An idea to get longer battery life in a laptop.

Adam Jackson ajax at
Mon Jan 4 10:34:41 PST 2010

On Sat, 2009-12-19 at 10:55 -0800, Samuel Löf wrote:
> Hi everyone, I have an idea for longer battery life in laptops.
> I have a laptop with a big 17" widescreen, the screen uses 1680X1050 I
> guess that the screen consume a large amount of the battery power. 
> What if one could lower the resulution without filling the whole
> screen? For exemple  1280X800 with 125 pixel black border up and down,
> and 200 pixel black border right and left.
> If I'm right the screen has orignally 1764000 (1680x1050) active
> pixels and with the lower resolution 1024000 (1280x800) that is 740000
> less active pixels and probaly result in longer battery life. This is
> probaly not a setting one would like to use all the time but I think
> it would be a good "emergency mode"
> I have no idea if this is possible, is it? Is it a good idea?

There's a number of things that draw power here.

One is the scanout engine.  This will use power proportional to the size
of the screen in pixels.  A smaller real framebuffer will use less
power.  This means that merely faking a smaller framebuffer by drawing
to only a subset of the screen will not reduce power consumption.

Another is the display itself.  Depending on the fabrication process of
the LCD, different color states will have different power requirements.
Typical LCDs are such that white is the least-power color, since you
have to twist the crystals to block the (white) backlight from shining
through, and twisting requires applying a voltage.  So if you do border
the screen, you'd want to set the border color to whatever the
lowest-power border color is for your display.

If you can't set a border color, you could try scaling the display up to
fit the screen, but now you're in a tradeoff.  The scanout engine will
use less memory bandwidth (and thus less power), but will also have to
turn on the scaler, which requires more power.  Probably not a win, plus
it'll look ugly.

But really, the biggest power consumer by far is the backlight.  Turn
the backlight down.

- ajax
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