Estimating Light Pollution in Wisconsin
Application of DMSP Data Analysis
David S. Liebl, April 2001

Many are familiar with photographs of the earth at night taken from space. While the jewel like patterns of light from cities, oil fields, and even fishing fleets are entrancing, these also represent wasted light shining into space.  Analysis of data from the Defense Meteorological Satellite Program can provide insights into the amount of energy wasted by the poorly designed exterior lighting that causes this sky glow. 

Syuzo Isobe and Shiomi Hamamura of the Japan National Astronomical Observatory have estimated light energy loss to space for a number of cities around the world. Using their measurements as a basis, we can estimate the amount of wasted light energy, and the electrical energy used to produce that light, for the state of Wisconsin.  

The results of this estimate suggest that 0.73 million megawatt hours of electrical energy are consumed each year to provide lighting that shines into space.  This is 1.1% of total electrical sales in Wisconsin (1999 = 64millionMWh).

Is it possible to completely eliminate this source of wasted energy?  Exterior lighting depends on the reflective properties of surfaces to provide adequate illumination.  As long as roadways, walkways and building surfaces are open to the sky, there will be light reflected upward into space. 

However, poorly designed and installed lighting that creates glare, over-illumination and up-light can be eliminated.  Our experience with retrofitting lights to eliminate glare and sky glow suggests that perhaps 25% (ca. 200,000 megawatt hours) of this energy could be conserved through good lighting design.  Lighting that illuminates only that which is meant to be seen, with light levels appropriate to the installation, can minimize energy loss and sky glow to the extent possible. 

Method:
Using data from Isobe & Hamamura , and population data from the US Census Bureau and municipal web sites, a per capita light loss value was calculated for the following American cities: 

City	Annual Light Energy Loss	Population	Annual Light Loss Per Capita	
	106kWh	thousands	KWh per capita	
Phoenix, AZ (metro)	55.0	2,000	27.5	
Tucson, AZ (metro)	13.2	750	17.6	
Sacramento, CA	14.9	370	37.8	
Stockton,  CA	3.68	210	17.5	
Modesto, CA	3.03	165	18.4	
Fresno,  CA	6.66	350	19.0	
Salem,  OR	2.39	108	22.1	
Eugene, OR	2.09	113	18.5	

From this, a nominal value of 20kWh per capita of annual light energy loss to space was derived.  Using 5.5 million as the population of Wisconsin, this gives an annual light energy loss of 110,000 megawatt hours.   Using a value of 15% efficiency for converting light energy to electrical energy, we can estimate that approximately 0.73 million megawatt hours of electrical energy are used each year to create the light seen from space.

This estimate is consistent with other estimates of the scale of light pollution.  Hunter and Crawford (IAU Colloquium #12, 1991) cite the following:

•	146 kWh per capita (based on 1000 lumens per capita)
•	1.6% of all electrical uses (extrapolated from street lighting)
•	3.1% -3.8% of all electrical uses (based on estimates of outdoor lighting use)
•	3.6% of all electrical uses (based on municipal lighting costs)
•	

The authors then use a “conservative” estimate of 2.5% of all energy uses as exterior lighting, 30% of which is wasted to the night sky.  They conclude that the United States wastes 17.4 billion kWh annually from poorly designed exterior lighting.