In the middle of the night, when most Americans are sound asleep, their lights and appliances off, a power hog is wide awake and running at nearly full throttle: the boxes that operate their cable or satellite television service.

The seemingly innocuous appliances — all 224 million of them across the nation — together consume as much electricity as produced by four giant nuclear reactors, running around the clock. They have become the biggest single energy user in many homes, apart from air conditioning.

Cheryl Williamsen, a Los Alamitos, Calif., architect, has three of the boxes leased from her cable provider in her home, but she had no idea how much power they consumed until recently, when she saw a rating on the back for as much as 500 watts — about the same as a washing machine.

A set-top cable box with a digital recorder can consume as much as 35 watts of power, costing about $8 a month for a typical Southern California consumer. The devices use nearly as much power turned off as they do when they are turned on.

“I could yank the power supply cord,” Williamsen said, “but that’s not a very consumer-friendly way to reduce energy consumption.”

The boxes have been at the center of a battle between the cable industry and conservationists who believe the devices could be far more efficient.

“It is a classic case of market failure,” said Andrew McAllister, a member of the California Energy Commission. “The consumers have zero information and zero control over the devices they get.”

The industry agreed recently to voluntarily reduce the power consumption of new devices, which it said would save consumers $1 billion annually. But experts say the deal will provide only a fraction of the potential gains and take years to realize.