Light out versus power in
A lumen is a unit of light output: how much visible light a bulb emits in total. A watt is a unit of power: how fast the bulb draws electricity. The Federal Trade Commission's shopping guide makes the point that matters: because bulbs have become so much more efficient, wattage is no longer a guide to brightness, and lumens are. More lumens means brighter, fewer means dimmer, whatever the technology inside the glass.
The ratio of the two, lumens per watt, is the bulb's efficacy. A 60-watt incandescent giving 800 lumens manages 13.3 lumens per watt; a 9-watt LED giving the same 800 lumens manages 88.9. Same light, a sixth of the electricity.
Lumen equivalents of the old wattages
The Department of Energy's purchasing guide gives the light output of the common incandescent sizes and the wattage range of ENERGY STAR LED bulbs that match them:
| Light output | Incandescent | LED |
|---|---|---|
| 450 lumens | 40 W | 4.5 to 7 W |
| 800 lumens | 60 W | 5.9 to 10.5 W |
| 1,100 lumens | 75 W | 9.5 to 14 W |
| 1,600 lumens | 100 W | 10.5 to 18 W |
So "a 60-watt equivalent" on an LED box means about 800 lumens, and the honest way to compare two LEDs is to ignore the equivalent and read the lumens: two bulbs both sold as 60-watt equivalents can differ in output, and in the watts they draw to produce it.
The Lighting Facts label, line by line
In the United States the label is required on general service lamps by the FTC's Energy Labeling Rule, and its contents are fixed by regulation:
| Line | What it states |
|---|---|
| Brightness | Light output in lumens, rounded to the nearest five |
| Estimated yearly energy cost | Based on the bulb's wattage, 3 hours of use a day and 11 cents per kilowatt-hour |
| Life | In years, to the nearest tenth, at 3 hours a day |
| Light appearance | Correlated colour temperature in kelvin on a warm-to-cool scale from 2,600 K to 6,600 K |
| Energy used | Wattage |
| Mercury | A disclosure if the bulb contains mercury, as compact fluorescents do |
Two lines are easy to misread. The yearly cost is only comparable between bulbs because every label uses the same 3 hours and 11 cents; your own bill depends on your rate and your hours. And the life in years is the rated life divided by 3 hours a day, so a bulb in a hallway that burns 12 hours a day will last a quarter of the printed years.
What a bulb costs to run
Yearly cost = watts × hours per day × 365 ÷ 1,000 × price per kWh
On the label's assumptions, for the two bulbs that give 800 lumens:
| Bulb | Electricity per year | Cost per year at 11¢/kWh |
|---|---|---|
| Incandescent, 60 W | 65.7 kWh | $7.23 |
| LED, 9 W | 9.9 kWh | $1.08 |
Multiply by the bulbs in the house and the saving is no longer small change. The electricity cost calculator takes your own tariff and hours, and the kW vs kWh guide explains the unit the bill is in. The power converter and energy converter handle the units if the data sheet is in something other than watts.
Choosing a bulb
- Start from lumens. Decide how much light the fixture needs, using the bulb it replaces as the guide: 450 lumens where a 40 W bulb was, 800 where a 60 W was, 1,600 where a 100 W was.
- Then read the watts. Among bulbs with the lumens you want, the lowest wattage is the cheapest to run.
- Pick the colour temperature for the room. Lower kelvin numbers give warmer, more yellow light; higher numbers give cooler, bluer light. The label places each bulb on the 2,600 K to 6,600 K scale, so two bulbs can be matched by that line.
- Check the fitting and the dimmer. The lumen figure says nothing about whether the bulb dims or fits an enclosed fixture; that is on the box, not on the Lighting Facts label.
- Plants are a different question. Lumens describe light as the human eye weights it, not what a plant uses; the grow light cost calculator works from the fixture's wattage and hours for that case.
Frequently asked questions
What is the difference between lumens and watts?
Lumens measure how much light a bulb gives out; watts measure how much electricity it uses. The two were interchangeable only while every bulb was an incandescent of the same efficiency. The Federal Trade Commission puts it directly: lumens measure brightness, more lumens means brighter, and wattage no longer tells you how bright a bulb is.
How many lumens replace a 60-watt bulb?
About 800 lumens. The FTC says a traditional 60-watt incandescent bulb produces about 800 lumens and that an LED can give the same 800 lumens from about 9 watts. The Department of Energy's table puts the LED range for 800 lumens at 5.9 to 10.5 watts.
What does the Lighting Facts label show?
Six things, set by the FTC's Energy Labeling Rule: brightness in lumens, estimated yearly energy cost, life in years, light appearance on a scale from warm to cool in kelvin, energy used in watts, and whether the bulb contains mercury. The cost and life figures assume 3 hours of use a day and 11 cents per kilowatt-hour.
What do 2700K and 5000K mean?
Correlated colour temperature, the "light appearance" line on the label, shown on a scale from 2,600 K to 6,600 K. Lower numbers are warmer, more yellow light; higher numbers are cooler, bluer light. It is a matter of taste and use, not of efficiency: two bulbs with the same lumens and watts can sit at opposite ends of the scale.
How much does an LED save over an incandescent?
On the label's assumptions, a 60-watt incandescent costs $7.23 a year to run and a 9-watt LED giving the same light costs $1.08, a saving of $6.14 per bulb per year before the longer life is counted. At your own rate and hours, the electricity cost calculator gives the exact figure.
Sources
- Federal Trade Commission: Shopping for light bulbs (lumens measure brightness, watts measure energy use; a 60-watt incandescent gives about 800 lumens, an LED the same from about 9 watts; the fields on the Lighting Facts label)
- 16 CFR 305.23, Energy Labeling Rule, lighting products (the required contents of the Lighting Facts label; yearly cost based on 3 hours a day and 11 cents per kWh; life in years at 3 hours a day; light appearance in kelvin on a 2,600 K to 6,600 K scale)
- US Department of Energy, FEMP: Purchasing energy-efficient light bulbs (the lumens-to-watts table for incandescent and LED bulbs; ENERGY STAR lamps required to last 10,000 hours or more)