LEDs, or light-emitting diodes, are the most efficient light bulbs on the market. They use up to 85 percent less energy than traditional incandescent bulbs, the ones with a thin wire filament inside, and can last up to 25 years. LEDs are also much more efficient than halogen bulbs and slightly more efficient than compact fluorescents, the other common options on the market, according to the Energy Saver office at the Department of Energy. (The office declined to comment for this article.)

There are about six billion screw-in sockets in homes across the United States.Two billion to three billion of those still have an inefficient incandescent or halogen bulb in them, said Noah Horowitz, a senior scientist at the Natural Resources Defense Council. If every home in the country switched those sockets over to LEDs at once, he said, we’d save an estimated 38 million tons of carbon dioxide emissions a year. He based his estimate on a report by the Appliance Standards Awareness Project that used Department of Energy data.

That’s equal to the emissions from about seven million cars. And, it would save money because it means the United States would spend about $14 billion less on lighting annually. On average, every household would save roughly $100 a year on its electrical bill. Precise cost savings would depend on a few variables, like electricity rates where you live, said Eric Hittinger, an energy expert at the Rochester Institute of Technology.

The upfront cost of LEDs tends to be higher than traditional bulbs, but that is rapidly changing as technology improves. And, over their lifetime, LEDs are almost certainly a better value because of the energy savings and because they don’t have to be replaced as often.

“In energy, we don’t get a lot of free lunches,” Dr. Hittinger said. “But LED lighting, switching to LED lighting, is basically a free lunch.”