"The hardware used to display a static image on the screen is
the same regardless of whether the image was generated with the
graphics card's 2D or 3D hardware. Regardless of the number of
graphical effects used on the desktop, the common case is for the
desktop to be static. Composited and traditional desktops will
generally consume the same amount of power. Summary: Don't offer
the choice of disabling compositing when on battery. It reduces
functionality for no power benefit.
"A black screen takes up as much power on a TFT as a white one.
A screensaver that draws a black screen saves no power and does
nothing to protect the screen. Summary: If the user has not
requested an animated screensaver, turn the screen off immediately
rather than drawing a black screen.
"Displaying an image on screen requires copying that image out
of graphics memory. Each memory access takes a certain amount of
energy. Reducing the amount of memory to be accessed is a good
thing from a power management perspective. Some modern graphics
chipsets (including just about any recent one from Intel) support a
mode where the contents of the screen are compressed in memory. The
compressed copy can then be read, saving energy. The mechanism for
this is generally a trivial run length encoding of the screen
contents on a line by line basis. Continuous horizontal blocks of
colour will compress well, while horizontal gradients will not.
Summary: Try to design desktop backgrounds with vertical gradients
rather than horizontal ones.