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:
 

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 outdoor 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 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 compact 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 any incandescent fixtures, as mentioned below.

Mercury Vapor (MV): 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, especially 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 wattage 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, remarkably 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 entire fixture. Use a more efficient light source, such as MH, HPS, or LPS, and use a high quality fixture, one that directs the light output to the areas needed and one that is glare free. A cost analysis study will show remarkable energy saving potential.

Metal Halide (MH): These lamps are used for both outdoor and indoor applications. Metal halide and fluorescent are now in a dead heat as the most energy efficient of the "white light" sources. Metal halide lamps produce light when an electric current flows through the gas within the lamp envelope. They are about twice as efficient as mercury lamps. Use this light source at night when it is necessary to render colors close to their daytime appearance. 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 accompanied by glare.

Fluorescent: Like metal halide, fluorescent is about four times as efficient as incandescent lighting. Fluorescent is commonly used for indoor applications, but outdoor usage is increasing. The best fluorescent and compact fluorescent (CFL) sources have several advantages over metal halide: longer life, a much shorter warm-up time to full brightness, ability to switch them on and off several times each night without significantly shortening bulb life, and a white light that is spectrally much less polluting than that produced by metal halide. Disadvantages are: high brightness CFLs are not available, light output is diminished at low temperatures, and a lamp may not even start at very low temperatures.

High Pressure Sodium (HPS): Its main usage is outdoors, for street lighting, parking lot lighting, and other such applications. It is generally more energy efficient than metal halide and is a good choice when true color is not critical. The light produced is an orange-gold color. It's very common in 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, for higher wattage lamps, are larger than the equivalent fixtures for HPS or MH. However, the LPS fixture 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 fluorescent fixtures to add a little white light. For equivalent fixtures (ones that offer the same amount of light and 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 savings is possible. Remember also that if the installation is glare free, a lower light level 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 can all be a major factor in conserving energy. Devices range from a simple on/off switch to computers programmed to control lights automatically. Turn lights 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.

Maintenance: Finally, do 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 by up to 50 percent.

*Cost Comparison Example: (Assume that a well-designed fixture is being used in these cases, so that the light output by the lamp is being efficiently utilized. 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 producing about 8000 lumens, quite a lot of light. These 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 of electricity (KWH). The total wattage of the system includes the wattage used by the lamp and the ballast together. It is easy to see the potential savings achieved by utilizing efficient lamps.
 

*See IDA Information Sheets 4 and 26 for additional energy saving facts.