mgroves

Much hay has been made about the CFL (compact fluorescent lamp) “bulb”, which includes a lot of “green” talk and a lot of talk about saving money on your electricity bill.

I won’t spend any time on the “green” arguments, because doing so begs a lot of questions, but what are the economic benefits of using CFLs, if any? I did an analysis like this for Hybrid cars, especially the “hacked” hybrid, some time ago. Now might be a good time to revisit that analysis to discover that, no, hybrids still don’t make sense (in general).

Let’s start with some facts and try to do an apple-to-apple comparison. The 13 watt CFL is the “equivalent” in terms of light of a 60 watt traditional bulb. Since the 60 watt bulb is the most common, I will use these bulbs as comparision.

Additionally, I will use the cheapest price of these bulbs: 60 watt bulbs you can get 4/$1.00 ($0.25), 13 watt CFLs are 12/$19.76 ($1.65). CFLs have longer lives too: I’m using 750 hours of life for 60 watt bulbs and 6000 hours of life for CFLs. The only other data we need is the number of hours per day that the bulb will be on, and the cost of electricity. I’m going to assume 5 hours a day, because I’m picturing the lights in my living room. I found the cost of electricity by kWh here; for Ohio it’s 9.51 cents/kWh.

I will give you the spreadsheet if you want to tweak these values (if you live outside Ohio, can get cheaper/longer lasting bulbs, whatever), but just go with me for now.

Here’s the results:

This means that if you plan to use a light source for more than 62 days at 5 hours a day, you are better off using a CFL.

But not every light in your house is on that long. So at what threshold should you use a CFL? Assuming a 1-year planning horizon, here is the answer:

This means: if you are using any light source in your house for more than 0.4 hours per day, every day for a year, you should probably use a CFL.

The amount of savings here isn’t drastic: at 5 hours a day, you will save $7.51 a year. At 0.5 hours a day, you’ll save $0.17 a year. Any light that’s on for less than that (every day for a year) will lose you money.

In the long run, it’s always better to us CFLs, as they will always outpace a light bulb, even if it takes a few years. So, if you are going to live where you are for a while, or you are comfortable with taking all your CFLs with you when you leave, it makes sense to get CFLs for everything. Otherwise, get CFLs for high usage areas, and use old bulbs for low usage areas.

Some additional comments: my current supply of standard bulbs (in use and in the package) does not enter into this analysis, because I consider them a sunk cost. Additionally, there might be some macroeconomic benefit in the long run by buying CFLs because it will allow companies to take advantage of economies of scale. That is beyond the scope of this analysis.

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