The biggest electricity drain in many U.S. homes is not the refrigerator, or the computer, or even air-conditioning systems. It's the little cable TV set-top boxes. One high-definition DVR and cable box use an average 446 kilowatt hours a year, about 10 percent more than an energy-efficient refrigerator. That's because their drivers run 24 hours a day even when you turn it "off." There's little pressure on companies to provide more efficient boxes, and you can unplug your sets when not in use, but the boot-up time will take a while.