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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?

enter image description here

Hack-R
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Matka
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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.

Level River St
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John Rennie
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    LEDs are only ~10% efficient? I thought they were much better than that! – jamesqf Apr 29 '16 at 17:19
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    @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. – Mason Wheeler Apr 29 '16 at 20:45
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    @SenorO From reading that, they seem to be concerned about the amount of subjective warmth per watt. – user253751 Apr 29 '16 at 23:02
  • @Mason Wheeler: I wouldn't say they get very hot. The ones I'm using right now (60W equivalent) are cool on the light-emitting portion, uncomfortably warm at the ceramic base, but nowhere near the burn-your-fingers temperature of an incandescent. – jamesqf Apr 30 '16 at 05:39
  • Heat is not wasted energy (in winter). – ProfRob Apr 30 '16 at 19:56
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    @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. – jamesqf May 01 '16 at 05:43
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    @jamesqf, the monochromatic LEDs are more efficient. But to get white light a luminophore is needed and that reduces the efficiency quite a bit. – Jan Hudec May 02 '16 at 08:00
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    @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). – isarandi May 02 '16 at 11:15
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    Lumens in general is most of what a consumer cares about. "How bright is this bulb". Sure, kelvin too for the color. I'm quite surprised the industry hasn't moved toward what seems a more meaningful metric, lumens/watt. That number would highlight the immense efficiency of LED over incandescent. – Greg Bogumil May 02 '16 at 21:24
  • Even though the LED is only 12.7% efficient, that is more than six times as efficient as the incandescent. And the incandescent puts out 7.5 times as much heat to produce the same amount of light. That's why incandescent bulbds feel hot and LEDs don't. – stannius May 03 '16 at 00:04
  • @SeñorO How does one get an efficient space heater? Mine is only 100% eta (Q/W)! – Aron May 03 '16 at 04:20
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    @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) – user253751 May 03 '16 at 05:20
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    @isarandi: A space heater takes electric energy and converts it to heat energy. A heat pump (aka air conditioner) takes electric energy and uses it to import heat energy from outside. 1W of electric energy can get you about 3W of heat energy (depending on efficiency and temperature gradient). So instead of wasting 100W on an incandescent bulb, it's better to spend 20W on a LED and 30W on AC, for the same effect. – Meni Rosenfeld May 03 '16 at 13:16
  • @jamesqf The light-emitting portion does get hot - but that's not the bulb in an LED. The base carries most of the heat to the heatsink at the bottom of the bulb, that's why the bulb itself is relatively cool, while the heatsink can easily burn you. Sure, it's a lot less Watt/area, but it's quite hot. This isn't a design to prevent the bulb from heating - it's just necessary to make the LED work well in the first place, since the efficiency and lifetime drops like crazy as the chip gets hot. – Luaan Aug 02 '16 at 11:36
  • @MeniRosenfeld Yes, but a lightbulb is quite a bit cheaper. Heat pumps (and AC) are still very uncommon where I live - they're just way too expensive, even with the rather large subsidies involved. Granted, the fact that ACs aren't common here is part of the reason - people just aren't familiar with the technology, and there's not a lot of suppliers... :) – Luaan Aug 02 '16 at 11:46
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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

https://i.stack.imgur.com/1nSqG.gif

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%.

