# Old Wive's Tale ...



## Turbo DV8 (Dec 23, 2006)

... except it's the husband who believes this, not his wife! I have an old-timer in-law who is supposedly retired from the electronic industry. He believes with dead seriousness that every time you flip a light switch on in your house, it costs 10 cents! Not leaving it on, mind you, just the act of turning it on. Has anybody ever heard this urban legend before? He leaves the incandescent lights on in his house because he believes it is cheaper than turning them on and off. Besides possibly senility, I can only think of one reason he might have picked up this line of reasoning. As with many laughable beliefs, it may have a seed of truth at it's root.

Isn't it true that when you first turn on an incandescent bulb, it's low filament temperature allows a huge surge of current on the order of milliseconds? After that, the nominal current is power over voltage. I mean, if you measured the cold resistance of the filament and divided it into line voltage to get the current, the result would be some astronomical figure. But, of course, the filament heats up and cools off almost instantly. Once on and heated, the resistance increases to the point which limits the flow of current and thus power dissipation. 

So, if you could maintain that initial high current (maybe 10 amps?) I could see how after an hour it would cost 10 cents, which is about what a kWh costs. But I try to tell my in-law that according to his logic that it costs 10 cents every time you turn a light on, then if I flip his light switch on and off quickly 4 times per second for one minute, that's 240 cycles, and he should see an instant $24 increase in his electric bill next month! Or keep it up for five minutes and see a $125 increase. But he won't let me do it!

Has anybody ever heard of this wive's tale, or have any idea where it's roots come from, or is my elderly in-law just getting to that "point?"


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## Paul_in_Maryland (Dec 23, 2006)

It's hard to believe that someone who believes such a myth could have been in the electronics industry! The myth in my house is that I'm costing us an arm and a leg by equipping the house with about thirty 2W luminous green electrolumiescent nighlight, or by neglecting to turn off my closet light. As I invariably reply, "Let's do the math." "Want to save money?" I reply; "turn off your (300-400W) computer at night." My PC is set to shut off after 4 hours; now my wife's is, too.


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## PhotonWrangler (Dec 23, 2006)

The Mythbusters recently covered this one. They tried all sorts of lights - incandescent, CFL, regular fluorescent and even metal halide. They built an automatic power-cycling rig and measured startup current and running current, and then calculated the overall cost. Long story short - myth busted.


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## 2xTrinity (Dec 23, 2006)

> ... except it's the husband who believes this, not his wife! I have an old-timer in-law who is supposedly retired from the electronic industry. He believes with dead seriousness that every time you flip a light switch on in your house, it costs 10 cents! Not leaving it on, mind you, just the act of turning it on. Has anybody ever heard this urban legend before? He leaves the incandescent lights on in his house because he believes it is cheaper than turning them on and off. Besides possibly senility, I can only think of one reason he might have picked up this line of reasoning. As with many laughable beliefs, it may have a seed of truth at it's root.


Assuming that electricity costs 10 cents/kWh, and that the act of switching a light on and off over the period of one second costs 10 cents, those light bulbs would have to be consuming 36 megawatts! I think the fuse would blow first 

In one respect however switching lights on an off rapidly might cost more than leaving them on, that is if the light is something that is sensitive to being switched on and off, such as a fluorescent or HID light, so it might reduce the lifespan of the bulbs. However, the point at which electricity cost of leaving the light vs the wear and tear/replacement cost on a fluorescent lamp is probably several minutes, not hours. 

In the case of incandescent lights, the bulbs aren't nearly as sensitive to cycling, and cost next to nothing to replace. They also consume a lot of power, so it always makes sense to turn those off and on when not in use for any length of time.


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## Turbo DV8 (Dec 23, 2006)

Paul_in_Maryland said:


> The myth in my house is that I'm costing us an arm and a leg by equipping the house with about thirty 2W luminous green electrolumiescent nighlight...


 

In my case, equiping the house with those electroluminescent nightlights did cost me an arm and a leg. I bought into them very early, I am thinking nearly ten years ago. Even at Costco they were quite an expensive novelty. Don't remember the brand. I purchased a multi-pack. Within a year most of them had begun to grow black splotches inside the luminescent material. Within another year, they were all worthless, as light output had dimished from initially dim to next to nothing. The initial cost was never recouped in savings. There was a recall on them by the manufacturer for this problem. They claimed the problem was improper sealing which allowed moisture to get in and grow a type of mold (the black splotches). I never had any of mine installed in a humid environment. I never went back to those electroluminescent nightlights after that. I just stick with regular 4w bulbs in a photoelectric switch base. Still costs only pennies a month to operate, the bulbs are dirt cheap to replace, and they put out way more light than the electroluminescent lights ever did.


