Microsoft has today attempted to counter criticism that its new Windows operating system represents a power-hungry " environmental disaster" with the launch of new research that suggests the system's new power management functionality could slash the energy demands of PCs and notebooks.

The study from PC Pro Labs compared the energy usage of Windows Vista and Windows XP based on "real world" workplace usage patterns and found that systems running Vista would use up to £46 less energy over the course of a year. It claimed that this is equivalent to a saving in CO2 emissions of 45 tonnes for a business running 200 PCs.

The savings were calculated based on a survey of 800 visitors to PC Pro's website which found that users' PCs typically spend 40 percent of the time undertaking "light tasks", a further 30 percent sitting idle, and 30 percent doing medium or very intensive tasks such as photo editing or encoding. The survey also found that almost a third of users leave their PC on overnight and a quarter leave it on over the weekend.

The research applied these circumstances to three HP desktops - HP Compaq dc7700: Intel Core 2 Duo CPU; HP dx5150: AMD Athlon 64 X2 CPU; and HP Compaq D530S: Intel 2.8GHz Pentium 4 CPU - running Vista and XP and found that Vista's default sleep mode, which automatically sends a PC to sleep if it is inactive for 60 minutes, could save firms up to £46 per PC per year compared to Windows XP.

It also concluded that Vista's ability to return from sleep mode in two to three seconds, compared to around five seconds for XP, meant this resulted in no real disruption for users and that new group policy functionality meant administrators could relatively easily change power management settings to save up to £50 per year per PC.

Contrary to many critics' expectations the study also found that when the new Aero user interface was activated it had a negligible impact on power consumption, compared to running Vista with it deactivated.

So is Vista really Microsoft's "most energy efficient operating system to date"? Has the company been picked on by environmentalists keen to find a high profile target for their anti-IT campaigns? Well, yes and no.

The key conclusion from the report is that Vista is far more energy efficient than XP based on the assumption that OEM's, third party software, or employees using XP don't intervene and turn their machine off or apply sensible stand-by settings.

If the study had compared Vista with a well-managed XP estate where energy efficiency policies are enforced and everyone turns off their PC when it is not in use then the new operating system's victory would have been far less clear cut.

But leaving aside the statistical problems with PC Pro's online survey of website visitors, we all know such workplaces are rare and in an environment where people steadfastly refuse to embrace energy efficiency best practices Vista's default sleep mode and group policy functionality should indeed ensure it is more energy efficient than XP.

As Paul Stoddart, product manager at Microsoft, observed: "Changing user behaviour is far harder than changing technology, so if you have a technology that takes away the expectation that users have to turn off their PC at the end of the day that will lead to significant [energy] savings."

In short, getting staff to behave responsibly is still the cheapest and most effective way of enhancing PC's energy efficiency, but it is sadly easier said than done and Vista will provide a simple way of addressing the problem automatically.

However, even if Vista is more energy efficient than previous models this does nothing to address environmentalists' central concern that the operating systems' stiff hardware requirements will lead to a glut of perfectly workable PCs being thrown out.

Speaking at the launch of Vista earlier this year, Tony Roberts chief executive of Computer Aid International predicted that 10 million PCs could be discarded in the UK in the next two years as people upgrade to Vista compatible models. "If you imagine each of these 10 million machines contains lead and many other toxic chemicals then we really are storing up an environmental disaster," he said.

Similarly, Sian Berry of the Green Party argued that Vista posed an " offensive cost to the environment" adding that "future archaeologists will be able to identify a 'Vista Upgrade Layer' when they go through our landfill sites ".

Beyond fears over e-waste there are also climate change concerns attached to this predicted mass hardware upgrade. According to the book Computers and the Environment by Ruediger Kuehr and Eric Williams a PC uses only 25 percent of its total power consumption when in use with the remaining three quarters used up in the manufacturing process. As a result it could be argued that any energy savings Vista delivers when it is in use are likely to be more than offset if the operating system is being deployed on a box fresh PC that has been bought to replace an older model that was working fine, barring its inability to support the demanding Vista OS.

Such a scenario is likely to be widespread according a recent survey from Softchoice which found that half of business PCs in North America would not be able to meet basic Vista requirements and that 97 percent could not meet the requirements for running the premium edition.

However, Microsoft dismisses fears over a glut of otherwise unnecessary upgrades as unfounded. "The idea that Vista is massively power hungry is just not true," said Stoddart. "This research used a PC that was three years old [the HP Compaq D530S] and a high percentage of PCs bought up to three years ago are Vista compatible… I dispute the idea that you need a new high end PC to run Vista."

Cynthia Crossley, director of Microsoft's Windows client business unit, added that most companies already had their upgrade patterns in place, implying that the emergence of Vista would not force them to get rid of machines they would otherwise have held on to.

It is clear from this latest research that Vista is greener than some environmentalists have claimed and that companies that deploy the technology and use the power management functionality correctly could save money and reduce carbon emissions. However, the energy savings enjoyed while the system is in use is just one part of the environmental jigsaw surrounding Vista and to suggest that it will not result in some unnecessary hardware upgrades is more than a little disingenuous.

The power management functionality is an admirable green feature, but when the minimum CPU requirements for XP were only 75 percent greater than the version it replaced, while the minimum requirements for Vista are a whopping 243 percent larger than they are for XP then Microsoft should be wary of suggesting that the environment was a top priority for Vista's developers.