Measuring a visit to Blackle.com against Google.com, Hansen said he didn't see much of a difference in power consumption, an observation that has been by others.

The idea that your Web browsing habits could save energy "sounds kind of silly," said Harry McCracken, founder of , a Web site covering personal technology. "Maybe [it's] something you should only worry about after you've sold your car, ensured you never leave a light on unneccesarily for even a millisecond," he said via instant message.

PC users could save much more power by simply turning off their PCs or putting them into sleep mode when they're not in use, said Mark Bramfitt, a principal program manager with Pacific Gas and Electric. "People don't recognize how much a PC uses," he said. "It's on the order of 100 watts or more. And if you're leaving that on around the clock, that's a big energy use."

Corporate users could save, on average, about 200 kilowatt hours of energy per PC each year by using sleep mode, he said. Home users could save three times as much power.

Bramfitt called Hansen's research "interesting," but he added, "I think there are bigger fish to fry."