December 7, 2014

Should I Throw Away Two Light Bulbs?

We have four small recessed lights in the ceiling of our hall.    Originally all four were incandescent bulbs.   They when I used up the last of my remaining incandescents I shopped around for replacements.   The incandescent bulbs were around $3-4 each.   I found LEDs for $10 each and decided to try one.   I bought one Sylvania 6-Watt R20 Medium Base LED Flood and it worked fine.    So then I bought 3 more to eventually replace them all with LEDs.    Another bulb burned out so I now have 2 LEDs and 2 incandescent.

The lights in question are in a main hall so they're on most of the time.   I figure an average of 10 hours a day almost all year.   We'll ball park it at 350 days a year.   Thats 3500 hours of use annually.  Could very well be less than that but I'm just estimating here.   If I replace the 35W incandescents with the 6W LEDs then I'll save 29W each.    That comes out to about roughly 100 kWh a year or $10 in annual savings.    

I haven't yet replaced the two remaining incandescent bulbs because I'm waiting for them to burn out before I replace them.    It just seems a waste to remove a working bulb like that.  

The LEDs are rated for 25,000 hours of use.    The incandescent bulbs are expected to last just 2500 hours.   I think I'm getting longer life out of the incandescents but can't say for sure.  

Lets look at the cost of the bulbs in 100 hour chunks: 

There are two components to the cost.   The cost to buy the bulbs and the cost of the electricity to operate them.

Cost to buy :
LED = $10 / 25000 = 4¢ / 100 hr
incandescent = $3 / 2500 = 12¢ / 100hr

Cost to operate :
LED = 6W * 10¢ kWH = 6¢ / 100hr
incandescent = 35W * 10¢ kWH = 35¢ / 100hr

If I was at the store comparing these two options then there would be no contest.    The LED wins hands down.  It costs 10¢ / 100 hr vs 47¢ for the incandescent.

However I have a partially used incandescent installed right now and my decision is between : option A) leave the LED on the shelf and continue to use the incandescent and option B) remove and throw away the partially used incandescent and install the LED in its place.

The electricity cost is clear.    Option A will cost me 35¢ and Option B will cost 6¢.

How I account for the cost to buy the bulbs in Option A versus Option B isn't clear to me.  For some reason I can't seem to wrap my head around this.   I've already bought both bulbs so that money is spent and its a sunk cost.   But if I use one or the other bulb it will save me buying another bulb which is minimum 6¢ /100hr to operate.  Huh?   I've confused myself.   Maybe a helpful reader can help me untangle this. 

In any case no matter how I cut it I'm pretty sure the 35¢ /100hr electricity cost of running the incandescents outweighs everything else.   Say I figure that throwing away the incandescent is wasting 12¢ for the incandescent bulb and then using the LED costs me 4¢ for the cost to buy that plus the 6¢ in electricity.  Add that all up and throwing away the incandescent and using the LED instead comes out to 22¢ total.   But continuing to pay electricity for the incandescent is 35¢.   So I seem to be spending a minimum of 13¢ / 100 hr to continue to operate those incandescent bulbs.

Bottom Line : yes I should throw away two light bulbs

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1 comment:

  1. Unless you can sell the incandescent bulbs, their purchase cost is irrelevant to this analysis. Assume that the incandescent bulbs have 2000 hours left, and that the LED bulbs you replace them with will always have the same cost, power rating, and expected lifetime, so that the hourly cost is the same. Over the next 27000 hours your options are:

    throw out bulbs: cost is 10c/100hour*270 = $27.
    keep incandescent: cost is 10c/100hour*250 +35c/100hour*20=$32

    So you save $5 by throwing the bulbs out, even if they still have most of their lifetime left. If you want to read more about this, the search term is "sunk cost".

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