I have been on a mission to slow replace all the bulbs to LEDs. At first, I use to judge the LED bulb's efficiency by looking at the % Energy Saving, that all bulbs have at the front of the box. However, I realised, its not a good measure of the bulb's efficiency.

So now I look at the two measures : wattage & lumens and divide the lumens by wattage to get the efficiency of the LED bulb I am purchasing. As an example, I try to go for a bulb which is atleast 100 lumens / watt. Unfortunately, most of the LEDs I have already installed are well below that (going around 85 lumens / watt).

My question is, is this the correct way to judge? How does one tell which are the most efficient bulbs on the market?

  • 1
    Color Rendering Index (CRI) is important to me, but maybe not to everyone. I'm not sure how it impacts efficiency though.
    – Dotes
    Nov 8, 2017 at 3:58

2 Answers 2


Watts per lumen is the way I evaluate With more lumens per watt is the best way because you have to look into the numbers to figure out how it was calculated for savings based numbers. I have seen 2 LED lamps with the same voltage & lumens show quite different saving numbers. The last thing to think about is the color frequency. For my home and plant I go for higher numbers for 2 reasons, 5k+ helps to reduce SAD and looks more like daylight. Down around 3k looks yellow/ orange and althow some lamps may score higher energy savings at a lower # this is based on incandescent lamps that match this color. the "warmer 6k lamp" used less power and to the human eye looks brighter the older technology led were advertised as more cost savings based on lower initial cost (due to older technology). Added per request; I live in Oregon where SAD is a problem because of cloudy sky's a broad spectrum higher color frequency light ( in the blue spectrum) helps reduce the affects when I worked at HP the plant was relamped with 6k lamps to combat this issue. Some rooms use only yellow light to prevent the photo masks from being exposed a test was done in 1 area where the operators that worked in the photo area spent at least 15 minutes with direct exposure to 6k & 6.5k at higher than normal light levels and the "sick time" for the area dropped after that they did the plant relamp and 3-4 years later the average "sick time" across the entire site was reduced by 3% according to the site health nurse.

  • Most helpful. Thank you, however, would you please elaborate on the SAD lighting, I believe @jeff above is referring to the same. From my understanding, the higher then index, the whiter the light, while SAD is suppose to be more like the sun so would have a lower number, around 2500-.
    – Kayote
    Nov 8, 2017 at 4:17
  • Generally LEDs will have more lumens/watt at higher color temperatures, since internally they make blue light and it takes less phosphor to make a bluer light (6000k) than a yellower one (2700k). Nov 8, 2017 at 7:34

I see 5 things to aim for.

Product quality. Quality matters, as the weak point in any screw-in LED lightbulb replacement is the power supply. The house label cheapies use cheap capacitors and are bound to fail a lot sooner. The internal LEDs proper will outlive us all.

Aiming. Up until LED, all lights made a sphere of light. But if you look at any lighting application, you don't want a sphere - you want a cone or wedge. LEDs natively do make a cone of light of about 160 degrees. In the past we tried to salvage some of the wasted light with reflectors, but those aren't very efficient. Optical lenses are very efficient, and work well with a 160 degree input. So for instance in a 60 degree downlight, almost all the LED's lumens can be brought to bear, where you were lucky to get 1/3 of the incandescent's lumens.

Some LED screw-in lightbulb replacements are built to emit a sphere of light. This is wasteful in most cases.

Lumens per watt. A lumen measures how much light the bulb puts out, in a 360 degree sphere. Incandescents do get credit for the lumens they waste in the wrong direction. This means an LED which emits a cone of light, will be more intense in its active direction than one which emits a sphere of light. So for instance a built-in LED downlight fixture of 1000lm will be much brighter than an incandescent downlight fixture that takes a common Edison bulb of 1000lm.

Lumens per watt is the light efficiency. It is adjusted for how humans see light. but it should be multiplied by the aiming factor.

Color Temperature. This is how blue or yellow the light is. If you've driven at night in the past few years, you notice some headlights are much bluer than most. LEDs can be any color temperature, except they are more efficient at the bluer lights.

  • 2700k is a hearth or gaslight color
  • 3000k is a halogen color (barely different)
  • 3500k is some yellower (greener) fluorescent lights
  • 4000k is the standard for fluorescent lights
  • 5100k is "daylight" and typical of a cloudy day.
  • 6000k is blue-sky daylight.

Color Rendering Index or CRI. This is a totally different factor than color temperature. This represents how balanced the light appears to the human eye - how natural looking, and is a score from 0 to 100. If you remember old fluorescent lights which made things look really "blah", that is bad CRI. Remember those yellow streetlights (high pressure sodium)? Those have a 20 CRI. The older deep yellow streetlights (low pressure sodium), their CRI went into negative numbers! The early LEDs that looked jarring had a CRI problem.

In an effort to ban fluorescent lights, the government now requires they have very good CRI. The industry responded by making them even better CRI - as high as 98. I pay abut $1.60 a tube for 90 CRI fluorescents, and they look really good. 80 CRI LEDs are readily available and 90 CRI can be had. Incandescents were 99 CRI, being very similar to an ideal.

  • A very informative answer, thanks for the details.
    – Kayote
    Nov 9, 2017 at 16:54
  • I am looking at LED lighting a lot since reading these immensely helpful answers, however, I am struggling to find the CRI ratings on bulbs. How do you find / calculate the CRI of an LED bulb?
    – Kayote
    Nov 12, 2017 at 6:51
  • 1
    @Kayote I just looked at my "150W" GE 3-way, and no CRI stated. It's harder to find than I thought on LEDs, I would search for a manufacturer data sheet. The better ones will test and will say. Very easy to find on fluorescent tubes. Nov 12, 2017 at 7:47
  • Thanks for the info. I was puzzled how everyone here was getting the CRI readings from. I visited Queen Elizabeth Birmingham hospital and I couldn't help but notice how wonderfully soft & pleasing the lighting was, even though it didn't appear to be LEDs. CRI is definitely something I am focusing more on.
    – Kayote
    Nov 12, 2017 at 8:21

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