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PCs are power guzzlers. But just one simple tweak could reduce standby power consumption by up to a factor of four

More speed means more power. Although things are gradually changing, the simple fact is that modern PCs - especially high-end rigs - routinely come fitted with 450W power supplies. This does not mean that they always consume 450W - the power rating is the maximum the supply is able to give if required - but you may have noticed processor manufacturers recommending a minimum 350W supply. It is unfortunate but true that, as far as PC power consumption is concerned, we have been going backwards, not forwards, over the last decade. Hard disks, graphics cards and, in particular, CPUs (see Processor power consumption, opposite) have all been steadily consuming more power. But if you pay a little more attention to the issue, you can reap definite financial rewards.

When being good is not good

The power consumed by a PC varies from minute to minute. There are simple factors such as whether the optical drive is spinning or has been powered down, but the main variation comes from the processor utilisation. This has serious implications. Over the past few years, for instance, the concept of Internet grid computing has become hugely popular. The first grid scheme to hit the headlines, back in 1999, was the SETI@home project, which processes and analyses the huge quantities of data collected by the SETI (search for extra-terrestrial intelligence) organisation. SETI@home and other worthy causes, including protein-folding calculations for cancer research, use 'spare' CPU cycles when the user's computer is idle and the screensaver is running (or optionally run all the time in the background).

Unfortunately, while looking for aliens and, in particular, cures for cancer are admirable pursuits, the idea of spare CPU cycles only applies in one sense. In terms of energy, they are not spare at all, since the amount of power required by a modern processor is proportional to how hard it is working. And we are not talking about a couple of watts here either. We tested a 3GHz Pentium 4 system and found that its power consumption while idle was around 110W. But then we started up SETI@home and watched the consumption increase by an enormous 90W as the CPU, chipset and RAM subsystems swung into saturation from the intensity of the calculations.

So even if you leave your PC on and idle all the time - although that's bad enough in terms of energy conservation - it is even worse if you set it to churn through hefty calculations. This is coupled with the competitive aspect that has emerged in grid computing, with teams of users competing to 'win' and be at the top of the computing league table. So not only has it led to more energy being used in computers that would have been left on anyway, but more computers have been left on that would otherwise have been turned off in order to accumulate number-crunching points.

The secrets of standby

So, when you are not using your PC during the day - if you are in a meeting, for instance - you should set it up to go into power-saving standby mode after a sensible time-out period of about 20 minutes. This is easy enough: just go into Control Panel and double-click Power Options to set it up.

However, it is not quite as simple as that if you want maximum power savings. By default, many PC motherboards come configured in their most compatible state. In other words, the manufacturers - quite rightly - ship the board ready to work with older peripherals and operating systems. But this can have a devastating effect on power consumption.

Most motherboards these days can enter one of two power-saving states, defined by the ACPI (Advanced Configuration and Power Interface) specification. The first of these, called the S1 state, puts the computer into a 'hot standby'. To quote the ACPI specification, all the hardware 'maintains context'; in other words, it remains powered up. This is a 'safe' standby state, since legacy hardware that does not properly support ACPI should have no problems - it remains powered. The S3 state, however, is a deeper sleep state requiring more thorough hardware and software support. In S3, the PC's memory remains powered - so that you can restart exactly where you left off without having to reload Windows or any of the applications and documents you were working on - but almost the whole of the rest of the system is powered off.

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