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The Webfooted Astronomer - November 2000


How Much Water Is Needed to Run a Light Bulb?

EVER wonder how much water is needed to run a light bulb? Dark Skies Northwest Chairman Bruce Weertman did, so he asked Craig Sprankle of the Grand Coulee Power Office. Since a watt is an instantaneous measurement of electricity, you normally deal in watt-hours, kilowatt-hours, or megawatt-hours. For instance, a 150-watt light bulb will continuously use 150 watts of electricity. If you turned it on for one hour, it would use 150 watt-hours.

At Grand Coulee, with a full lake behind the dam, 1.1637 gallons of water will produce one watt-hour. Therefore, the 150-watt bulb would require 150 x 1.1637 gallons or 174.555 gallons to operate for one hour.

Using 8,760 hours in a year, the bulb would use 8,760 x 174.555 gallons or 1,529,101.8 gallons of water per year. There are 7.48 gallons in a cubic foot. Dividing 1,529,101.8 by 7.48 gives you 204,425.37 cubic feet of water needed to operate the 150-watt bulb for one year.

When dealing with volumes of water, you normally use the measurement acre-feet. One acre-foot is an area one acre square, covered one foot deep in water. One acre-foot contains 43,560 cubic feet. So, 204,425.37 cubic feet divided by 43,560 equals 4.7 acre-feet of water needed to operate a 150-watt bulb continuously for one year from power produced at Grand Coulee, assuming the lake is full and no power is lost in transmission.

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