Speedstep, C1E and similar technologies are great for the average CPU, which spends most of its time doing hardly anything at all. If you've a system that gets much heavier use, though – a server, perhaps – then you'll need something stronger. And undervolting is the ideal candidate.

It works like this. Your CPU receives a set voltage from the motherboard. This allows it to run at the given speed, but there's often a sizeable margin of error to make sure that your system can cope if the processor comes under heavy load.

And that's why you'll almost certainly be able to reduce your CPU's voltage, running it at a lower temperature, cutting your fan speeds and reducing your system's power requirements.

Sound risky? Well, there are potential problems. Cut the voltage too much and your system may not boot at all (be sure you know how to reset the BIOS settings in your CMOS RAM before you start).

Even if your system starts, it may crash later. If you want to try this then you must use a stress tester like ORTHOS to confirm that your PC is stable at the reduced voltage.

If you make undervolting work, the results are very worthwhile. High-voltage CPUs like the AMD Phenom II X4 955 or Intel Core 2 Extreme QX9650 in particular can see falls of more than 35W under load and perhaps 7W when idle, which for an always-on, highly active PC could save you more than £16 a year.

And keep in mind that undervolting isn't about reducing performance, so your system should be as fast as it always was – just more energy-efficient.

You could try undervolting by simply reducing the VCore (CPU core voltage) setting in your BIOS set-up program. That can conflict with options like SpeedStep, though, which already reduce voltage when they're able to, so a better approach is to use a tool like RMClock which lets you set a custom voltage for each of your CPU multipliers.

Check the Undervolting Guide at Notebook Review for step-by-step instructions, and take the time to explore RMClock's power management profiles – the tool has a lot to offer.

Graphics cards

Processors are power-hungry enough, but the real energy hog in many PCs is the graphics card.

High-end models like the GeForce 9800 GX2 consume around 90W for 2D activities and around 220W for 3D graphics under load, and even lesser cards will be responsible for a high proportion of your PC's electricity bill (Tom's Hardware has a list of the power requirements for a range of various graphics cards).

You can minimise this effect by underclocking the cards, though. Try reducing the processor core and memory clockspeeds to the minimum allowed by your system.

This will drastically cut 3D performance, but Windows and 2D applications will most probably work just fine. And you can always restore the faster default settings if you want to run something that requires all of your graphics card's considerable power.

To try this on an Nvidia system, right-click the desktop and choose 'Nvidia Control Panel'. Click 'Profile | Save' to save your current settings, entering an obvious name such as standard.nsu.

Click 'Adjust GPU Settings' in the Performance setting, drag the Core Bus and Memory Bus settings to the extreme left and click 'Apply'. Then click 'Profiles | Save', enter a name like energy saver.nsu and click the 'Save' button.

This tweak alone cut our test system's idle power use by 15W, and consumption under load by 49W (that's about a third). Of course, performance will be affected too, falling by around 75 per cent in our case. Gulp.

To get around this, click 'Adjust Custom Rules' and create a few rules to suit your needs. You could choose to load the energy saver profile when you run Word, say, or the faster standard settings when you launch a game. Just spend a few minutes setting things up and you'll soon be reaping the financial benefits.

If you have an ATI card, check your driver for similar options or use a third-party overclocking tool (these will happily help to you underclock, too). RivaTuner and ATITool are both worth a look.