I was recently introduced to a Google search wrapper called www.blackle.com. It's sole claim to fame is that it displays a Google-like search page, but with a background color of black. Why? Blackle.com claims that black pixels require less energy to display than white pixels, so if everyone who uses Google were to see a black screen instead of white, the world would collectively save upwards of "750 megawatt-hours per year" of electricity.
Those are grand claims. While the sentiment to save energy and reduce environmental impact is well placed, I was skeptical.
So, I dug out my only CRT monitor from the closet and hooked it up to my Kill-A-Watt power meter and my laptop's external video connector. For a white screen, I used an Outlook message editor window, maximized. For a black screen, I used a cmd prompt window, maximized.
Nokia 447x 21 inch CRT: Black screen: 75 watts. White screen: 101 watts.
On the surface, these results appear to support the claims and pie-in-the-sky global estimates.
However, there's a catch.
Look at the results for an LCD monitor:
Samsung 21 inch LCD model 204B: Black screen: 36 watts. White screen: 36 watts.
LCD screens use fluorescent (or recently, LED) backlights to illuminate the screen. The backlights consume the same amount of power regardless of whether the LCD crystals are showing black pixels or white pixels. If anything, LCDs have to work harder to show black pixels because they are flooded with white light. In a CRT, black is the default state and the CRT has to work to make a white pixel.
LCD screens have been outselling CRTs for many years now. Laptops surpassed desktop sales years ago. I don't know if there are already more LCD's in the field than CRTs, but it's clearly the case that LCDs are growing while CRTs are in decline. Odds are, you're reading this text on an LCD screen.
While it's true that a black pixels consumes less power than a white pixels on a CRT screen, pixel color has no effect on LCD power consumption. Given that LCDs are a large and growing (and possibly majority) portion of the global monitor population, the power savings claimed by blackle.com is a case of diminishing returns.