32
\$\begingroup\$

Generally I see efficiency related to the lumens per watt, but what is the actual typical efficiency of LEDs in terms of electrical energy in to optical energy out? What sort of conversions apply?

\$\endgroup\$
  • \$\begingroup\$ There is no typical, it varies widely between manufacturers, colours, output power and operation. \$\endgroup\$ – PlasmaHH Aug 25 '17 at 9:33
  • \$\begingroup\$ @PlasmaHH Yes - but I would like some data with colors, output powers etc as parameters. For example, what is the most efficient color LED? Is IR or UV more efficient than visual etc \$\endgroup\$ – Dirk Bruere Aug 25 '17 at 9:58
  • 1
    \$\begingroup\$ 'I read somewhere' (which is always a bad start) that the theoretical maximum luminous efficiency, when calculated using the photometric weighting used for white light lumens, is 250 lumens/watt, on a 100% conversion of energy to light. If true, that means 100 lumen/watt of the best LEDs is pretty good, and there's scope for small improvements, but no large factors. Monochromatic sources should give better figures. \$\endgroup\$ – Neil_UK Aug 25 '17 at 9:58
  • \$\begingroup\$ @DirkBruere: that is some serious research effort to do, I doubt anyone would do it here. \$\endgroup\$ – PlasmaHH Aug 25 '17 at 9:59
  • 2
    \$\begingroup\$ As Arsenal noted, top performance commercially available white LEDs have around 200 lumen/Watt "efficiency" and somewhat more than 50% of the input electroical energy leaves the LED as light. [!!!!] \$\endgroup\$ – Russell McMahon Aug 25 '17 at 12:59
31
\$\begingroup\$

To make things clear let's define what we are talking about.

There are two terms which are mixed up pretty often:

  • Luminous efficiency:

The luminous efficiency is a dimensionless quantity which is derived from the luminous efficacy. It is simply the quotient of luminous efficacy of the source and maximum possible luminous efficacy of radiation.

  • Luminous efficacy:

This is the value you see more often. It usually has the unit of lumen per watt. And gives the luminous flux per power, which is a useful quantity to see how much light we will get with a given power.

With this we have to be a bit careful as well. Because the power can be the radiant flux of the source or the electrical power. So the former can be called luminous efficacy of radiation, and the latter luminous efficacy of a source or overall luminous efficacy.


Now the problem arises, that we cannot see all colors equally well. And lumens are actually weighted based on the response of our eye:

CIE 1931 Luminosity.png
Public Domain, Link

So with this, you can create some values of upper bounds (based on the redefinition of the unit candela). This would be the luminous efficacy of radiation.

Which are:

  • Green light at 555 nm: 683 lm/W
  • Maximum for CRI=95 at 5800K: 310 lm/W (based on truncated black body radiators)
  • Maximum for CRI=95 at 2800 K: 370 lm/W

For more see here.

If you lower the color rendering index (CRI), you can achieve higher values. But not higher than 683 lm/W.


So how efficient are LEDs?

Here we have values of luminous efficacy of a source.

Well there is a race of efficiency. Cree posted a press release with a laboratory LED of 303 lm/W at 5150K. The CRI was not mentioned, I guess it is lower than 95, but based on the data above that seems like it would have a luminous efficiency of something like 80% to 90%.

Of course your average available LED has less. 100 lm/W would be around 25% to 30% and the new 200 lm/W chips announced recently (as of August 2017) reach 50% to 60%.

Note that the above is for photopic vision (day-vision), things change with scotopic vision, but that's usually not so interesting.


If you really want to get into the guts of it, you'd have to take the spectrum of the LED and find out what the highest theoretical maximum for that spectrum is (based on the weighting curve) and then you can calculate the value.

As each and every LED has a different spectrum it is hard to get this data easily.


I hope I haven't made a mistake here, because I always find the topic a bit confusing no matter how many times I revisit it.

