International Dark-Sky Association (IDA)

3545 N. Stewart Ave., Tucson AZ 85716 U.S.A.



Efficient Outdoor Lighting

IDA Information Sheet No. 52 (April 1992)

The efficient and effective use of electrical lighting outdoors can offer major energy and cost savings. This information sheet suggests some of the things that can be done. Most of the suggestions apply to indoor lighting as well, where they also offer significant savings.

There are several clear cut measures that can be taken to improve energy savings. New, much improved light sources are now available which provide considerable more light per unit of energy. Most newer fixtures offer better light control, putting light where it is needed rather than wasting a great deal of the light produced by the lamp. Replacement of older fixtures and lamp[s with the newer, improved ones can greatly improve efficiency.

lamp efficiency is measured in lumens per watt. A lumen is a unit for measuring the amount of light; a watt is a unit for measuring the amount of electrical energy used. The lamp that gives the most lumens per watt is the most efficient lamp. The table below lists the lighting efficiency of some of the common lamps used for outdoor lighting:

Type of Lamp          Lumens per watt         Average Lamp Life (hours)

Incandescent                8 - 25                   1000 - 2000
Mercury Vapor              13 - 48                  12000 - 24000+
Fluorescent                33 - 77                  10000 - 24000
Metal Halide               60 - 100                 10000 - 15000
High Pressure Sodium       45 - 110                 12000 - 24000
Low Pressure Sodium        80 - 180                 10000 - 18000

Incandescent: It is the most common type of lamp used in homes, indoors and outdoors. It is the most energy inefficient of the common lamp types. It produces light by electrical energy heating a filament of fine wire that glows white-hot when the current flows through it. It produces a great deal of heat relative to the amount of light, only 10 percent of the energy goes to producing light. It has been called a heat source that happens to produce some light at the same time. The 40 watt bulb is often adequate for most lighting applications, such as a porch light, especially if it is used in a fixture that actually controls the light output rather than scattering it everywhere. Many of the existing fixtures are very inefficient and waste a good deal of the light (remember that the lamp is inefficient too). Replace incandescent lamps with more energy efficient lamps in good fixtures. One can use the mini-fluorescent lamps in an efficient fixture, or for even more efficiency use a low wattage LPS lamp in a well shielded fixture. Consider also time clocking and incandescent fixtures as mentioned below.

Mercury Vapor: It is commonly used for a number of outdoor applications, such as "security" lighting, as well as indoors for some applications. It has a relatively long life compared to most other lamps,, expecially compared to incandescent. These lamps are a quartz tube filled with mercury gas under pressure. Light is produced when an electric current passes through the mercury vapor. Like all such high intensity discharge (HID) lamps, a "ballast" is required to start and to operate the lamps at the correct voltage and current levels. For savings, one can and should use the lowest possible wattages for the application. Many of the existing fixtures have a great deal of associated glare due to lack of adequate light control. With a good fixture, less light is wasted and lower wattages can be used. In a glare free lighting environment, remarkable low light levels still give excellent visibility. It is in the high glare areas, even with much higher levels of lighting, that we have difficulty seeing well at night. When replacements are indicated, one should replace not only the lamp but the fixture. Use a more efficient light source, such as MH, HPS, or LPS, and use a high quality fixture, on that controls the light output to the areas needed and one that is glare free. A cost analysis study will show remarkable energy saving potentials.

Fluorescent: These are about four times as efficient as incandescent lighting. They are commonly used for indoor applications. Energy savings are possible here by using lower wattage lamps (using more efficient lamps T-8, for example), disconnecting ore removing some of the fixtures (ballasts must be disconnected too, rather than just removing the lamp), replacing the existing ballasts with more energy efficient ones (electronic, solid state, or energy savings ballasts), or redoing the entire lighting system with more energy efficient fixtures.

Metal Halide (MH): These lamps are used for both outdoor and indoor applications. They are currently the most energy efficient of the "white light" sources. They produce light when an electric current flows through the gas within the lamp envelop. They are about twice as efficient as mercury lamps. Use this light source at night when it is necessary to render colors closely to what they would appear in the daytime. As with all light sources, one should not use more wattage than is necessary for the application. "More light" is not always better. In many applications, such overkill is counterproductive to visibility, especially if it is also producing glare.

High Pressure Sodium (HPS): Its main usage is outdoors, for street lighting, parking lot lighting, and other such applications. It is more energy efficient than metal halide and is a good choice when true color is not critical. The light output is an orange-gold color. It's a very commonly used throughout the U.S.

Low Pressure Sodium (LPS): This light source is the most energy efficient of all, and it is an excellent choice when used with a quality fixture that controls the light output. The light is produced from glowing sodium gas within a tube, and so the LPS fixtures is an excellent choice for street lighting, parking lots, and security lighting. There is no color rendering at all, but adequate color rendering is quite possible with system designs that also use a few MH or florescent fixtures to improve color rendering. For example, in equivalent fixtures (ones that offer good light control), a 175 watt mercury vapor fixture could be replaced by a 100 watt HPS or a 55 watt LPS. The 35 watt LPS is equivalent to a 200 watt incandescent. It is easy to see that considerable energy saving is possible. Remember also that if the installation is glare free, a low level of overall lighting offers excellent visibility. More is not always better.

Lighting controls: Controlling when and where the lights are used, how long they are on, and how bright they are all can be a major factor in conserving energy. Devices range from a simple on/off switch to computers programed to activate light automatically. Turn light off when not needed. Use individual controls rather than lighting large areas off of one switch. Use timers. Don't burn outdoor lights in the daytime. Use photo-sensors when possible. Some of the newer applications use motion sensors for room light control, and such systems are also feasible for outdoor applications.

Finally, don not forget lamp and fixture maintenance as a factor. Keep the fixture clean from dust and dirt. Such contamination can reduce light output in some cases up to 50 percent.

Example of Cost Comparison: (Assume that a well designed fixture is being used in these cases, so that the light output by the lamp is not wasted by an inefficient fixture. A bad fixture could be wasting more than 50 percent of the lamp's light.) Compare a 175 watt mercury (These are generally found in poor fixtures!) to a 100 watt HPS and a 55 watt LPS lamp. All of these lamps are outputing about 8000 lumens, quite a lot of light. They are wattages that would commonly be used for residential street lighting. We assume 4100 burning hours per year, from dusk to dawn, and 8 cents (U.S.A.) cost per kilowatt-hour (KWH) of use. The total wattage of the system includes the wattage used by the lamp and the ballast together. It is easy to see the potential saving gained with efficient lamps.

Lamp Wattage   Total Wattage   KWH Use/Yr  Oper $/Yr  100 lamps   10000 lamps

   175              208           853        $68.22     $6822       $682,200 
   100              130           533         42.64      4264        426,240 
    55               80           328         26.24      2624        262,400 

See IDA Information Sheets 4 and #26 for additional discussions of energy saving facts.

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