Ad blocking remains a controversial practice. Online publishers hate it because they depend on ad revenue. Google has warned about the practice in its financial filings.

But AdBlock Plus, a Firefox add-on that blocks online ads, has long been one of the most popular downloads among Firefox users. Almost 270,000 people download it each week.

The rationale for ad blocking goes beyond personal preference, a desire for speed, or avoidance of obesity, a condition some consumer groups associate with advertising.

Blocking ads improves security. Finjan, a computer security company, last year said that 80% of malicious code it detected was found in URLs categorized as "Advertising." Other researchers like Harvard's Benjamin Edelman have also documented the security risks posed by ads.

Security researcher Robert "RSnake" Hansen, CEO of SecTheory, offers another reason to block ads: doing so saves energy.

In an informal study posted on his company's Web site, Hansen has analyzed the power used by his PC while browsing popular Web sites using both Microsoft Internet Explorer 7 and Firefox 3. The data he collected shows "how significant dynamic client side technologies and advertisements are to overall power usage of the average consumer," his report says.

Rich media, in other words, impoverishes the planet. Or something along those lines.

"The number one most abusive technology appeared to be Flash banner ads," Hansen's report says. "... While other technologies can and did cause power spikes, they caused issues far less often than Flash, making it the least 'green' technology we came across. However, JavaScript, Java, VBScript and Silverlight all easily could have caused problems, and they should not be discounted as possible culprits for power consumption."

Using Firefox 3.04, with AdBlock Plus and NoScript, a browser add-on that blocks client-side scripts, Hansen realized a power savings of 11 watts, based on average readings taken at the ten Web sites that, thanks to their use of rich media, draw the most power.

These power-hungry sites, according to Hansen's numbers, include: 1.) myspace.com, 2.) gamespot.com, 3.) go.com, 4.) espn.go.com, 5.) bestbuy.com, 6.) disney.go.com, 7.) weather.com, 8.) gamefaqs.com, 9.) tribalfusion.com, and 10.) amazon.com.

Hansen said that Microsoft Internet Explorer was comparable to Firefox and that ads and scripts made the difference in terms of power consumption.

While 11 watts may not be much in the grand scheme of things, every bit counts. "If you add up all of the rich media across the Internet, you're talking about megawatts of power," Hansen said.

Hansen acknowledges that unplugging equipment when it's not in use represents the best way to conserve energy. "But while the browser is turned on, there do appear to be ways to surf in a manner that can reduce the power impact," his study concludes.

For online companies -- particularly for Google, which promotes energy conservation even as it serves energy-hungry rich media ads -- Hansen's findings may prompt some soul searching.

But at a time when businesses are cutting jobs and trying to make money any way they can, improving the energy efficiency of online ads probably isn't at the top of corporate to-do lists.