CuriousOne
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    Would it be possible to add a graph an example LED light spectrum for comparison? – Oliphaunt Apr 29 '16 at 07:12
  • Spectra are available at https://en.wikipedia.org/wiki/Light-emitting_diode --- I suppose that the commercial white LEDs are phosphor-based, so: https://upload.wikimedia.org/wikipedia/commons/thumb/4/48/White_LED.png/350px-White_LED.png – Rmano Apr 29 '16 at 09:12
  • 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. – Ehryk Apr 29 '16 at 14:17
  • 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. – nhinkle Apr 29 '16 at 16:14
  • @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. – CuriousOne Apr 29 '16 at 16:50
  • @CuriousOne I clarifying that the power ratings aren't overspecced needlessly - though they may be an upper bound not a median for safety reasons, it wasn't a marketing thing to increase sales or anything. For LED lights, I somewhat agree they are marketing these really hard; the other thing they don't mention is that they are more directional, and a '40W' equivalent LED may not provide as much illumination in all directions as a 40W incandescent, and that also isn't mentioned. – Ehryk Apr 29 '16 at 16:55
  • @nhinkle: This is limited to a single sample of personal experience, but I rarely ever achieve the same amount of subjective illumination with the promised reduction in power ratings. Usually I have to install a higher than "promised" grade of LED lamp to replace an incandescent. That's OK. The prices of LEDs have come down so considerably recently that it doesn't matter any longer and soon enough we can completely do without this advertising gimmick (which is all this is), as people will learn using the actual luminosity specs, rather than the power consumption. – CuriousOne Apr 29 '16 at 16:56
  • @Ehryk: Of course the lighting industry is competing per marketing... what else can it compete with? That hasn't changed since the days of the incandescents and those of us with a little industry experience completely understand that one can not make a product without making a profit, which, whether we like it, or not, requires an effective marketing department. Been there, done that. :-) The real problem is, indeed, the radiation pattern, but the end user has to compensate for that, no matter what the specs, so better plan in a slightly higher than promised power replacement. – CuriousOne Apr 29 '16 at 17:01
  • @nhinkle: Re "...LEDs can actually reduce the total lighting requirement", I've found that to be especially true when using "bright white" or "daylight" LED bulbs, which more closely match natural light - rather than a whale oil lamp, which is what "warm white" seems to be trying to match. – jamesqf Apr 29 '16 at 17:25
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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.

jasev
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    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). – Knut Gjerden Apr 29 '16 at 09:54
  • @Knut I'm guessing you mean what in English are called 'electric fires' (device for heating) not 'ovens' (device for cooking). – peterG Apr 29 '16 at 12:30
  • @peterG almost, I think. Google told me electric radiator, whilst electric fire appears to be ... an electric fireplace. The Norwegian word is panelovn, if you do an image search you see what I mean. Like a radiator, but (potentially) much slimmer and nicer looking than a radiator. – Knut Gjerden Apr 29 '16 at 13:33
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    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. – Brian Knoblauch Apr 29 '16 at 15:30
  • @Knut - agreed 'radiator' is a better fit. – peterG Apr 29 '16 at 15:59
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    @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. – nhinkle Apr 29 '16 at 16:16
  • @BrianKnoblauch wire two '75W equivalent' LED lights to a hair dryer, and use that in your garage or under the hood then! You get 15x the heat output of a 100W light bulb, with more light! – Ehryk Apr 29 '16 at 16:57
  • @nhinkle: Gravity - it's not just a good idea - IT'S THE LAW! :-) – Bob Jarvis - Слава Україні Apr 29 '16 at 18:53
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    @mhinkle: Right, but they don't sell 100W electrical resistance heaters for under a dollar at every hardware store. – Nate Eldredge Apr 30 '16 at 06:50
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    @ Nate Eldredge: Maybe not the "under a dollar" part, but they do sell reasonably priced electric resistance heaters, e.g. http://www.homedepot.com/p/Frost-King-6-ft-Electric-Water-Pipe-Heat-Cable-HC6A/202262328 And the "under a dollar" is only the (breakable and subject to burnout) bulb, not the cord or fixture. – jamesqf Apr 30 '16 at 17:46
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    @BrianKnoblauch http://www.amazon.com/Pecute-Infrared-Ceramic-Emitter-Reptile/dp/B00ORM0JK8/ (Lasts a whole lot longer than a 100W bulb too.) – Solomon Slow May 01 '16 at 03:00
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    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. – jwg May 01 '16 at 22:37
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    @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. – Knut Gjerden May 02 '16 at 07:51
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    @jwg yes, and many people do, but many don't. It is an expensive investment, light bulbs are not. But as part of some environmental act in the EU, it is now not possible to buy incandescent light bulbs any more because they are measured only in term of electricity in and light out. Had people considered more than light as the only product of an incandescent bulb that could be different. – Knut Gjerden May 02 '16 at 07:59
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    @nhinkle: Well, you don't need lights in the summertime in the Nordic countries :) In all but the most southernmost parts, there is practically 24/7 daylight between May and August. – Juha Untinen May 03 '16 at 08:15
  • Worth noting is that the reason they do this is because the populace and light bulb manufacturers have conflated the luminous flux and power consumption for decades, so consumers are used to thinking about their bulbs in terms of power and not luminous flux. I'd be interested in the history of how that came about, but that's not part of this question (and is probably off topic for the site). – jpmc26 May 03 '16 at 09:50