Itís time to untangle the Christmas lights. Every year, we pull endless strands from the attic, only to find half of them not working.

When I was a kid, my dad just bought packs of giant bulbs, and it was our job to swap out the duds. Now we throw the strands away and buy more. Itís such a waste.

Light pollution, however, is wasting energy on unnecessary light. Incandescent bulbs, like the old-fashioned Christmas lights, use only 10 percent of incoming energy to produce light. The rest is wasted on heat. Try to change one when it burns out. But light pollution isnít just about energy waste. Urban sky glow washes out starry skies, light trespasses into neighboring yards, and glare can reduce nighttime visibility. In the Lowcountry, light pollution has a direct impact on sea turtle hatchlings.

The International Dark-Sky Association has established guidelines to reduce light pollution ( IDA-approved fixtures are hooded to direct light toward the ground to minimize waste and reduce sky glow. Although holiday lighting and low-voltage landscape lighting are exempt from these regulations, a maximum of 1,600 lumens is allowed outdoors on residential properties. Regardless, low-voltage landscape lightsí output is less than 1,600 lumens.

Weíre accustomed to wattage when describing bulb illumination because, for the past two centuries, weíve used incandescent lights. Even though we know how bright an incandescent 60-watt bulb will be, wattage is a measure of electrical power, not illumination. Lumens is the measure of light relevant to humans.

Halogen bulbs, improved incandescent bulbs, are commonly used in low-voltage lighting. A 10- or 20-watt halogen light will produce a warm spectrum of light suitable for up-lighting most medium-size trees. A 20-watt halogen generates about 350 lumens. A 35-watt halogen would be used to up-light a large tree, and that generates 650 lumens.

Incandescent bulbs are being phased out by the curly designed fluorescent bulbs, otherwise known as compact fluorescent lights (CFL). While they are much more energy-efficient, they contain mercury. If you read the cleanup procedures for a broken CFL, you might get nervous. Fluorescents, though, arenít commonly used in low-voltage landscape lighting.

In the past couple of years, light-emitting diodes have emerged in popularity. Many Christmas lights are LEDs. They are extremely energy-efficient, converting 90 percent of input energy into light. A 6- to 8-watt LED will generate about 800 lumens, equivalent to a 60-watt incandescent. In other words, it uses 7.5 times less power to produce the same amount of light. While they are significantly more expensive, the cost can be offset over time.

Solar lights use LEDs precisely because they are energy efficient. Most solar lights, however, operate more like beacons of light and lack the ability to illuminate a tree or sidewalk. The solar panel also requires full sun. That can be difficult if lights are located in shade.

Until recently, halogens have ruled landscape lighting systems. For a halogen lighting system, a 300-watt transformer (power supply) would adequately light many residential properties. In theory, this can power 30 10-watt halogen lights. In actuality, itís less than that due to voltage drop and transformer efficiency. While halogens are still popular, LEDs are becoming relevant. Compare a 3-watt LED to a 10-watt halogen that has the equivalent lumen output. Theoretically, 100 3-watt LEDs can be used on the same 300-watt transformer. However, only 30 3-watt LEDs are needed to get the same lumen output; therefore, a less expensive 100-watt transformer can be used. Money also can be saved with LEDs because smaller gauge wire can be used.

One criticism of LEDs is the light quality. While halogens provide warm light, LEDs can be harshly white. But even this has improved as filters have been used to alter LED output quality as good as halogens. And those Christmas lights you pull out of the attic? Probably LEDs.

Tony Bertauski is a horticulture instructor at Trident Technical College. To give feedback, email him at