Instead of trying to put a CFL in every socket, savvy homeowners interested in energy efficiency are increasingly pursuing a more nuanced strategy: Choose the right technology for each application to deliver optimal performance and cost-effective energy savings.
The most familiar (but perhaps the least useful) way of comparing lightbulbs to each other is wattage, which tells you how much power is consumed but tells you nothing about how much light the bulbs will provide—or whether you will like their light quality. Other key terms include:
Wattage Equivalent. Most energy-saving lightbulbs claim wattage equivalent, often in bold, colorful text at the top of the package. Ignore this! The federal government declined to regulate how manufacturers calculate and report wattage equivalency, so the claims products make are all over the map and often deceptive. It’s smarter to shop on the basis of measured light output instead (see the “Lumens Equivalent” table for comparative information).
Lumens are the measure of the absolute amount of light a bulb provides. An integrating sphere is one measuring tool that’s used. It first captures the bulb’s total light output in all directions across all the different wavelengths of light. Then it weights the resulting values to reflect the human eye’s sensitivity to each wavelength, summing up all the weighted values to give an overall measure of “useful” light output. Dim bulbs may only deliver 200 lumens or so, while really bright ones can deliver 2,500 lumens or more.
Efficiency. If one lighting technology can deliver more lumens of light per watt of power consumed, it is said to be more energy efficient. Lumens per watt is the figure of merit for efficiency, but that almost never appears on product labeling or packaging, so you have to calculate it from the values that are provided separately. For example, say a standard 60 W incandescent is rated at 750 lumens—that’s 12.5 lumens per watt. Compare this to a 14 W compact fluorescent rated at 900 lumens—that’s 64.3 lumens per watt. Efficiencies can range from as little as 5 to more than 100 lumens per watt, depending on the technology you choose and the amount of light you need.
Lifetime is now reported in years on product labels and assumes three hours of operation per day (a little higher than typical usage according to utility studies). Also, remember that the difference between a projected lifetime of 20 years and 25 years on two products is probably not meaningful, given the uncertainties in the accelerated lifetime testing process and the degree to which new lighting products will continue to improve between now and then. From a practical standpoint, the warranty a manufacturer offers is more useful; the highest-quality products usually offer a 10-year warranty.
Color rendering index (CRI) tells you how accurately a bulb renders a particular subset of colors (primarily pastels). A CRI of 80 or greater is usually recommended by lighting experts, but there is debate in the lighting community about the merits of paying extra money for products with a CRI greater than 90—most users can’t tell the difference under typical household lighting conditions.