Lightbulb Energy Savings, part I

Posted: September 1, 2011 in analysis

The United States is set to phase out the current generation of incandescent light bulbs and replace them with high-efficiency alternatives (mainly compact florescents).  Complaints about this over-regulation have come from many quarters and with many justifications.  According to the naysayers, CFLS give off poor quality light, take a long time to warm up to full brightness, are hazardous to dispose of or if they break, don’t really last longer than incandescents, and don’t save any money because the waste heat from incandescent light bulbs actually helps heat your house in the winter.

Wait, what?  I’m not so interested in analyzing the others (suffice it to say that I have an apartment lit solely by CFLs and have encountered none of those problems) but I find it hard to believe that the waste heat from incandescent light bulbs is actually cost effective.  Bloggers argue this point here and here, among other places, and a more traditional news source discusses it with respect to colder climates here.

I have a basic logic-thermodynamic argument that this can’t be true, but I’ll save it for the end of the article.  Let’s step through the energy expenditures and costs for heating a home in the winter (and not forget cooling it in the summer), and see what we get.  I’ll use my home climate of Saint Paul, Minnesota, which is the coldest major US metropolitan area.  Because I don’t live in a house, and therefore don’t pay my own heating bill, I will borrow the numbers from a home of a friend of mine, but understand, I’m not talking about doing actual experiments here to test different theories.  I’m just using these numbers to frame the mathematics of my argument.

Incandescent Lighting

My friend heats his home with a natural gas fired furnace.  Last heating season, he burned about 840 therms of natural gas to heat his home.  He also heated it incidentally in a number of other ways; running appliances, computers, and of course lighting.

He estimates that, during the 210 days of the heating season, his household used around 800 watts of incandescent lighting for 5 hours a day.  Incandescent light bulbs are 90-95% efficient as heaters, so that comes out to an additional 27.2 therms of incidental heating.

He paid $0.65 per therm of natural gas, for a marginal total of $546.  This does not include the line charges, because those do not vary with consumption.  He paid $0.09 per kilowatt hour for electricity, for a total of $76 in electricity for lighting.  He also needs to replace lightbulbs when they burn out.  If he uses GE Reveal 60w bulbs, he’ll go through 14 @ $1.25 ea. during the season, which costs another $17.50

Now, heating a house is a complicated thing.  As I mentioned above, there are other heat sources in the house, like the refrigerator coils, stove, computers, TV, and so on.  Heat is also escaping from the house, mitigated by the window seals, insulation, and so forth.  But the key thing to understand about this analysis is none of that matters.  What we know is that, between lighting and the furnace, the house needed 867.2 therms of heating last season.  If we could go back in time and change only the lighting, the difference (plus or minus) would be reflected entirely in the furnace, since nothing about the refrigerator, computer, or insulation would change.

Compact Florescent Lighting

So now we pretend that this house was lit all last year completely by CFLs.  He uses about 21% of the electricity as before, cutting his electrical bill to $16.50.  He pays $7 each for high quality lights, but needs to replace less than two a  season, on average, for an annual cost of $12.25.

These CFLs still give off heat, but the amount is greatly reduced to only about 4 therms.  This means that, instead of 840 therms of natural gas, the house needs 863.2 therms to keep it at the same temperature.  This costs $561.

So, the incandescent house costs a total of  $639.50 to heat and light for the winter, and the CFL house costs $589.75.  That pretty much says it right there.  In the coldest major city in the US, only considering the heating season, it is still considerably cheaper to use CFL lighting than incandescent.

I could continue with the elaborate math and show how the extra heat is unwanted in the summer and saves air conditioning costs, but there’s no need.  Even if you never ran the AC, the heating deficit caused by removing incandescent lights only negates a small fraction of your savings.

Next time, we’ll look at LEDs, thermodynamics, and other parts of the country.

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