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Help me save a friend's marriage! :-)

They have 2 light fixtures in their bathroom. One is incandescent with 6 75W bulbs, and the other is a fluorescent fixture with a 22W bulb that looks like it probably a type T9 bulb.

Whenever she goes into the bathroom, she turns on the incandescent fixture, because she heard that it is expensive to turn on an fluorescent fixture.

In contrast, whenever he goes into the bathroom, he turns on the fluorescent fixture because he knows that 6 75W bulbs uses much more energy than 1 22W bulb.

But she argues he winds up using more energy because of the initial draw on the fluorescent bulb.

He argues that she winds up using more energy because of all those incandescent bulbs, and the heat they create.

I suggested that maybe they replace the incandescent light fixture, but they both said that's not an option right now (they are saving to eventually buy a nice LED one). Then I thought maybe they should replace the fluorescent one, but I questioned whether or not the cost (and waste if they didn't find a new home for the old fixture) would be worth it.

What do others think? How much energy does a fluorescent T9 bulb use on start-up? Should they use the fluorescent fixture for long bathroom visits, but the incandescent for short visits? How do you define long and short? Would it be worth it to replace the fluorescent fixture?

All in all, I say they are pretty happily married, if this is their biggest argument! :-)

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  • To answer conclusively, we'll need to know at least what type of ballast is used in the fluorescent fixture (though exact model number would be better). How old are the fixtures?
    – Tester101
    Apr 15, 2015 at 11:16
  • The ballast limits the amount of power that can be delivered to the lamp 22w is the limit, compared to 450w of the incadesents the math is simple the incadesent lamps draw close to 20 times the power all the time.
    – Ed Beal
    Apr 8, 2019 at 13:47

3 Answers 3

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I once performed a direct study on this subject. I built an apparatus to power-cycle a T12 fluorescent lamp every 60 seconds and an identical lamp every 12 hours. I used both recording watt-meters and standard household electric meters to measure the power consumed.

I expected that the power consumption of the filaments during preheat would be different from the power consumption of the ionized arc. Probably more, but possibly less. I was surprised to find that even after 96 hours I detected no measurable difference.

I could only guess that the ballast limited the filament current just as it limited the ion arc current.

This was so long ago that there was only one kind of ballast available, what we now call a dumb ballast. I don't know what the result would be with a modern fixture.

I also applied an oscilloscope to the line side of the ballast and I could see a power spike that lasted less than 1/120 second. I couldn't get a good idea of its magnitude. I theorized that the meters used moving parts and could not practically respond to an event of such short duration.

My conclusion was: Yes, a fluorescent lamp does consume a power surge on startup -- but you don't pay for it!

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  • This is correct a ballast is there to limit the current, the strike that some think is a high draw is actually a high voltage low current.+
    – Ed Beal
    Apr 8, 2019 at 13:49
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They should use the fluorescent and leave it on if they both are going to use the bathroom within a few minutes of each other. the energy cost of turning it off and on is saved within a few seconds. http://energy.gov/energysaver/articles/when-turn-your-lights

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  • Thank you for the excellent reference. It has great info about CFL bulbs. Are T9 fluorescent bulbs considered CFL's? What about the long "shop light" fluorescent bulbs... are those considered CFL's?
    – RockPaperLz- Mask it or Casket
    Apr 13, 2015 at 9:09
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    CFL's are called Compact Fluorescent Lights ... Think of the curly-Q bulbs and you've got it. If it's straight and long, it's not a CFL. All fluorescent bulbs work off of the same principles, though, so don't get hung up on a name.
    – Pᴀᴜʟsᴛᴇʀ2
    Apr 13, 2015 at 11:19
  • @Paulster2 Thanks. T9 bulbs are neither curly-Q nor straight and long; they are circular. In that article, can one simply replace "CFL" with "any fluorescent bulb"? I wonder why they specifically wrote "CFL".
    – RockPaperLz- Mask it or Casket
    Apr 13, 2015 at 20:52
  • @RockPaperLizard ... I didn't see the "right" T9 bulb, sorry. But there is definitely a difference between the CFL and the T9. I would be curious as to why they were calling a T9 a CFL ... nothing too compact about them, lol.
    – Pᴀᴜʟsᴛᴇʀ2
    Apr 13, 2015 at 20:57
  • @Paulster2 Who is calling a T9 a CFL bulb?
    – RockPaperLz- Mask it or Casket
    Apr 13, 2015 at 21:04
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The fluorescent-start issue is not about energy. It's about tube life.

Fluorescents are a type of "discharge light" meaning an electric arc runs down the length of a tube of vapor. Many lights are: the most obvious is neon, but also sodium, mercury and metal halide.

To start the tube, the ballast must initially strike the arc. It does this with a spike of high voltage across the electrodes at both ends. They have a chemical coating. Normal operation wears the coating, but starts wear it a lot more. If your fluorescent fixture is frequently turned on-off, the tubes will fail sooner. That said, tubes are cheap - cheaper than the electricity needed to run the equivalent incandescents (which also fail sooner from frequent starts, and fail sooner in any case.)

Also, smarter ballasts greatly ease startup wear by preheating the bulbs for a soft start, however, this takes a second or two to start up. I don't mind using a programmed start ballast on a motion sensor.

The best answer is LED. They are not degraded by frequent cycling, in fact, turning them on/off 500 times a second is one common way to dim them.

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  • Also worth noting is that fluorescent bulbs all contain mercury, which is a biohazard. They require special hazmat disposal. They release toxic mercury onto nearby surfaces if they break.
    – RockPaperLz- Mask it or Casket
    Mar 16, 2016 at 2:23
  • According to Nicholas French (I did not confirm this information), Not all FL bulbs contain mercury. Older bulbs, yes. But there has been a big push towards the manufacturing of FL and CFL without mercury for many years now, hence the green ends on T12, T8, and several others. That doesn’t justify disposing of them improperly, however. It does mean though that those won’t deposit mercury into the ecosystem when disposed of improperly and therefore aren’t QUITE as environmentally unfriendly as their predecessors. (edited slightly)
    – RockPaperLz- Mask it or Casket
    Apr 8, 2019 at 6:31
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    The green lamps I have purchased still have Mercury in them but much lower levels, by law I must recycle them since I used to use several hundred a year, moving to LED's better light less maintenance and big savings on the power bill.
    – Ed Beal
    Apr 8, 2019 at 13:56
  • @EdBeal Thank you for the additional information.
    – RockPaperLz- Mask it or Casket
    Apr 8, 2019 at 18:23

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