HowTo: Compare LED Lumens and Watts for a Bulb

Summary:

Lumens are not Watts. They really have no relationship except for the specific item (bulb or LED). The number of Watts is not a good way of comparing LED bulbs to Incandescent bulbs because they consume a lot of power (watts) that do not convert to light (Lumens).  Only a small portion of the radiated energy of the burning filament in the bulb is in the visible light range. Much more of it is in the Infra-Red or, heat range.

Incandescent Bulbs Produce Watts and Lumens

Typical Household Incandescent Bulb

Typical Household Incandescent Bulb
(Image: Wikipedia)

Have you ever noticed how hot a standard incandescent bulb gets? Even the smallest night light bulbs get hot to the touch.  It’s been reported that only about 5% of the energy radiating from that bulb is visible light. We measure the energy consumption of bulbs in watts.

Typically, in a 120V AC (alternating current) household electrical system, the night light bulb consumes either 4 or 7 watts of power. Watts is a unit of power derived from the volts multiplied by current. So a 60 Watt bulb is drawing 1/2 amps of electricity (120 x 1/2). Do you need to know this? Not really unless you are calculating the size of your circuit breaker load since breakers are rated in amps (i.e. 15A or 20A, etc. breakers.)

The light output of any lamp fixture or bulb is measured in Lumens.  Lumens gives a numeric value to the total amount of light flowing out of the light fixture or bulb in all directions. Different manufacturers rate their bulbs slightly differently due to manufacturing differences.

Guide to lumen output of bulbs.

  • 4 watt night light = 20 lumens.
  • 25 Watt bulb = 180 lumens
  • 40 watt bulb = 400 lumens
  • 60 watt bulb = 800 lumens
  • 75 watt bulb = 1100 lumens
  • 100 watt bulb = 1700 lumens.

Notice how you get more lumens for every watt as the bulbs get larger. This means that more energy is released as heat in the smaller bulbs compared to the larger bulbs resulting in them being less efficient.

Light Emitting Diodes and LED Bulbs:

LEDs operate at much lower voltages. Typically, the white LEDs operate at 3.2 to 3.6 VDC (Direct Current.) The more recent prototype high brightness LEDs are delivering up to 200 Lumens per watt but the current production LEDs are closer to 120-130 lumens per watt. This is so much more efficient considering that the 100 Watt incandescent bulbs put out roughly 17 lumens per watt.

LED bulbs don’t produce the stellar Lumen per watt ratings of a single LED chip because the bulbs must convert the 120 VAC of your household electricity, to much smaller DC voltages. This requires a power supply inside (think of your cell phone charger as a power supply that converts 120 VAC to 5 VDC.) Power supplies are not 100% efficient and consume 10-25% or more of the power from the household electrical system before sending it to the LEDs. So that means maybe 75% of the power is consumed by the LEDs within the bulb.

LEDs are also have directional light where bulbs are omnidirectional.  This means that there must be several LEDs in a single LED bulb package to send light all around the LED bulb. Some LEDs can provide light in a 120° angle so just three of them could shine light out in a 360° circle. Click here for a detailed technical discussion of LEDs and Lumens and how they are determined.

If you think of a round bulb shining like the sun, a regular incandescent bulb puts out light all around the sides (equator) and up and down (north and south poles). This means that more LEDs are needed to shine light both outward (around the equator) and up and down (north and south poles). Now we are complicating the circuitry to hook up all the individual LEDs to the power supply. This also reduces power efficiency due to the electrical losses of the components. Remember, an incandescent bulb only has one component: a filament.

LED Bulbs

Some Of The Different LED Bulbs On The Market

Individual LEDs are really electronic chips with special solid state materials and phosphors to change electrical movement (electron flow) into light movement (photon flow). This conversion also generates heat due to normal current flowing through the solid state materials and other resistive paths.

So, in a more perfect world, if we could eliminate the power supply and have LEDs run directly off of the AC source, we could greatly improve the efficiency of even the most efficient LED bulbs today. That work is going on in research labs right now.

Dimming:

LED bulbs are difficult to dim. Many won’t work with a standard light switch dimmer and the dimmer’s package will tell you if they work with LED bulbs. This is because pf the power supply that is inside the bulb. There are dimmers on the market made for LED bulbs that also work for incandescent bulbs. [Ed. Note: The same problem exists with CFL lamps.]

Cost:

Expect to pay more for any LED bulb (and LED dimmer). Some of the cost justification comes from comparing how much energy you’ll save over the lifespan of the LED bulb which is normally rated in the 30,000 to 50,000 hours ranges. The range is wide because the lifespan of an LED range is always under debate. There comes a point when the LED produces less light at the same current draw. Some argue that when it reaches 70% of the light output it originally had, that it is the end of life for the LED. Others argue that it should be 50%. Some labels display the amount of light after 25000 hours of use (see the DOE label below).

Recommendations:

Look for the Lumen output on the label for any LED bulb. Typical packages list the equivalent size incandescent bulb they are meant to replace too.

Although somewhat technical, also look for the CRI or color rendering index. The higher, the better as this means it gives more accurate reflective color off of many of your everyday household items. A 100 CRI would be roughly equivalent to an incandescent or halogen bulb. Most LED bulbs fall below 90 and colors look a bit different. There is a comprehensive Wiki article explaining CRI for LED bulbs here.

DOE Label

Department of Energy (DOE) Lighting Label

FTC Label

FTC Lighting Label

The Federal Trade Commission (FTC) and the Department of Energy (DOE) are working on standard labels for bulbs that currently look like these two.  The DOE adds CRI information (Color Accuracy) and other technical information (like Lumen Maintenance) that set LED bulbs apart from incandescent bulbs. You’ll probably find products with either label on the market.

Again, the Lumen rating will steer you into installing the correctly sized light bulb for the task it is supposed to handle. In common with both labels is also the Light Color or Appearance. Notice that “Warm” is more yellowish toward the red end of the spectrum while “Cool” is nearer pure white toward the blue end of the spectrum.  This will affect the overall ambiance of the room in which the light is installed.

This article should help you understand how you might go about comparing LED bulbs with standard incandescent bulb.

- Bart

 

 

 

 

 

 

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