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Introduction

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Power supplies are the unsung heroes of modern PCs. While the latest and greatest CPUs and graphics cards merrily suck up more and more power and nab all the headlines in the process, it's your poor old power supply unit (PSU) that's actually got to keep it all going. So, today we're going to give the PSU some much deserved airtime, so to speak.

We've gathered together 10 top quality PSUs and put them to test, looking first at whether they are up to the task, then how noisy and hot they get, what extras you get in the box, and of course the all important price. We've chosen the 500 - 700W range as this is what we would consider the sweet spot for most performance PCs, giving you plenty enough power to use a top end CPU and graphics card with plenty of head room left over for multiple hard drives, optical drives, sound cards and other miscellaneous items. Before we get onto the testing, though, we should briefly explain what it is we'll be looking at.

What is a power supply?

Power supplies are slightly deceptively named as they give the impression of creating power out of thin air. Instead it would be more accurate to call them power converters; they take the 230V supply from your wall and convert it down into the different voltages required by the various components of your PC.

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P (Power) = I(Amps) x V (Volts)

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Modern power supplies are capable of supplying six different voltages, though some of these are only included for the sake of backwards compatibility. Nowadays both the CPU and GPU use 12V so this is the most important one to keep an eye on. Other components on your motherboard make use of the 5V and 3.3V lines so these are also relatively important. The others can essentially be ignored but we've of course tested them for brevity.

So when a power supply is rated at 700W what it actually means is it can safely convert 3Amps at 240V to some combination of 12V x I1 + 5V x I2 + 3.3V x I3 = 700W. Except it isn't actually quite as simple as that.

As more and more components used the 12V line the number of amps being drawn sky-rocketed. So in order to safely supply the amount of current required by the 12V components, it was stipulated that multiple 12V 'rails' should be used, each with a maximum load of 20A. For instance there will be one rail for the CPU, one for the graphics card, and one for the motherboard. However, just to really complicate things, some manufacturers have chosen to ignore this suggestion so they still use one massive 12V rail, as is the case with the OCZ Fatal1ty supply we're looking at today that has a max 56A 12V rail.

As well as this, there will be 'sub-limits' to the initial 700W total. So, say, the 3.3V and 5V rails combined can only deliver 130W, while the 12V rails cannot exceed 480W total. By adding all this lot up you get to the final total power rating.

The stickers on the sides of PSUs indicate exactly what loads it can handle.

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