After moving into my current home, I discovered that the previous owners had left dozens of light bulbs for the various fixtures in the house. I was happy to know that I wouldn’t have to restock for a while. In the interim, compact fluorescent light bulbs have become inexpensive, and LED bulbs have begun to become economical as well. While I have realized for some time that CFLs are a good investment with a short payback period, I have yet to replace my bulbs. At some level, it feels wrong to throw out all those light bulbs. What is the real return on throwing out a working bulb and replacing it with a CFL?
I calculated the payback period in days when replacing a 60W bulb with a CFL, assuming $0.1 per kWh electricity and $0.97 per CFL, which is what I paid at Home Depot last weekend . I performed the calculation for a variety of usage assumptions, and this graph shows the results:
The payback on moving to CFLs is quite fast, a few weeks for high usage bulbs, and several months for bulbs used only one hour per day.
The first graph begs the question – how frequently does a light bulb need to be used to justify replacing it with an incandescent? Assuming that a 10% return on investment is desired, that the CFL will last 5 years , and that electricity costs $0.10 per kWh, I calculate that you should replace any bulb used more than 9 minutes per day .
That’s a pretty low bar, lower than I expected. As CFL prices have dropped, and light quality has improved , there aren’t many arguments left for sticking with incandescents. And for the lazy, switching to CFLs will decrease the frequency of light bulb changes, resulting in lower effort as well.
Conclusion: Throw out your light bulbs and replace them with CFLs today. The quality of CFL light output is now pretty close to incandescent, and you are burning money every day you wait!
I replaced roughly 40 light bulbs last weekend, in the middle of writing this post. For the most part it’s worked out – the light quality is decent, but the CFLs still take some time to get to full intensity, and I may have to replace a few that flicker due to dimmers on the switches.
Here is my calculations spreadsheet on Google Docs.
 While this was a sale price, CFL prices have been falling steadily and the standard price at HomeDepot.com is still only $1.25 per bulb (see the 12 pack of 60W-equivalent TCP brand bulbs available at this writing).
 Many CFLs are warrantied for 7-9 years, and claim 8000-12,000 hours of working life. Five years is thus a conservative estimate, but takes into account the fact that CFL quality control is still an issue, so that some percentage of bulbs will be defective.
 The calculations in my spreadsheet are linear with respect to purchase price – if you pay $2 for a CFL instead of $1, then you should replace all bulbs used for more than 18 minutes a day, and so on.
 That CFL light quality has improved is my personal opinion – look around on the web, and you will find hundreds of articles disparaging CFL light quality. I think they’ve come a long way, however, and the soft-white (2700K) bulbs available now do an acceptable job imitating incandescent soft-white bulbs.