Which Bulbs to Replace with CFLs

In the previous post I compared the long term costs of buying and operating a Compact Fluorescent Light bulb (CFL) with the same costs for a traditional incandescent light bulb. I showed that it makes good sense to install CFLs in light fixtures that are on at least 4 hours per day. But what about all of the lights we only turn on for a few minutes or a couple hours every day? In this post I'll present a straight forward recommendation for which incandescent bulbs you should replace with CFLs immediately, which ones you should replace when the existing bulbs burn out, and which ones don't make sense to upgrade at all (from a cost perspective).

In order to make my recommendation table below I modeled the payback period of each light bulb. This means that I computed the amount of time it would take for the electricity bill savings to make up with difference in price between a new CFL, a new incandescent, and operating an existing incandescent. For instance, I discovered that the savings in electricity from replacing a working 100 Watt incandescent bulb that is on for 3 hours every day with an equivalent CFL bulb pays for the cost of the new bulb in 4 months. After that 4 months, the future savings are like money in the bank. Don't worry if this is confusing, the table should make things clear.

To use this table first print it out by click on it to open a large printable version. Print in landscape. Then use it to take an inventory of the incandescent lighting fixtures in your house. If you have, for instance, a lamp with standard 60 W incandescent bulb that is on for 3 hours every day, put a mark in the appropriate box (the intersection of 3 hours and 60 Watt "normal"). Once you've filled out the chart for your whole house follow the color coded recommendations to start saving immediately. Or, if you're not that motivated, just take a look at this chart next time a bulb burns out or you're planning a trip to the hardware store. Keep in mind that 5 minutes per day is the same as 30 hours per year... maybe appropriate for those dining room lights that only come on for holidays.

You can also use this guide to make sure the CFLs you already have are installed in the best places. If you have some CFLs in the red zone, consider moving them to lighting fixtures in the green zone.

To give you an idea of the savings we're talking about, using this table I spent a one-time amount of $75 to buy the bulbs that will save me $16 every month for the next several years (giving me a payback period of less than 5 months).

Some important elements of this model are the cost data and assumptions used. Most importantly, the model is built using the average cost of electricity in the US of $0.08/kwh. For light bulb prices I tried to find the best prices from national hardware stores and online lighting stores. I've included the CFL prices in the table.

My recommendations are based on an observation that lighting technology is advancing fast enough that it may not be prudent to upgrade to CFLs wherever the payback period is longer than 2 years. In some cases in the above table the payback period is more than 200 years!