I've read that old fluorescent light bulbs produce less light over time.

Do they also use less electricity, or do they use the same amount, but produce more heat?

  • Most likely, they produce less visible light and more UV light. What's getting tired is the phosphors which convert UV to visible. Feb 4, 2018 at 20:27
  • @Harper Thanks. (If you have a source for that that would be appreciated.) If that's true - that also means they grow less and less safe with time.
    – ispiro
    Feb 4, 2018 at 20:58
  • the UV probably does not make it through the glass
    – jsotola
    Feb 4, 2018 at 21:48
  • @jsotola I don't think it's that simple. (Example)
    – ispiro
    Feb 4, 2018 at 22:47
  • @ispiro, your citation includes the source information you asked Harper for.
    – fixer1234
    Feb 5, 2018 at 0:11

3 Answers 3


All lamp manufactures have the lamp curves, as the lamps are started it damages them a little and when running they are degraded but slowly. The industry standard is the lamp is at end of life when the light output is below 50%. Lamp life curve The ballast controls the wattage consumed this stays the same for the life of the lamp until the contacts are eroded and cannot strike. another lamp life example

  • Thanks. But what happens to the extra watts? Heat? More UV?
    – ispiro
    Feb 5, 2018 at 23:01
  • you will usually see the ends turning black because of the heat, I change lamps based on light meter readings this has helped to protect the ballast. The ballast prevents discharge lamps from drawing more power but provide the higher voltage to allow them to strike as the lamps age the voltage required to strike the lamp increases, I did notice that the short cycle time is 3 hours for phillips not 4 as I had commented below.
    – Ed Beal
    Feb 5, 2018 at 23:06
  • The black stuff on the ends near end of lamp life are not from heat. The cathodes inside are coated in a dust called emissions. This emission helps with creating the initial arc inside the tube. Each time the lamp turns on, some of this emission gets flung off the cathode. It lands on the edges as this black stuff. en.wikipedia.org/wiki/Fluorescent_lamp#Emission_mix
    – Swanson
    Feb 9, 2018 at 20:34
  • The erosion of the electrodes causes the black exactly as the electrodes are aged they also become shorter causing a higher strike voltage. You can call it anything but that is what is hapening
    – Ed Beal
    Feb 9, 2018 at 21:17

I have never heard of the wattage being affected by age. Temperature yes. I had to get this information directly from a high-bay light manufacturer as it is not published. A T5HO ballast used the listed wattage when the test chamber was at 20 degrees Celsius. But when the test chamber temp was increased to 25 degrees Celsius the lumen output increased as did the wattage. This explains why so many T5 fixtures had an efficiency greater than 100%. They used the ballast wattage at the lower 20C and the fixture lumen output at the higher 25C.

The lumen loss lines up well with the dirt loss. As the lamp gets covered in dust, the light output decreases. This would not affect the wattage. At around 20 months, the light loss factor of fluorescent is about 0.9. This is also about the dirt depreciation factor at 20 months. https://ouc.bizenergyadvisor.com/BEA1/OMA/OMA_Lighting/OMA-12

There may be some change in wattage as the mercury gets embedded in the glass over time. Affecting lumen output and fixture wattage. But I think the primary culprit is dust on the lamp and fixture surfaces which won't affect the fixture wattage.

  • The light output actually decreases lamps are considered to be at there end of life at 50% . short cycling decreases the lamp life I believe 4 hours is considered the minimum on time with some manufactures using +12 hour cycles to show longer life.
    – Ed Beal
    Feb 5, 2018 at 22:09
  • "Efficiency greater than 100%?" Really? They GENERATE power back into the grid?
    – JRaef
    Feb 6, 2018 at 2:05
  • Light fixtures do not generate any power, they consume power. Before LED fixtures became standard, other fixtures had an efficiency rating. If a bare lamp created 2,900 lumens, and this fixture with two lamps had an output of 5,000 lumens. then the fixture has an efficiency of 5,000 / (2 * 2,900) = 0.862 or 86.2%. Nothing can be 100% because the lamp has to be held by something that would absorb some light. When T5HO lamps got popular a lot of the linear, indirect manufacturers started showing fixture efficiency greater than 100%. They found a way to manipulate the math.
    – Swanson
    Feb 9, 2018 at 19:57
  • @Ed Beal - Yes, end of life is considered when 50% of test lamps fail. Cycling of on/off does affect the lamp life. But this was not the original post's question. They wanted to know about wattage of the lamp as the lumen output decreases near end of life.
    – Swanson
    Feb 9, 2018 at 20:09
  • See my answer I covered the end of life wattage, I did answer that part of the question also.
    – Ed Beal
    Feb 9, 2018 at 21:34

It's a waste, here's two lesser-considered reasons why:

Regardless of the actual lumens/watt of any particular old CFL, they are dimmer than new ones. That means the consumer will inevitably turn on additional lamps to achieve the desired brightness. This doubles the amount of power used for the task, even if the per-unit is the same old vs new.

Considering the electronics of the driver, things get more complicated than just phosphor wear. The capacitors used on CFLs, esp cheap CFLs are prone to failure. These are relatively high-voltage parts, which are expensive, leading manufacturers of bulbs, especially cheap ones, searching for the cheapest unit. They don't need to last 10 years, so they are MTBF-spec'd for the task.

As they dry, they lose capacity. As the capacitors lose capacity, the CFL driver must work harder to regulate output voltage. This causes more heat, and we all know where heat comes from; power. I don't have before/after measurements on like bulbs, but i know that old ones (even if completely dark) still use power, unlike a tungsten filament.

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