The Physics
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Power of a Streetlight

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Frequently Asked Questions. Institutions of Lighting Engineers. 2004. "The lamps used in streetlights vary in both size and consumption (typically between 35 and 250 Watts) depending upon whether they are lighting a residential area, main road or a town centre." 35–250 W
"It is generally assumed that the average wattage of a streetlight is about 80 watts." Average: 80 W
Outdoor Lighting Handbook. International Dark-Sky Association. December 2002. [see table below] 18–400 W
Lurkis, Alexander. The Power Brink: Con Edison – A Centennial of Electricity. New York: Icare Press, 1982: 16. "The Central Station was located at Twenty-fifth Street. It contained two Corliss steam engines, boilers, and 'two dynamoelectric generators side by side, the first of which supplied the Broadway circuit and the second the Fifth Regiment Armory' and lights in the building. The lamps were '…equal to 1000 candlepower'" 18.8 W
Maurath, Joe. High Pressure Sodium Lamps. Streetlight Evolution, May, 2004. "General Electric introduced the first high-pressure sodium (HPS) lamps that became commercially available in the United States in 1965 in the 400-watt size…. By about 1970, 100 and 250-watt sizes came on the scene, offering more flexibility among installations." 100–400 W
"By 1980, lamp sizes had ranged from 35 to 1,000 watts; with most types attaining 24,000 hour estimated operating hour life, creating much competition with mercury lamps of the time…. Efficiencies of the late 1990s range from 64 lumens per watt for the small 35-watt lamps to 140 lumens per watt for the 1,000 watt size. Common HPS lamp wattages are 35, 50, 70, 100, 150, 200, 250, 400, and 1,000. Medium base lamps are available through 150 watts; all are otherwise produced with mogul screw threads." 35–1,000 W
Factor Lamp Type
Incandescent Fluorescent Metal Halide High-Pressure Sodium Low-Pressure Sodium
Wattage 25-150 18-95 50-400 50-400 18-180
Output (lumens) 210-2700 1000-7500 1900-30000 3600-46000 1800-33000
Efficiency (lumens/watt) 8-18 55-79 38-75 72-115 100-183
Lumen Maintenance (%) 90 (85) 85 (80) 75 (65) 90 (70) 100 (100)
Lamp Life (hours) 750-2000 10000-20000 10000-20000 18000-24000 16000
Energy Use high medium medium low lowest
Color Rendition good good good moderate none*

Most of us take for granted the fact that we can easily find our way outdoors at night. It's hard to believe that prior to the introduction of electric streetlights in the late 1870s, this task was far more difficult and dangerous.

The earliest electric streetlights were illuminated by arc lamps. Arc lamps contained two carbon rods that, when touching, completed an electric circuit. When the rods were pulled apart, the current continued to flow across the gap in an arc that created light.

By 1912, arc lamps had become obsolete thanks to the invention of incandescent streetlights, which had a longer lamp life and were easier to maintain. Incandescent lights were suspended on poles that curved into a "gooseneck" shape that resembles today's streetlights. Similar to modern incandescent lights, these gas-filled streetlights contained coiled tungsten that glowed when heated by the current passing through it.

In time, many incandescent streetlights were replaced by fluorescent ones. Fluorescent lights produced cool white light that allowed for excellent color rendition. Their other advantages included high luminous efficiency and long lamp life. Fluorescent streetlights were popular during the mid 1950s up until the 1970s when they began to be replaced by high pressure sodium and metal halide sources of lighting. The latter two remain the most commonly municipal and industrial lighting sources today.

The measure of the amount of energy used by a streetlight (or any device) per unit of time is known as power. The derived unit for power is the watt = joules/second. Power is equal to the product of the current and the voltage running through a device:

P = VI

Power can also be expressed in terms of the resistance of a device:

P = I2R


P = V2/R

Different streetlights consume different amounts of energy. Incandescent lamps typically range from 25-150 watts, fluorescent lamps range from 18-95, metal halide lamps range from 50-400 watts, and high pressure sodium lamps range from 50-400 watts.

Marina Avetisyan -- 2004