Though it's not one of the top 100 most popular sites, and therefore not part of his study, the biggest power hog that Hansen came across was Mattel's Barbie , which easily used up a whopping 100 percent of the CPU resources on his 1.5GHz Windows XP machine with 1G byte of memory. "Barbie was absolutely pegging the CPU," he said.

Just how much power do these sites require? Hansen said that when he used Firefox's NoScript and Adblock Plus plugins to disable the moving graphics on the worst of the sites, the power consumption of his computer dropped by about 10 watts.

That's a fraction of the power used by your average lightbulb, but it's still more than green computing consultant Tom Block expected. "Ten watts seems like a lot when all we're doing is pulling a Web page," said Block, who is president of .

Hansen said Web developers can be more environmentally responsible by reducing the amount of animation on their sites, but he admitted there may be a downside: less traffic, because Web surfers respond to flashy graphics.

Hansen isn't the first to look at what Web sites can do to reduce power consumption on their visitors' computers. Last year the Web site launched, that people could save energy by switching their home page from Google.com to a page with a black background such as Blackle.com. The idea was that a black Web page required less energy to illuminate than a white one.