\$\endgroup\$
  • \$\begingroup\$ So if your off the shelf LEDs are between 25-50% efficient, and your off the shelf LEDs purport to use 5-10 watts of electricity, and your off the shelf incandescent bulbs purport to use 50-100 watts of eletricity (for similar lighting), does that mean incandescent bulbs were 2.5-5% efficient? \$\endgroup\$ – Sidney Aug 25 '17 at 16:43
  • 5
    \$\begingroup\$ @Sidney Yes, something around that. Incandescent bulbs put out something between 10 to 15 lm/W at around 2500 K to 3000 K, with the value above of 370 lm/W luminous efficacy of radiation, you end up with around 3% to 4% luminous efficiency. Yeah, pretty bad. \$\endgroup\$ – Arsenal Aug 25 '17 at 16:49
  • \$\begingroup\$ Assuming the price of the Cree chips is swamped by the total power consumption over the expected lifetime, then this result essentially finishes a 700 year effort by mankind to reduce the cost of lighting: ourworldindata.org/light -the best possible result is only a 10% improvement. \$\endgroup\$ – user14717 Aug 25 '17 at 20:09
  • \$\begingroup\$ A follow up question concerning color LED and a hopefully correct answer can be found here: electronics.stackexchange.com/questions/325949/… \$\endgroup\$ – Arsenal Sep 1 '17 at 22:43
7
\$\begingroup\$

As noted in the comments, it depends.

Older LEDs often have a lower efficiency than newer types.

Some bulbs have more efficient electronics to convert the mains voltage to the DC voltage needed for the LEDs.

But for a given LED light bulb you could make an estimate as often the amount of power needed by an incandescent light (with a similar amount of light output) is printed on the box. According to Wikipedia the average efficiency of an incandescent light bulb is 2.2 %.

Lets take the Ikea "LEDARE" E-27 600 lumen light bulb as an example:

Equivalent power for incandescent bulb: 48 W Actual power used: 8.6 W

So that means this bulb claims to be 48 / 8.6 = 5.6

times more efficient than an incandescent bulb so that would result in:

5.6 * 2.2 % = 12.3 % efficiency.

For this Ikea Ledare lamp.

Note that this is the total efficiency so the efficiency of the electronics times the efficiency of the LEDs themselves.

Proper LED driver electronics should have an efficiency of 85 - 99 % (that is my personal guess !) So the actual efficiency of the LEDs will be slightly higher than the 12.3 % I just calculated.

That's assuming all numbers given by Ikea are true of course.

\$\endgroup\$
  • \$\begingroup\$ Thanks. However, I am more interested in the actual LED itself rather than entire light fittings \$\endgroup\$ – Dirk Bruere Aug 25 '17 at 9:56
4
\$\begingroup\$

You need 1/683W of power to generate 1 lumen of light. It means that efficiency is somewhere around 12%. This is how it goes:

Firstly, let's assume that we have a light that radiates equally to all directions. By definition, 1 candela is 1/683W (550nm monochromatic light). 1 candela radiates to 1 steradian angle, which is 1/4π (8%) of the full sphere surface area. So, you need 4π/683W to produce 1 candela to all directions and the total luminous flux is 4π = 12,6 lm.

Which power is equal to 1 lm? You get it by dividing 4π/683W by 4π and the end result is (4π/683W) / 4π = 1/683W. Essentially, you need 1/683W of power to create 1 lumen luminous flux (1/4π = 0,08 candela to all 1 steradian angles).

Using above figure you need 900 times 1/683W = 1,32W to produce 900 lumen luminous flux.

My real light bulb, procured from a local store, state 900 lm and 11W of electric power. I assume that it radiates light equally to all directions. Using previous figures, electrical efficiency of the bulb is 1,32 W / 11 W = 0,12 which is equivalent to 12% efficiency.

\$\endgroup\$
  • \$\begingroup\$ You are introducing quite a bit of error in assuming that you light bulb is emitting monochromatic green light. As stated in my answer the maximum lumen per watt depends on the light temperature and the color rendering index. @2700K, which is a typical value for household bulbs, you end up with a value of 370 lm/W, so your bulb would be 22 % efficient. \$\endgroup\$ – Arsenal Jan 31 at 22:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.