Laptop displays consume 30% of the total energy supplied by the power supply or the battery and represent a substantial efficiency paradox by themselves. On the one hand, they cut power use and physical size by perhaps 50 to 80% relative to the external cathode ray tubes (CRTs) that preceded them. On the other hand, they still exhibit profound, fundamental inefficiencies in their basic design. The power that feeds the display starts at the AC wall plug and is converted in the power supply to DC. But displays require AC power to operate their fluorescent backlights, so the portion of the power that runs the display must undergo a second inefficient conversion from DC back to AC in an inverter, at an efficiency of perhaps 80% to 90%. Of that AC power, about 30% to 40% is successfully converted to visible light in the cold cathode fluorescent backlights, with the rest becoming heat.
Then, because of the inherent opacity of most liquid crystal technologies, 95% of that light is absorbed in the crystals themselves, rather than passing through them to emerge as useful visible light of a particular color.38 In total, then, perhaps only 1% (84% * 30% * 85% * 35% * 5%) of the energy content of the electricity drawn from the wall is actually available in the pattern of visible light emitted from its display that we call “information.” If, to be generous, we consider only the efficiency of the entire chain of components in the display system, rather than the computer as a whole, we would still have to conclude that the system only converts about 2% to 3% of the electrical energy consumed by it into visible information.
(ThinkPad love: ThinkPads were the most efficient laptops tested by the NRDC. In particular, the T40 had the highest performance and the lowest energy usage. The IBM power supply was the most efficient among those tested; if everyone switched to IBM power supplies, the savings would be between 210 and 520 GWh, depending on usage. Apparently, ThinkPads also have unusually efficient screens; switching to ThinkPad screens would save an additional 260-550 GWh.)