Every advert I come across for LED bulbs advertise them as the equivalent of a higher W incandescent bulbs.

This makes no sense to me, if the room requires 40W to lighten it up then it'll always require 40W of energy. How is it possible for 6W of energy to do the job? What am I missing here?

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3 Answers 3


A 40W incandescent light bulb has a luminous efficiency of 1.9%. That means only 1.9%, or 0.76W, of the energy consumed by the bulb ends up as visible light.

LED bulbs have an efficiency of around 10% - the efficiency depends on the design and can be as high as 15% or as low as 8%. So a 6W LED bulb will produce between 0.9 and 0.48W of visible light.

The claim that a 6W LED bulb produces as much light as a 40W incandescent bulb requires the efficiency of the LED bulb to be 12.7%, which is well within the range of efficiencies that LED bulbs can achieve.

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    $\begingroup$ LEDs are only ~10% efficient? I thought they were much better than that! $\endgroup$
    – jamesqf
    Commented Apr 29, 2016 at 17:19
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    $\begingroup$ @jamesqf LED lights still get very hot. Any heat generated by an electrical or mechanical device (other than a heater, of course) is wasted energy. $\endgroup$ Commented Apr 29, 2016 at 20:45
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    $\begingroup$ @Rob Jeffries: Yes, the heat is largely wasted, because the same amount of electricity used to run a heat pump can produce several times as much heat. $\endgroup$
    – jamesqf
    Commented May 1, 2016 at 5:43
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    $\begingroup$ @jamesqf How could that be? A given amount of electric energy is converted to light and heat, both of which are useful (say, in winter). The only kind of "waste" here is that alternative energy sources for heating are cheaper (like natural gas). $\endgroup$
    – isarandi
    Commented May 2, 2016 at 11:15
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    $\begingroup$ @isarandi You can use a 50W light bulb, use 50W of electricity, and get 2W of light and 48W of heat. Or you can use a 10W LED bulb and a 40W heat pump, and get 2W of light, and 8W + (40W*3) = 128W of heat for the same amount of electricity. (3 being a made up number of course) $\endgroup$ Commented May 3, 2016 at 5:20

The advertising suggests that the new 6W bulb generates as much light as a 40W incandescent light used to. Most of the energy in an incandescent light bulb is converted into infrared radiation that we can't see or it's at the red end of the spectrum where the human eye is not very sensitive. This


is a typical incandescent spectrum and you can see how little of the emissions fall into the visible. All the power outside of the narrow visible band is wasted.

In comparison, a modern LED lamp contains a blue emitter chip and a yellowish phosphor, the spectral contributions of both can be seen here:

enter image description here

Please note that the emissions in this graph only cover the visible waveband, which is not the entire physical truth, of course, the graph is just cutting out the emissions in the deep infrared due to the low temperature heat that the bulb is still emitting. Despite this omissions, the total conversion efficiency of electrical energy to visible light is still much higher than that of the incandescent light.

Having said all of that, some of the practical results with this kind of advertising will be disappointing. For one thing the industry has a bad habit of over-speccing their products (that is also the case for the incandescents!), for another, the radiation patterns of many of these lights are different and they may produce brighter light in one area but then fail to illuminate the entire room. I would expect to put, at least, 50% more "equivalent" lighting power in with the new lights than with incandescents to get similar results. That is still a significant energy savings. If you can trade 9W of LED lights for 40W of incandescents, that's savings of roughly 75%.

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    $\begingroup$ Would it be possible to add a graph an example LED light spectrum for comparison? $\endgroup$
    – Oliphaunt
    Commented Apr 29, 2016 at 7:12
  • $\begingroup$ Spectra are available at en.wikipedia.org/wiki/Light-emitting_diode --- I suppose that the commercial white LEDs are phosphor-based, so: upload.wikimedia.org/wikipedia/commons/thumb/4/48/White_LED.png/… $\endgroup$
    – Rmano
    Commented Apr 29, 2016 at 9:12
  • $\begingroup$ Incandescent power ratings are important - they need to be matched to an outlet capable of supplying that wattage, or there can be fire risks. Over time, though, the buying public started equating power ratings (not over-specced) with brightness (not correct to do), and so we have these "40W equivalent" for people who still think of brightness in terms of incandescent power ratings. $\endgroup$
    – Ehryk
    Commented Apr 29, 2016 at 14:17
  • $\begingroup$ I disagree that you'd need more lighting. There are plenty of commercially available LED fixtures which have equal or superior light distribution to incandescent bulbs. In some applications LEDs can actually reduce the total lighting requirement. $\endgroup$
    – nhinkle
    Commented Apr 29, 2016 at 16:14
  • $\begingroup$ @Ehryk: The power ratings are usually correct, as you said, but the promised brightness and lifetime of both incandescents and LED lights is often enough "pushing" the truth a little. As with all advertising... buyer beware. The main problem for buyers of LED lights is that the industry did not, as it should have, disclose that the base electronics of these lights is heat sensitive. When used in the wrong sockets and lamps (old designs made for incandescents), the temperatures will often be too high and the electronics will fail early. $\endgroup$
    – CuriousOne
    Commented Apr 29, 2016 at 16:50

You're mixing power needs with luminous effects.

According to the advert you posted, that LED bulb consumes 6W (power) to get a luminous flux of 500 lumens (lumen is a photometric unit, like candela or lux): https://en.wikipedia.org/wiki/Luminous_flux

On the other hand, an incandescent bulb would need to consume 40W (power) to get the same luminous flux.

That is what the advert is telling you. You don't "need" 40W to lighten up a room, that is the power you invest to get the light. LED bulbs have a better return of investment because they are based in a completely different physical phenomenon (incandescence vs. photoelectrical effect).

EDIT: Of course, as CuriousOne pointed out, the advert is probably overstating the advantages of the product. That is what they do, after all.

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    $\begingroup$ I just have to specify that "LED bulbs have a better return of investment" of light. From an incandescent bulb you would also get warmth. This is not always desirable, but in cold regions (such as my home in Norway), incandescent light bulbs used indoors gives you point heating of a room which is slightly more efficient than a few electrical ovens placed along walls which is the usual setting. Btw, in Norway in general light, heating and cooking are all electrical so they end up on the same bill, thus indoor LEDs just shift the energy around the bill (if your ovens are on). $\endgroup$ Commented Apr 29, 2016 at 9:54
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    $\begingroup$ Going to miss the 100w IC light bulbs... They're a relatively cheap, safe, and easy way to keep some heat in engines in cold hangars/garages during deep freezes. $\endgroup$ Commented Apr 29, 2016 at 15:30
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    $\begingroup$ @KnutGjerden these coincident effects work against you in the summer though, especially in homes with air conditioning. Also, bulbs aren't any more efficient at heating than an electrical resistance heater is. If anything the wall heaters are better. Heat rises, and lights are typically on the ceiling. Most spaces don't need the top of the room heated because the people stand on the floor, not on the ceiling. $\endgroup$
    – nhinkle
    Commented Apr 29, 2016 at 16:16
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    $\begingroup$ The argument about incandescent bulbs being cheap electrical heaters ignores the fact that if you are heating your home mainly with electricity, you could use a heat pump which is vastly more efficient. These are widely available in Norway. $\endgroup$
    – jwg
    Commented May 1, 2016 at 22:37
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    $\begingroup$ @nhinkle you are right, but air condition is not common in homes in Norway. And even the summer does not get very warm many places. The majority of the year, people heat their homes. April 2015-2016 the average temperature where I live was 5.9 C. $\endgroup$ Commented May 2, 2016 at 7:51

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