A brief guide to Power Supplies (PSUs) - Wattage and PSU calculators, Efficiency, Cabling and Reviews
- Created on 17 April 2010
Of the desktop PCs bought today the majority are pre-assembled by the like of Dell and other large manufacturers, with everything built in to a case giving no choice over core components such as the PC Power supply (also known as PSUs) which can have considerable bearing over the longevity and ultimately stability of a system in regards to both the PSU itself as well as any comoponent reliant on it, which means all of them! If you are thinking of building a new machine yourself though, or need to upgrade/replace the PSU in an existing shop bought PC, there are various factors that you should bare in mind:
Wattage: The overall power rating of a PSU is measured by its wattage and power supplies are available in a range of power ratings up in to the 1000W ranges. Now the important thing to remember here is that you need a PSU that is capable of powering the components of chosen system. This will be specifically dependent on the number of drives you have installed (hard drives, CD, DVD or Blu-ray), the type and number of graphics cards you intend to use, the mainboard you will be using (single or dual processor erc.) and the amount of cooling you wish to install (fans need power too!). Based on this you can estimate your requirements either by adding up the individual components requirements or you can estimate using a handy online PSU calculator such as those provided by Antec, Enermax, Extreme Outer Vision. The calculators tend to give you a recommended wattage for your requirements, so the obvious answer would be to go out and buy a matching PSU for installation in to your system. Whilst this might save a certain amount of money, there are plenty more things to consider, not least of which is the difference between max and peak wattage ratings.
Maximum wattage ratings relate to the maximum continuous power rating of the PSU, whereas the peak wattage ratings equates to the peak wattage that the PSU can provide (generally over short time periods, such as startup where a little extra power is sometimes required). This is a difference that some cheaper OEM manufacturers do not always clearly differentiate on their packaging: you would not want to run any PSU at its' peak wattage for any length of time as it is not deisgned to run under that duress. Additionally as stated above, the output from the PSU calculators tends to be a minimum recommended wattage, so for piece of mind, our recommendation is to purchase a PSU that is capable of powering at least 20% more than this minimum maximum power in order to prolong the life of the PSU by not running it at its full potential. If you are intending to upgrade however at some point, whether that be a graphics card or a new hard drive it is worth considering buying something with a higher rating that will allow for future upgrades.
Efficiency: Typical efficiencies for PSUs tend to be in the range of 60-70%, meaning that of the power drawn from the mains, 30-40%, is lost as heat, meaning increased temperatures inside cases and noise levels are observed. Unfortunately, a lot of OEM and unbranded PSUs still seem to follow this form. This doesn't necessarily mean that they won't work well, however the efficiency of your computer will not be optimum (costing you more money to run) and the associated temperature levels inside your case add more pressure on to any cooling scheme that is in place, essentially creating a noiser system by giving cooling fans more work to do etc.
The latest certification for PSUs is through the 80 PLUS program. A lot of big brand computer manufactures (DELL, HP, etc. for a full list please see here) have signed up to the 80 PLUS program and certified PSUs are manufactured to guidelines that specify that PSUs must have at least 80% efficiency or higher. Power supplies manufactured to these specifications tend to cost more, but in turn should cost you less over the lifetime of your machine due to their enhanced efficiency and also put less thermal pressure on your machine resulting in quieter and more stable computing.
Cabling: PSUs come in modular (cables can be disconnected) and non-modular forms (cabling is hard wired). Pricewise the non-modular forms are cheaper (as there is less cost in producing them) but make installation a little more difficult as space has to be found for any spare cabling. In contrast, the modular PSUs are more expensive yet are far easier to install due to the fact that there aren't spare cables trailing around and generally give a much tidier system build. Most manufacturers tend to draw a compromise with their top PSUs, with the motherboard connectors (ATX12V cables and 20+4 pin motherboard connectors) hard wired (non-modular) and the SATA (HDDs and optical drives) and PCI-E cables (video cards) connected in a modular design. The choice really comes down to personal preference, if you only intend to install the PSU and leave it then a non-modular design should be fine, wherease if you intend to continue to pull apart and upgrade your machine a modular PSU would probably be worth the investment.
Other considerations: Circuitries to prevent damage resulting from short circuits (SCP), over voltages (OVP), over power (OPP), over temperature (OTP), and over current (OCP) are generally incorporated into most PSUs as standard to fulfill safety regulations and all for sale in Europe incorporate some form of power factor correction (either active or passive) that helps provide a more efficient power distribution. The better of these two (but more expensive) is active PFC that provides a more "smoother" delviery of power to a computers components, and this is the form of PFC found in the higher end of the market. If you intend to run multiple graphics card configurations, consider purchasing an SLI-certified power supply that meats the rigorous demands of such a setup.
Summary: When deciding on a power supply, first check the minimum wattage and then buy something that gives you at least 20% headroom (e.g. if your minimum recommended wattage is 400W, buy a PSU that can provide at least 480W) or more if you intend to upgrade in the future and make sure that the PSU rating is for maximum wattage, not peak wattage.
Definitely consider purchasing an 80 Plus certified PSU, not only are they better for the environment, but they should also save you money on your electricity bill and also prolong the life of your machine.
For those who like to fiddle around on a regular basis inside their machine consider a modular PSU, as there will be less wires to get in your way and clutter the case bay.
If you want to run SLI or multiple graphic card systems you will need and SLI certified PSU, the majority of which have a single +12V rail (which allows the PSU to cater better for more powerful graphics card) which we would recommend for such configurations to avoid potential power problems.
Lastly, read reviews of power supplies (we have a few listed below, to which we are constantly adding) and if possible stick to well known brands rather than oem or unbranded supplies that are cheaper and tend to lack the features of the slightly more expensive branded counterparts.