Power supply units are responsible for delivering power to individual components within your PC. Few units offer their own surge protection, and as many components are fairly delicate, it is always advisable to plug your PC into the wall via a surge protection device of some sort.
Thankfully, power supply units are fairly reliable, and the majority outlive the working life of the PC. However, with newer components requiring more power than ever, typical supply ratings have increased significantly in line with the demand. Unfortunately, this means more heat, and whilst good quality supplies will still comfortably see your PC through its lifespan, cheap supplies have become synonymous with early failure.
When looking for a new or replacement power supply, there are a few things to bear in mind. The first and foremost is the form factor, and the most commonly used are ATX and the newer ATX-2. Older machines still use the AT form factor, though these are becoming less common. To confirm what you currently have, simply take the case off of your machine and look for a sticker on the power supply, which should tell you everything from form factor to rating in watts.
The next thing to consider is the rating, which is quoted in watts. It is never advisable to go for a lower rating supply than you currently have, as it simply may not be able to deliver enough power to all components. In many cases, going above your current rating will be beneficial, as it will be able to handle additional hardware, as well as run at less than full capacity, hence lowering heat generation and extending its lifetime. Typical supplies are around the 350-400 Watt region, though it is not uncommon to come across supplies or 500 Watts or more. If you have a new machine with an up to date graphics card and multiple hard and optical drives, then you may be advised to go for a higher rated supply of around 500 Watts, though older machines should cope perfectly adequately with a 350 Watt supply.
You will also need to check that the supply you are looking at has the appropriate connectors. Newer supplies may be quoted as being ATX12V compliant, which means they are suitable for power hungry Intel Pentium 4 and AMD Athlon systems. These are compatible with older systems, as this merely indicates that the supply has the additional 4-pin 12V connector required on the aforementioned systems.
It is always worth noting what connectors are available. Most supplies have adequate connectors for a hard drive, optical drive, floppy disk and so forth, though if you have multiple drives as well as a new graphics card, it may be worth looking for something with a little higher rating. You can piggyback power connectors, but you shouldn’t do this too much to avoid overloading the power supply.
Finally, always aim for as higher quality power supply as you can afford. Cheap supplies are generally best avoided, and whilst you needn’t spend an excessive amount, getting one from a reputable brand will pay dividends in the long run. You may also wish to look at silent power supplies, particularly if your machine is located in a quiet environment, though again this will all add to the cost.