From Luke Walker:
From the lights out department – did you know that a cathode ray tube (CRT) monitor uses about 74 watts to display an all white web page, but only uses 59 watts to display an all black page? Yes, there all still plenty of these still in use, particularly in China and Latin America. Worldwide, about 25 percent of the monitors currently in use are cathode ray tubes, which means that they waste energy displaying white backgrounds. This can add up for sites with a global audience.
Take at look at Google, for instance, who gets about 200 million queries a day. Let’s assume each query is displayed for about 10 seconds; that means Google is running for about 550,000 hours every day on some desktop. Assuming that users run Google in full screen mode, the shift to a black background will save a total of 15 (74-59) watts. Now take into account that about 25 percent of the monitors in the world are CRTs, and at 10 cents a kilowatt-hour, that’s about $75,000/year, a goodly amount of energy and dollars for changing a few color codes.