More than 80% of Americans subscribe to pay TV, and the 160 million set-top boxes installed in their homes cost $3 billion to operate largely because the boxes never go to sleep, an environmental group Tuesday reports.

"When not displaying or recording video content, U.S. television set-top boxes (STBs) continue to use almost as much power as when they are in use," said Noah Horowitz, senior scientist at the Natural Resources Defense Council, a Washington-based advocacy group that studied the devices. "The consumer, who pays the electric bill, has little choice about what television set-top box the service provider installs and how much energy it uses."

The study found that today's average new cable high-definition digital video recorders (HD-DVR) use more than half the energy of an average new refrigerator and more than an average new flat panel TV. Two-thirds of their total energy consumption – the equivalent annual energy output of six coal-burning power plants – occurs when they're not in use.

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In contrast, the study says cellphones are able to use extremely low levels of power when not in use, primarily to preserve battery life. It says similar innovations are beginning to appear in Sky Broadcasting's set-top boxes in Europe and it urges pay-TV service providers in the U.S. such as Comcast, Time Warner, DirecTV, Dish Network, Verizon and AT&T to do the same. It also encourages consumers to ask providers to supply them with boxes that meet ENERGY STAR Version 4.0.