Possibly the most overlooked part of any system today, however it is quite possibly the most important. PSU can cause problems when people upgrade parts of the system. Adding an extra HDD, an extra optical drive or even adding a few USB devices can put up your power consumption enough so that your existing PSU can't handle it and you will get problems ranging from frequent system crashes to (worst case) mobo or component death...
So, what do you look for when buying a PSU and why shouldn’t you just buy a PSU and case package?
Obviously every component part of your machine uses power. The amount of power a specific part uses can be measured in Watts. The more components you have in your machine the more watts your PSU needs to put out. To find out the correct wattage needed for your system you can look here
Or you can use this general rule of thumb:
High End System (Server, Video Editing, Top end games machine etc) 600 - 950 Watts
Med Spec System (General games machine, Light video editor, Mutli drive system etc) 450 - 600 Watts
Low End System (Word processor, Occasional internet browser etc) 300 - 450 Watts
These are just very rough guides and I would recommend using the website above. What to look for:
Basically there are a lot of abbreviations and jargon when looking at buying power supplies. Most however are brand related and offer no true meaning other than an addition to the model number.
However there are a couple that you should look out for: PFC: Power Factor Correction:
Basically the power coming out of your home socket is not always 'clean'. There can be variances in frequency and power spikes that will reduce the life of your power supply unit and cause damage to connected components if not corrected. Active PFC is more efficient than Passive PFC but is more expensive. A lot of older / cheaper PSUs don't have PFC. Hold up time:
This refers to how long a PSU will continue to provide power after the AC input has ceased. It is measured in milliseconds. The higher the number, the better the PSU. Efficiency:
This figure is determined by how much of the energy input to the PSU is actually used and how much is wasted. Obviously the higher the efficiency rating the better as these will save a little on the electric bill
The efficiency is usually measured when the PSU is at peak output (under the highest work load) as this is when the most power is being drawn from it. Greater than 70% efficiency is recommended. Voltage Rails and Wattage:
The voltage rails should never stray more than 5% plus or minus the given figure. If a rail does drop or increase more than 5% its rated voltage it could potentially damage or destroy components. This isn't something that manufactures tell you. This is mainly because it happens with only a small percentage of PSUs. I would however check an old PSU from time to time as the older they get the more likely they are to stray. To test the rails use a Digital Multi Meter (only a couple of pounds at a DIY store)
As mentioned before higher end systems need more watts. Beware of cheap PSUs with a high wattage value as these are quite often peak values and are not a true reading of the constant output of the PSU. So simply a cheap PSU that has a wattage of 460W probably only puts out about 400W constantly. Amperage:
PSUs with only one 12V rail should really be pushing out between 15 - 24 Amps. PSUs that have dual rails should roughly be between 18 - 22 Amps on both rails. A lot higher than this can be of concern as the rail is drawn from the most and is connected directly to the mobo. Generic PSUs / PSU and Case bundles:
As I said at the start "why shouldn’t you just buy a PSU and case package?"
Well the main reason for this is the PSUs tend to be just a no name a value PSU with peak values given to increase how good they look. If you really like the case that comes with the PSU then fine, buy it. But throw the PSU in the bin, unless you are building a machine that will have very little in the way of actual hardware. Finally:
Remember if you are adding quite a bit to an existing system or upgrading an old one, don't forget about the PSU. If you’re not sure it can handle the output then buy a new one. Better to spend out the extra few pounds (or dollars) now, than have to replace expensive components later.