How much light do you want? The brightness of a bulb is measured in lumens. If you're used to comparing bulbs by their wattage, the U.S. Department of Energy offers simple guidelines for choosing a bulb with the right number of lumens. To replace a 40-watt incandescent, look for a bulb with about 450 lumens. To replace a 60-watt bulb, aim for 800 lumens. A 75-watt equivalent should have about 1,100 lumens, and a 100-watt equivalent about 1,600. These numbers are only averages, so you can choose a bulb with more lumens if you want brighter light or a lower-lumen bulb for dimmer light.
What kind of light fixture do you have? Not every bulb will work in every fixture. Some CFLs and LEDs, for instance, can't be used in ceiling fixtures where the bulb hangs upside down. Other bulbs can't be used in a fully enclosed fixture or in an outdoor fixture that's exposed to the elements. Check the package to make sure the bulb you choose is rated for your intended use. Also, check the size and shape of the bulb to make sure it will fit in your fixture. If you're using a light fixture in which the bulbs are visible, such as a ceiling fan, you may prefer the smoother look of an enclosed bulb rather than a spiral-type CFL. For recessed can lights, floodlights or reflector-type bulbs are your best bet; they're designed to direct the light out of the fixture and disperse the heat that can build up inside.
Where will the bulb be used? Many LED bulbs are less than stellar at distributing light in all directions, so if you want ambient lighting, a CFL may be a better choice. However, LEDs are ideal for task lighting, under-cabinet lighting or any other use in which the light is focused in one direction. Another factor to consider is how long the light will be used at a time. CFLs tend to wear out faster when they're switched on and off frequently, so they're not great options for bathrooms or closets, which are usually lit for only a few minutes at a time. CFLs also take anywhere from a few seconds to a few minutes to reach their full brightness when you turn them on. In any area where you need full brightness right away, such as a stairwell, an LED may be a better choice.
Are you using a dimmer? A few CFL bulbs work with dimmers, but none of the best-rated CFLs in professional tests fit the bill. LED bulbs are dimmable, but many of them will work only with specific dimmers. One of the most common complaints we found about our Best Reviewed LED bulb, the Cree 60-watt Replacement LED Soft White, is that it doesn't work well with the dimmers people have in their homes, including some dimmers that Cree claimed were compatible with it. LEDs may also produce a buzzing noise when used with a dimmer. Check reviews to make sure the bulb you're considering works with the dimmer you have, or choose a less efficient halogen bulb.
Do you need a light for outdoor use? For an outdoor light, check the bulb's temperature rating. Some bulbs, especially CFLs, won't work in low temperatures. Even those that do work will take longer to warm up in cold weather. LED bulbs aren't affected by the cold, so they're a good choice for outdoor use.
What shade of light do you prefer? Most reviewers prefer the warmer light of a traditional incandescent bulb, a shade usually referred to as "soft white" or "warm white" on lighting labels. However, others prefer a bluer light that may be identified as "daylight" or "full spectrum." The Lighting Facts labels on all new light bulbs note the bulb's "color temperature," but the term is slightly confusing since a warm, yellowish light has a lower color temperature than a cool, bluish light. The CNET light bulb buying guide says it's less confusing if you think of the light as a flame: "It starts out yellow and orange, but when it gets really hot, it turns blue." A color temperature of 2,700K is closest to an incandescent, 5,000 to 6,000K is closest to natural daylight, and bulbs labeled "bright white" fall in between, around 3,500K.
How concerned are you about mercury? The small amount of mercury in a CFL bulb doesn't pose a threat to your health unless the bulb breaks. In a fixture that's out of reach, such as an overhead light, that generally won't be a problem. Table lamps and floor lamps are more likely to be knocked over, however, so you might avoid putting CFLs in these fixtures, especially if you have small children in the house. If a CFL does break, the U.S. Environmental Protection Agency offers tips on how to clean it up safely.
LED bulbs cost more up front than other bulbs, but their longer life and lower energy use can pay for them in the long run. A calculation on the Ecofrugal Living blog compares the costs of three types of bulbs over 25,000 hours of use: Traditional 60-watt bulbs would cost about $195 to buy and use, CFLs would cost around $44 and a single LED bulb would cost about $37. This makes LEDs the best long-term value, but it's also reasonable to choose CFLs while waiting for LED prices to drop further.
Incandescent light bulbs haven't disappeared completely. Although the 2007 energy bill set new efficiency standards that most incandescent bulbs can't meet, a few types were exempted from the new rules. Three-way bulbs, appliance bulbs, bug lamps and colored light bulbs are a few of the specialty incandescent bulbs that remain legal -- for now. The energy bill requires manufacturers to keep an eye on demand, and if there's a sudden spike in sales for any type of bulb, the U.S. Department of Energy will step in and set new efficiency standards for it. Even stricter standards for light bulb efficiency will take effect in 2020, requiring most bulbs to be 60 to 70 percent more efficient than incandescent bulbs. Most CFLs and LEDs can meet this standard, but today's halogen incandescent bulbs may fade from the market.