All those gadgets add up. It's not just refrigerators and air conditioners and electric heaters we need to worry about in our homes. The International Energy Agency estimates that worldwide, consumer electronics now represent 15 percent of household power demand, and that is expected to triple over the next two decades. Any gains made in the efficiency of electronic appliances have been offset by increasing numbers, and by the increasing tendency for devices to be 'always on'. To satisfy the demand from gadgets will require building the equivalent of 560 coal-fired power plants, or 230 nuclear plants, according to the agency.