current is about twice the steady state current.
But notice how short the inrush current is? If we zoom in and measure this time you will understand. See that time on the side (delta t) 48.00ms. That’s 0.048 seconds. That means as long as you are out of the room for for more than about a 1/10 of a second, it does indeed save money to turn the lights OFF!
I don’t know about you guys, but I usually leave the room for more than .1 seconds at a time… Usually… LOL.
This is worst case scenario. What about best case? Best case, the most efficient lighting is LED’s. Here is the overshoot in LED lighting. NONE. Period.
This means that it always saves money to turn them off when leaving the room, even when it is for 0.10 seconds.
Hope you found this informational. As a science dork, I just thought it would be fun to prove this myth either way.
Don’t forget to turn out the lights!!!
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{ 7 comments… read them below or add one }
Great work. The Mythbusters also covered this topic, as seen here: http://video.google.com/videoplay?docid=180639728096740394
But wait, you should also talk about how that inrush current is hard on the components of the lightbulb and will shorten the life of the bulb if done too many times. How about some experiments on that>
@Anonymous: Thanks for the Vid, I must have missed that episode.
I love mythbusters. SCIENCE RULES !
@Claire: Most components are rated several times what is needed and inrush current typically has little effect on the life of the bulb, be it incandescent or CFL or whatever. What DOES make a huge difference on it’s lifespan is the heating and cooling of the components. If you just turn a bulb on and leave it on it will last hundreds of times longer than turning it on and off. This is due to the expansion and contraction of the components causing mechanical failure. For example, the incandescent filament is made of tungsten, which works fine, but eventually all the heating and cooling causing the expansion and contraction will cause it to fail. Like bending aluminum back and forth, eventually it just breaks.
Thanks for the comments!!
Very cool! I love science and engineering. I went back to school and am studying electronics as of now… I am going to have to recreate the experiment for the fun of it! Thanks!
that was so educational. I guess my dh was right, he constantly reminds me to turn lights off..and I argue that it really doesn’t matter since I will be right back asap.
thanks again.
Anybody see “The Gods Must be Crazy”? It’s an old movie that takes place in the Australian Outback. Anyway, one of the thing the movie makes a point about is the Rhino and its self-appointed position as fire marshall. If it’s around a fire it wants to stamp it out.
At our house we call our youngest an Electric Rhino. Ever since she was little — well before she could even reach a light switch — she became obsessed with turning off lights in empty rooms. I just showed her this article. As I was explaining it to her, the biggest smile came across her face. She feels vindicated.
Hi Engineer,
This made my day and truly brings out the science geek in me! Saving money through empirical scientific experimentation. I absolutely love it. Fascinating, more please!