The Power Supply Unit (PSU) is more important than many people realise - the computer needs it to work at all and if it's not up to standard or is faulty, then your computer can crash without warning and generally be unreliable. When this happens, it's often not "obvious" that the power supply is faulty at all.

So, when buying a PSU for your case, or buying a case with one already fitted, you need to look out for the following:

• Is it ATX? If you are building a computer from new parts, then it will have to be an ATX PSU. The motherboard connector on the older AT PSUs will not fit current ATX motherboards, also the power switch on the case front operates in a different fashion.
• The amount of power it can supply: An average computer these days really needs a PSU rated at 250 Watts as a minimum, with a preferred minimum of 300 Watts. Please note that while some PSUs, notably some cheaper items, while rated at 250 Watts (for example), may not be able to supply that 250 Watts for a prolonged period of time. With this in mind, try and obtain one with a higher rating than you think you need.
• AMD or P4? There are 2 main CPU manufacturers currently vying for your hard earned cash - AMD and Intel. AMD produce the Duron and Athlon range, and Intel produce the Pentium4 (P4) and Celeron range. The PSU will need to be certified compatible for use with AMD (most if not all will work with AMD based machines), but more importantly, P4 based computers require an extra power connector for the motherboard. Most PSUs you will find at present will be compatible, but it's worth checking the small print - just in case.
• Noise: The noise level of a PSU can make a huge difference to the comfort of the user, especially after long periods of use. A quiet room is rated at 32db, which is something to compare against when making your choice.

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