How much does a light bulb cost?


There is of course, an easy answer to this… It costs whatever the sticker on the box says.  On the other hand, there are two other costs associated with a light bulb.


1)      Replacement cost.  A light bulb is no good if it doesn’t last.  In the long run, a $5.00 light bulb that lasts for 100 hours is cheaper than a $1.00 light bulb that lasts for 10 hours. 


2)      Energy cost.  You’d probably like your light bulb to light, and for that you need energy.  Energy consumption is measured in kilowatt hours.  If you burn a 60 watt bulb for 1 hour, you’ve consumed 60 watt hours or 0.06 kilowatt hours of power (remember, kilo = 1000).  Since the electric company charges you for every kilowatt hour you consume, a bulb that uses less power may also be cheaper in the long run than a bulb with a lower sticker price that consumes more energy. 


Please note:  When people think about bulb wattage, they usually think that it is a measure of light intensity.  In actuality, light intensity is measured in lumens.  A typical 60 watt incandescent bulb produces about 800-900 lumens of light.  A typical 100 watt incandescent bulb produces closer to 1700 lumens.  As you will see in this exercise, it is possible to decrease the number of watts that you use, and keep the same number of lumens.



In this exercise, we would like you to determine which of the following bulb is the least expensive to use to light your home.  We’ve gathered the following information from packages available at Home Depot.


Incandescent Soft White

            $1.49               60W                1000 hrs          840 lumens      4/pkg


Incandescent Soft White

            $1.49               100W              750 hrs            1690 lumens    4/pkg


Incandescent Soft White Long Life

            $2.79               100W              1125 hrs          1600 lumens    2/pkg


Incandescent Soft White miser

            $2.49               95W                750 hrs            1610 lumens    4/pkg


Incandescent Soft White miser

            $2.49               55W                1000 hrs          800 lumens      4/pkg


Fluorescent GE Super Long Life Soft White

            $13.99             15W                10,000 hrs       900 lumens      1/pkg


Fluorescent GE Long Life Soft White

            $9.99               15W                6,000 hrs         700 lumens      1/pkg


Fluorescent GE Long Life Soft White Bullet (has casing to make it look like an incandescent bulb)

            $15.99             15W                6,000 hrs         800 lumens      1/pkg


Fluorescent Lights of America Trilite

            $7.99               15W                10,000 hrs       840 lumens      1/pkg


Fluorescent Lights of America Soft White

            $8.99               25W                10,000 hrs       1500 lumens    1/pkg


Halogen GE  90W

            $5.49               90W                2000 hrs          1680 lumens    1/pkg


Halogen GE 50W

            $4.99               50W                2000 hrs          830 lumens      1/pkg


EvoLux LED Light Bulb

            $99.99             13W                50,000 hrs       900 lumens      1/pkg


Conserv-Energy CFL Minispiral,

$8.99               13 W               10,000 hrs       900 lumens     10/pkg

Conserv-Energy CFL

$5.79               23 W               10,000 hrs       1600 lumens    10/pkg


Conserv-Energy BR30 Flood Light

$8.89               15 W               10,000 hrs       750 lumens     6/pkg


 Lights of America, LED R30 flood

$10.99             3.5 W              30,000 hrs       ~500 lumens    1/pkg



Using Excel, please calculate how much it would cost you to keep each of these bulbs burning continuously for one year.  The easiest way to do this would be:


1)      Calculate the bulb price per hour of life.  For example, for the 60W Incandescent Soft White, the purchase price per bulb is:


$1.49 ÷ 4 (number of bulbs per package) = $0.3725


This gets you approximately 1000 hours of bulb life, so the cost of the bulb per hour is:


$0.3725 ÷ 1000 hours = $0.0003725/hr


2)      Calculate the cost to run the bulb for one hour.  For the same example, a 60W bulb running for one hour consumes 0.06 KWH (kilowatt hours) of power.  Our good friends at Utah Power currently charge $ 0.0613070 per KWH.  So the cost to run that bulb for 1 hour is:


0.06 KWH/hr x $ 0.0613070/KWH = $0.00367842/hr


3)      So the total cost to run a 60 W Incandescent Soft White bulb for 1 hour is:


$0.00367842/hr + $0.0003725/hr = $0.00405092/hr


4)      This may not seem like much, but keep in mind that there are 8,760 hours in a year.  So the cost to run that bulb for a full year would be:


$0.00405092/hr x 8760 hr/yr = $35.48/yr


Use an Excel spreadsheet to calculate and compare the estimated yearly cost of each type of light bulb listed above.  Since they produce very different light, separate out the 800 lumen from the 1700 lumen bulbs.  Hand in your results along with the answers to the following questions:


1)      Which types of bulb (incandescent, halogen or fluorescent), costs the least in the long term?

2)      Other than cost, what other factors might you consider when purchasing a light bulb?  Name at least two other factors.

3)      How does this exercise fit into the larger themes of the course; that is, how does determining which bulb is the most economical contribute to our study of sustainable systems?  Think about this question both from the point of view of the consumer and of society at large.