Lumens per Watt

Lighting technologies on today's consumer market span a wider range of choices than ever. As we migrate toward better energy efficiency, new metrics come to the forefront to help with decisions about what to buy. With the advent of compact fluorescent lamps (CFLs), the actual light output in lumens became an important characteristic for comparisons, since their power consumption in watts is considerably less than the power required by standard incandescent bulbs. Product specs and advertising are now including lumens per watt as a measure of efficacy, to indicate the amount of light produced with the power consumed by the device.

Many web pages now include tables that list this measure along with the actual light output and power required for a given lighting technology or device. For example, see this one from a utility company. A similar table appears in the middle of Home Depot's main page about light bulbs.

A review of such tables shows that incandescent bulbs trail the pack at only 10 to 17 lumens per watt. Fluorescent devices are much better, in the range of 30 to 110 lumens per watt. LED devices are shown in various tables and past web articles as being in the range of 20 to 60 lumens per watt. However, the LED technologies are improving rapidly. One of the companies actively developing this technology has reported achieving 200 lumens per watt. Another company making progress on the price is discussed in the article:
Silicon Is Key to Quest for $5 LED Lightbulb.

As noted in the article LED Lighting: Blue + Yellow = White :
"One of the goals of the U.S. Department of Energy's 2010 solid-state lighting road map calls for more than doubling the lumens per watt in commercial LED products by 2015."


Currently available LED devices are pricey, but moving lower, and are not yet at efficacy levels that surpass linear fluorescent bulbs. It should be noted, however, that linear (tube) fluorescent bulbs diminish in brightness with age and are usually specified with their beginning-of-life brightness. Also, the efficiency of the fluorescent fixture ballast is not necessarily included in the specs for the bulbs. And remember that fluorescent bulbs contain mercury. Be sure to recycle them.

A common application example may provide additional appreciation for the device specs. My friend Larry has a workshop that is illuminated using four twin-tube 48" fluorescent fixtures. Assuming 40-watt T12 bulbs and using the data from Home Depot, these fixtures would produce 8 x 2600 = 20,800 lumens and consume at least 320 W. If the same lumens were to be produced using common 100 W incandescent bulbs (1180 lumens each), there would be 17.63 bulbs, consuming 1763 W. One currently-available LED spotlight could provide 1100 lumens with only 15 W. Power for the 19 needed bulbs would total 285 W, but the cost is prohibitive -- $798. When LED devices are commercially available at 200 lumens per watt, the total power requirement for Larry's workshop would be 104 W.  Clearly it is not economical for Larry to replace his full set of fluorescent bulbs with LED devices at the present time. New construction, especially commercial, should look closely at using LEDs, however. The economics of using currently-available LED bulbs in place of incandescent bulbs and CFLs are discussed in the article: The Lightbulb That Really Is a Better Idea.

Hopefully, as incandescent bulbs vanish into history, LED devices will become more plentiful and less expensive so that we can enjoy better lighting efficiency while avoiding the toxic characteristics of fluorescent bulbs.


John