How to Calculate Lumens Per Watt

"Lumen" comes from the Latin word for "light." The "watt," however, is named in honor of James Watt (1736-1819), a Scottish inventor, in recognition of his contributions to the development of the steam engine.

An Increasingly Common Term

Lumens have come into everyday use largely as a result of the government mandate to phase out incandescent light bulbs in favor of compact fluorescents. Because the two types of bulb consume different amounts of power to produce a given amount of light, lumens offer an easier way for consumers to compare the new lightbulbs with the older ones they are replacing.

Lumens per watt (or LPW) is an efficacy rating given to light bulbs to determine how much light they produce for each watt of energy they use. When you shop for produce at the market, you look for the best deals for a pound of fruit or vegetables. When you're shopping for light bulbs, you should look for ones that produce more light with less energy. Light bulbs with high efficacy ratings mean more savings on your power bill. Calculating LPW requires using the simple mathematical formula. Does this Spark an idea?

Tips & Warnings

Fluorescent bulbs last longer than incandescent, but their lumen output decreases over time, therefore manufacturers give two lumen ratings for fluorescent bulbs: an initial lumen rating and a mean (average) lumen rating. The mean lumen rating is determined at 40 percent of the light bulb life. Buy light bulbs with the highest efficacy rating to get the most light from the least amount of electricity and decrease your monthly power bills.

What is a Lumen?

Lumens are a measure of the perceived power of light -- i.e. of light you can see. Waves of light that are beyond the spectrum visible to the human eye are not included in the lumen.

Lumens are the amount of light produced by a bulb. Find the number for lumen output on the light bulbs box.

How to convert lumens to watts

How to convert luminous flux in lumens (lm) to electric power in watts (W).

You can calculate watts from lumens and luminous efficacy. Lumen and watt units represent different quantities, so you can't convert lumens to watts.

Lumens to watts calculation formula

The power P in watts (W) is equal to the luminous flux ϕV in lumens (lm), divided by the luminous efficacy η in lumens per watt (lm/W):

P(W) = ϕV(lm)/ η(lm/W)

So

Watts = Lumens / (Lumens Per Watt)

or

W = Lm / (Lm/W)

Example

What is the power consumption of a lamp that has luminous flux of 900 lumens and luminous efficacy of 15 lumens per watt?

P = 900Lm/W / 15Lm/W = 60W

Luminous efficacy table

Light type Typical
luminous efficacy
(lumens/watt)
Tungsten incandescent light bulb 12-17 lm/W
Halogen lamp 16-24 lm/W
Fluorescent lamp 45-75 lm/W
MCOB LED lamp 100-130 lm/W
Metal halide lamp 75-100 lm/W
High pressure sodium vapor lamp 85-150 lm/W
Low pressure sodium vapor lamp 100-200 lm/W
Mercury vapor lamp 35-65 lm/W

What is a Watt?

A watt is defined as the amount of power needed to move a one kilogram object at a speed of one meter per second against a force of one newton. In simple terms, it is a measure of the amount of energy something either uses or produces. One watt is equal to 1/746 horsepower.

Wattage is the amount of electricity needed to light the bulb. The higher the wattage is, the brighter the light will be. You can find the bulb's wattage written both on the box and on the bulb itself.

Calculate Watts for LEDs

Did you ever wonder how many watts an LED is consuming? It is very easy to calculate. Simply take the forward voltage drop and multiply it by the current (remember Oms Law - P=I*E). So assume you have a bright white LED that is rated at 3.2 Volts forward voltage. You'll find this in the spec sheet. If you're running at 20 mA, total power is calculated by multiplying:

Power in Watts = Amps * Volts or 3.2 * 0.020 = 0.064 Watts!

Now you can see why these light sources are so energy efficient! If you had a 15,000 mcd LED with a 30° Viewing angle, you'd be delivering 3.2 lumens using 0.064 watts. That is 50 lumens per watt! Compare that to the 5 to 15 lumens per watt from regular incandescent lamps above.

Note:
To set the current at 20 mA:
Assume you will power it with 5 volts at 20 mA using a resistor in series with the LED. To set the current at 20 mA subtract the forward voltage from the power source (5.0 - 3.2 = 1.8 volts). We will want a resistor to drop 1.8 volts at 20 mA, so:
1.8 volts divided by .02 Amps = 90 ohms.

What is the Difference?

Because watts and lumens are measures of different things, there is not necessarily any correlation between them. For example, a conventional light bulb and a compact fluorescent light bulb may put out the same number of lumens, but the conventional bulb will consume three to five times the number of watts needed by the compact fluorescent.

Divide the light bulb's lumen output by the number of watts the bulb needs. For example, a 100-watt light bulb usually has 1800 lumens, so the LPW is 18.

To learn more about for light guide / lighting designer information

Share Page Back Top