“It may not seem like a big problem,” said Michael Goldschmidt, a housing and environmental design expert at the University of Missouri. But an average home, he said, racks up roughly a whole month’s worth of vampire power annually . “It’s a really big deal.”

The Natural Resources Defense Council puts the cost at about $165 per United States household, or $19 billion nationally. That translates to around 44 million tons of carbon dioxide, or 4.6 percent of the country’s total residential electricity generation. In an office, things like printers, computers and phone chargers can consume a quarter of a building’s energy.

You can get a general sense of the vampire power in your home by checking your energy meter during low-use times like late at night or early morning. Then, to see which household items are frequent phantoms, you could try a free smartphone app called Dr. Power. It has a library of more than 100,000 appliances and devices. Another option is a Kill-a-Watt meter, which measures how much energy individual appliances are using. They’re available at most hardware stores and can sometimes be borrowed at public libraries.

When it comes to reduction, Lisa Schmidt, chief executive of Home Energy Analytics, a California-based start-up that develops energy analysis software, recommends focusing on the higher consumption items first. “You may find one device is taking up 30 percent of the phantom load,” she said. “That’s the one you want to focus on.”

Home entertainment systems are a common energy suck, Ms . Schmidt said, especially surround-sound stereo systems. Heated towel racks are an increasingly common electricity hog. There are also things that people rarely use but keep plugged in, like the set-top box in the spare bedroom. Older items, like VCRs, also tend to be less energy efficient.