Were you aware a site with a white background (which I love) consumes more energy than a site with a black background? An all white web page uses about 74 watts to display, while an all black page uses only 59 watts.
“Image displayed is primarily a function of the user’s color settings and desktop graphics, as well as the color and size of open application windows; a given monitor requires more power to display a white (or light) screen than a black (or dark) screen.” Roberson et al, 2002.
Needless to say, Google is one of the most often used sites on the net today. According to Nielsen/NetRatings (“Researchers take time out to measure online audience,” Financial Times, July 16, 2007, p. 15) Google users spent 2,557,000,000 minutes on Google websites in May of 2007 (746M for Yahoo, 7,535M for MySpace, and 2,117 for Youtube). That’s 426,616,666 hours or roughly 4,865 years! Over the course of one year that would amount to 511,400,000 hours of Google website use. HeapMedia created Blackle, a black version of Google. Yes, HeapMedia itself has a black background but not all the sites they have created have a black background.
On a global scale a black Google homepage would save 8.3 Megawatt-hours (MWh) a day, 750 Megawatt-hours a year says a blog. Triple Pundit concluded that the savings were more on the line of 1534 vs 3000 MWh per year.
In fact, switching over to an LCD monitor from the traditional CRT monitor will also save a considerable amount of energy. So if you have an LCD monitor, a black Google may not really save a whole lot of energy.