Buying a light bulb used to be a no-brainer. Now it's a brain teaser; the transition to more energy-efficient lighting means choosing from a dazzling array of products.

We've long identified bulbs by their wattage, but that is actually a measure of electricity, not the brightness of a bulb. The amount of light a bulb generates is measured in lumens.

An incandescent 60-watt bulb, for example, gives off 800 lumens of light. And LED bulbs, which are more energy efficient than their incandescent counterparts, can deliver the same amount of light using as little as 10 watts.

The Environmental Protection Agency says that if every household replaced just one incandescent bulb with an "Energy Star"-rated LED or CFL (compact fluorescent), Americans would save close to $700 million per year in energy costs.

But with so many types of bulbs with different price points and life spans now on the market, many consumers are confused.

When we asked for your questions about light bulbs, we got an earful. So we called in Noah Horowitz, director of the Natural Resources Defense Council's Center for Energy Efficiency, to answer your most frequently asked questions.

(We should note that Natural Resources Defense Council, a nonprofit environmental organization, is a strong backer of energy-efficient lighting. It receives a small percentage of funding from government grants, including one from the EPA Energy Star program to accelerate the adoption of energy-efficient equipment.)

For even more information about light bulbs — the different types available, how long they last and what they cost over the life of the bulb — check out our guide to changing light bulbs.

Why do some CFLs die so quickly? The whole seven-year life thing seems random. I have some bulbs that last years but others that die within a year.

As not all CFLs are created equal, only buy those that have the Energy Star logo on them. Those bulbs are not only efficient but also meet the Environmental Protection Agency's rigorous performance requirements and must pass various tests including longevity. Switching your CFL on and off frequently may shorten its life. Additionally, CFLs may not turn on or reach their full brightness in really cold temperatures.

Everyone I've talked to says they just throw dead CFLs in the trash. Isn't this a problem for landfills? Are we going to start hearing about dangerous mercury levels in the ground and water in a few years?

CFLs have very low levels of mercury in them, now as low as 2 mg per bulb. Consumers should take advantage of free CFL recycling programs that are offered by leading retailers such as Home Depot and Lowe's. You should also be aware that while incandescent bulbs do not contain mercury, they cause a lot more mercury to be emitted into the environment from coal-burning power plants, since they use four times more energy than a CFL to produce the same amount of light.

I have at least three lamps that use a three-way bulb (50/100/150), and I like having the option in terms of brightness. Is there a CFL or LED version of the three-way bulb?

If you want to have different levels of light and use an energy-saving bulb, you have two great choices. If you have a three-way socket, you can buy a three-way CFL which will offer low, medium and high light output, just like your old incandescent bulb did. If your fixture is dimmable, almost all LEDs are dimmable and you can enjoy even more flexibility.

Are there CFLs or LEDs for candelabra bulbs? What about globe-shaped bulbs for my bathroom vanity?

The great news is there is an energy-efficient CFL or LED for just about every socket. These include candelabra or flame-shaped bulbs, as well as the round globe-shaped bulbs that are often used in the bathroom vanity over the sink. Candelabra and globe CFLs have been around for years, and LED models are coming on line now, too.

I have some sockets that take nothing higher than a 60-watt bulb. If I use an LED or CFL, can I use a brighter bulb? For instance, a 13-watt CFL is equivalent to a 60-watt incandescent. Is it OK to use a 23-watt CFL instead? That would give me the equivalent of a 100-watt incandescent.

Fixtures have a safety rating and one should not put in bulbs that exceed the labeled rating (such as "do not exceed 60 watts"). As long as you don't put a bulb that uses more than 60 watts in that socket you will be fine.

The good news is the energy-saving bulbs that replace a 60-watt incandescent will only use 10 to 15 watts, depending on the actual bulb you buy, and give off the same amount of light. If you want even more light, you can bump up to a 23-watt CFL that will give off as much light as the old 100-watt bulb did, while still staying below the 60-watt power cutoff. You should not, however, install a 100-watt bulb, as that could cause a fire hazard.

Are there some places in the home where you would recommend CFLs over LEDs or vice versa, such as in outdoor fixtures that are exposed to temperature extremes? Or in indoor lamps?

CFLs do not work well in cold climates and may not even start, so are not a good choice for the porch light or other outdoor sockets in cold climates. We recommend consumers select LEDs for use in recessed cans and downlights as they are better at serving as directional lights, and for sockets that are connected to a dimmer.

For those bulbs that are not used very often and are not switched on and off frequently, CFLs are probably your best bet. Conversely, put LEDs in hard-to-reach sockets, as they last up to 25 years (at three-hours-per-day usage), and you'll avoid the hassle of having to change the bulb for a really long time.

Are there dangers to putting LEDs into enclosed fixtures or in the recessed cans in my ceiling? Could they overheat?

The electronics inside the LED may fail if they are subjected to very high temperatures. LED reflector lamps are specially designed to withstand the high temperature environments found inside recessed cans, the downlights or circles in your ceiling. If you put an LED into an enclosed fixture, it may shorten its lifetime. Look for those that are labeled as being suitable for use in enclosed fixtures.

I don't understand color temperature. "Daylight" made walking down my hallway to the bathroom feel like walking into a jail. How do we determine, before buying, whether the light from the bulb is "good" — i.e., not too harsh, bright enough and sufficiently diffuse?

CFLs and LEDs come in different "flavors" of light. If you want to replicate the old yellowish white light that your incandescent gave off, look for bulbs that are marketed as "soft white" or "warm white." Conversely, if you prefer the light to have more of a bluish-white color, then select a lamp that is marketed as "daylight."

Before you go out and switch out all the bulbs in your home, we suggest you try one of each and see which one you like. While CFLs, when first introduced 20-plus years ago, did not give off pleasing light, today's CFLs are much improved and in many cases you'd be hard pressed to notice the differences from your old incandescent. Regarding LEDs, we find people love everything about them including the light quality, with the exception of the purchase price — which fortunately is dropping every day.

Many LEDs just don't dim smoothly and simply turn off before they reach a desired low level. Why? Will dimming improve?

Dimmable LEDs will work with most, but not all, installed dimmers. In a few cases, you might need to replace your dimmer and install one that is designed specifically for LEDs and CFLs, which use four times less power than the old incandescents did. In order to obtain the Energy Star label, dimmable lamps must dim down to 20 percent of full light output without a noticeable hum or flicker. As LEDs are still relatively new products, we expect future dimmable LEDs to perform even better.

I like LEDs, but I feel like I'm being gouged. Why are LEDs still so expensive, especially the brighter ones? When will the price come down?

The price of LEDs is dropping rapidly. The LED bulb that replaces the old 60-watt incandescent that used to cost $40 just a few years ago is now down to $10 or so today. [Note: NPR recently purchased 60-watt equivalent LEDs at Home Depot for less than $5. Noah Horowitz believes this price reflects an instant rebate from local utilities.] Also keep in mind that the $10 or $20 LED you just bought will save you $100 or more over the life of the bulb in the form of lower electricity costs.

Also, the individual LEDs are getting more efficient, which means manufacturers can use fewer LEDs in the bulb to deliver the same amount of light and they will need less aluminum as a heat sink, since there will be less heat to manage. All of this translates to lower costs. The LED bulbs that give off the same amount of light as old 75- and 100-watt incandescents cost more, because they require more LEDs and related materials. Their price, too, will come down with the efficiency gains and the economies of scale that come from higher production levels.

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