Our home's gadgets and appliances—from TVs, computers and coffee makers to clothes dryers—often suck up power even when we're not using them. These energy vampires take quite a bite of our utility bills. Now, we know just how much.

Smart meter data from 70,000 homes in northern California show that devices not being used—many in "sleep" or "standby" mode— consume electricity around the clock and account for nearly one fourth of a home’s power use. The biggest energy hogs include aquariums, pay TV set-top boxes, and hot water recirculation pumps.

The U.S. cost of all these idle loads—ranging from $165 to $440 per home—totals $19 billion annually and equals the output of 50 large power plants, says a study Thursday by the Natural Resources Defense Council or NRDC, an environmental group. Consumer electronics such as printers and gaming consoles account for half of this amount.

The cost isn’t just financial. The report, "Home Idle Load," says the electricity generated for these always-on devices represents a lot of carbon pollution that contributes to global warming but could be avoided.

The problem may be widening. While many devices are becoming more energy-efficient, there are simply more of them. Once purely mechanical devices, such as refrigerators, are going digital with electronic displays and controls. Increasingly, they’re also gaining Internet connectivity.

View Images The Xbox One and other gaming consoles are among many electronic devices that draw energy even when not in use. Photograph by Sean Dempsey, PA Wire, Associated Press

“The digital controls are not designed with efficiency in mind,” says report author Pierre Delforge, NRDC’s director of high-tech sector energy efficiency. “This is not rocket science,” he says, noting the technology exists to keep devices connected at half a watt or less.

He says the report, in addition to detailed audits of 10 homes and an analysis of 2,750 San Francisco Bay area homes, contains the broadest use ever of smart meter data to assess idle loads. “We were surprised by the huge number and variety of energy hogs,” he says in an interview, noting there was an average of 65 devices in the audited homes.

The report "is significant,” says Steve Nadel of the American Council for an Energy-Efficient Economy, a Washington-based research group. Though the data only covers northern California, he says its findings are indicative for other parts of the United States.

Delforge says consumers are using many of the same devices nationwide. Also, he says results did not vary much based on a home’s location, age, size, or number of occupants. Half of the smart meters were in mild climates along the Pacific coast while the remainder were in more severe inland climates.

What mattered most, he says, were the devices themselves. The report urges more incentive programs for consumers and mandatory efficiency standards for manufacturers. Delforge says the federal government’s voluntary ENERGY STAR program, which gives a yellow label to efficient products, is a “good start,” but more needs to be done.

Nadel agrees. “We can and should do more,” he says, noting that some—but not all—of the Department of Energy’s efficiency standards limit a product’s energy use in “standby” mode.

Consumers can take action, too: Nadel says he’s increased use of power strips at his home. Delforge says he audited his own home and found its idle load uses 70 watts—less than half of the 164-watt average for the 70,000 California homes evaluated. He says his biggest energy hogs are his computer modem, using 20 watts, and his electric car charger, 15 watts.

“For people who are motivated, they can reduce their idle load by half or more,” Delforge says. He recommends buying simple “kill a watt” devices to detect the biggest offenders. Even without such an audit, his report urges consumers take these five steps:

1. Unplug devices not in use or used rarely, such as a DVR set-top box in the guest bedroom, a second fridge in the garage, or the furnace in the summer (switch it off if hardwired).

2. Plug devices into a power strip, or consider installing a whole-house switch that remotely turns off controlled outlets with the flip of a switch.

3. Plug them into a timer. A digital timer is better than a mechanical one because digital timers typically have a lower standby load. Use a timer for hot water recirculation pumps, instant coffee machines, or towel heaters.

4. Adjust power settings. Set your computer to go to sleep after 30 minutes or less of inactivity. Turn it off when you’re dnoe using it. Disable the “quick start” setting for TVs if they use more than a couple of watts, and disable the “instant on” mode for game consoles if you don’t need it.