Something I did not know

From a laptop computer efficiency paper.

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.)

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5 Responses to Something I did not know

  1. Anonymous says:


    The AC/DC inefficiency is hardly unique to laptops & is one of the first things you tackle when trying to build really efficient datacenters. Plus think of how many different widgets in your home require power adapters of some sort. There’s probably a large green home conversion business opportunity in adding shared modular DC converters if someone can figure out how to make it work.

    • Anonymous says:

      Re: ac/dc

      Right. I do know about the AC->DC issue (and have to wrestle with a dozen rectifiers at home).

      But what I found really strange about this situation is how displays need AC (which I did not know, though which makes sense when I think about my home monitor that I just plug into a wall sans rectifier or big power supply), so the current has to go from AC to DC via the power brick and then back to AC just for the screen. It’s kind of the inverse (ha) of having an inverter in your car, and plugging your cell phone into it (DC->AC->DC).

      The other thing about 95% of the light being absorbed by the crystals was also interesting.

  2. Out of date info

    That paper is from 2003.

    Since then LCD screens have steading been switching from CCFL backlights to LED backlights. The newer backlights are brighter, give more accurate color, and consume rather less energy, and use DC power to boot. Most medium-to-high end laptops have LED backlights at this point, though the cost still hasn’t come down enough for them to be widely used in the bargain machines that constitute the bulk of the market.