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## PhotonWrangler (Dec 23, 2006)

I had a similar type of EL nightlight and it developed black splotches also, but I didn't think that was _mold_. Wouldn't a liviing organism have a difficult time growing in the presence of a constant 120vac electric field?


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## James S (Dec 23, 2006)

tee hee, as soon as the filament is heated up to temp, the power surge is over. How long does it take the bulb to actually turn on? You're heating how much tungsten? It's a TINY mass that needs to be heated up before it reaches it's stable point. So it's very silly indeed :laughing:


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## Sinjz (Dec 24, 2006)

I saw the MythBuster episode too. Basically for incandecents, you would have to be out of the room for less than half a second to save any money. Anymore and you're just wasting energy. With florecents it's about 30 seconds. They also took into account bulbs blowing and in thier very UNscientific test came up with a couple of years worth of turning on and off before the bulb blew.


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## Eugene (Jan 4, 2007)

If he came from the electrical (as opposed to electronic, a lot of people interchange the two terms) industry I could maybe understand. IIRC old time fluorescent with the simple ballast and starters consumed a little more at starting then operating. Also on multi phase motors they have to put large capacitors on the line to do power factor corrections and if those aren't the right value it can make the meter do funny things.
My computer (laptop) has a 65W power supply and thats even with enough overhead to run external drives and USB devices so even though I leave it on all the time its using approx 30W.


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## yuandrew (Jan 5, 2007)

^
And that reminds me of my Dad who thinks that just because my Desktop computer has a 400 watt power supply means it uses 400 watts all the time when it is running. In fact, it only uses about 160 watts plus another 40 for the LCD monitor.


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## Achilleus (Jan 6, 2007)

Go outside, read the meter. Go inside, flash the lights 1000 times. Go outside and read the meter. Report back findings.


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## hizzo3 (Jan 6, 2007)

Achilleus said:


> Go outside, read the meter. Go inside, flash the lights 1000 times. Go outside and read the meter. Report back findings.



i dunno you may have a huge electric bill after doing so....  

as far as turning on and off florescent lights, it depends on the quality of the ballast. cheap ballast dont like cycling very well.


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## PhotonWrangler (Jan 6, 2007)

Achilleus said:


> Go outside, read the meter. Go inside, flash the lights 1000 times. Go outside and read the meter. Report back findings.



Or just install The Clapper on all of your lights and have everyone in the family applaud vigorously.


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## Achilleus (Jan 7, 2007)

hizzo3 said:


> i dunno you may have a huge electric bill after doing so....
> 
> as far as turning on and off florescent lights, it depends on the quality of the ballast. cheap ballast dont like cycling very well.


 
But I think the OP was talking about incandescent lights specifically. 

Anyway, what if the guy had flashing christmas tree lights?


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## jtr1962 (Jan 7, 2007)

The husband (at the time anyway) of one of my mother's cousins told me how everytime you turned on the air conditioner you burned $1 worth of electricity. This was in the mid 1970s when electricity was probably around 5 cents per kW-hr. Even then I knew he was wrong but shut my mouth to avoid an argument. Just doing the math it's obvious-$1 bought 20 kW-hrs. To burn this much in the 10 seconds or so a compressor took to get up to pressure would mean a power input of 7.2 megawatts. I think that's about what a large, raging house fire puts out. If this were the case, the AC probably would have vaporized or at least melted instantly. Amazing how some myths go on thanks to an uneducated public.


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## winny (Jan 7, 2007)

As PhotonWrangler said, Mythbusters did break this myth, but with a quite incorrect method, judging from what you saw in the episode.

They did however come to the correct answer, electricity-wise, the time constant for incan bulbs is <1 second. They should have weighted in the cost for the bulb and electricity and found the break even-point, but I'll wait for that for the Mythbusters: Real engineers crack myths-version.

Anyhow, if only people would have learned to do integrals in school and knew E=P*t, non of these problems would exist. What an utopia!


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