Battery Equivalents and Replacements

Luminous Efficacy: Lumens to Watts Conversion

Different light sources emit different amounts of visible light using the same amount of power. Luminous efficacy is a measure that shows how well a light source converts one type of energy, usually electricity, into visible light.

Luminous efficacy is a ratio of luminous flux (given in lumens) and power (given in Watts) and is written as "lm/W." In short, more energy-efficient light sources will provide more lumens for the same amount of power.

Published: December 2, 2022.

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Luminous Efficacy Of Various Light Sources

The following comparison chart lists the luminous efficacy of various light sources given in lm/W, but also as a percentage of the converted energy.

Light Source Luminous Efficacy (lm/W) Luminous Efficacy (%)
Incandescent (Tungsten) 8-16 1.2-2.3%
Incandescent (Halogen) 17-24 2.5-3.5%
Fluorescent 60-100 9-15%
Light-Emitting Diode (LED) 75-210 11-30%
Metal Halide Lamp 65-115 9.5-17%
Low-Pressure Sodium Lamp 100-200 15-29%
High-Pressure Sodium Lamp 85-150 12-22%
Plasma Display 2-10 0.3-1.5%
Mercury Vapor Lamp 35-55 5-8%
Ideal Monochromatic Light Source (555 nm) 683.002 100%

As one can see, actual luminous efficacy differs from light source to light source, even among light sources that are based on the same technology.

Obviously, light sources based on heat or chemical energy, like an incandescent gas mantle lamp or limelight burner, are not considered.

As one can see, some of these light sources are obsolete, while some of them have rather limited use.

Luminous Efficacy of Household Light Sources


The following comparison chart lists typical luminous flux (lumens) for various light sources and their electric power consumption (given in Watts):

Lumens Incandescent
Fluorescent LED
90 15 6 2-3 1-2
 200  25   3-5  3-4
 450  40  29  9-11  5-8
 800  60    13-15  8-12
 1100  75  53  18-20  10-16
 1600  100  72  24-28  14-17
 2400  150    30-52  24-30
 3100  200    49-75  30-36
 4000  300    75-100  40-45

Very often, light sources used at homes are compared with a 100 Watts incandescent tungsten (Wolfram, W) light bulb that was a very popular light source until newer, more efficient light sources appeared, like fluorescent lights and especially LED lights.

Thus we can write:

  • Incandescent Tungsten 100W Bulb: 100W, 1600 lumens → 16 lm/W, ~2.3%,
  • Incandescent Halogen 72W Bulb: 72W, 1600 lumens → 22.2 lm/W, ~3.25%,
  • Fluorescent 25W Tube: 24W, 1600 lumens → 66.6 lm/W, ~9.75%,
  • LED 14W Bulb: 14W, 1600 lumens → 114.3 lm/W, ~16.7%

Note: these are some average, general values. For the exact values of your light source(s), check the documentation.

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Energy Saving Light Sources: Are They Worth It?

Incandescent tungsten light bulbs are still used for various reasons, one of which is that they are very cheap. But, with the advances in technology, LED light bulbs are cheaper and cheaper. But which one to choose?

If we compare incandescent tungsten light bulbs and LED light bulbs that are both producing 1600 lumens, we find out that an incandescent light bulb requires 100W of electric power, and an LED light requires ~14W.

Note: some manufacturers even offer LED light bulbs that are rated at 1600 lumens, 13W, or even 12W.

When those light bulbs are used 8 hours per day, 30 days per month, that is 240 hours per month.

During one month, these light bulbs require (assuming $0.15 for 1 kWh of electricity):

Incandescent Light Bulb → E = 240h * 100W = 24000 Wh = 24 kWh = $3.6

LED Light Bulb → E = 240W * 14W = 3360 Wh = 3.36 kWh = $0.504

So, if we replace one 1600 lumens 100W incandescent light bulb with 1600 lumens 14W LED light bulb, one can save ~$3 per month if those light bulbs were/are running for 8 hours per day.

The average price of a good 1600 lumens 14W (or even less Watts) LED light with a CRI in the 90+ range is 3-10 $US, rarely more, depending on the model.

So, a good LED light bulb pays itself off after 1 to 3 months.

If three months are required for a LED light bulb to pay itself off, during a single year, such LED light can save ~27 $US (9 months x ~3 $US per month), which is not a small amount, especially if more incandescent light bulbs are replaced.

Long Story Short: With a luminous efficacy of 100+ lm/W, home LED light bulbs are becoming the most popular light source at homes and apartments - just be sure to pick the light bulb with the right CRI (Color Rendering Index, 90% or more) and the light temperature that suits your needs.

For example, the "warm white" of 2700K is very close to the incandescent tungsten light bulb temperature of 2800K.