What Do I Need To Look For In A New Power Supply?

When shopping for your new power supply there are many factors to consider.  While opinions may differ on the order of importance, the following list is sequenced to account for both the practical and preferential considerations that are to be looked at:

1) Dimensions & Form Factor
Before anything else, consider the physical requirements you will have to meet in order to mount your power supply unit to your case.  People who overlook this requirement can sometimes purchase a unit that is either too big or too small for its housing.  As far as the dimensions are concerned, ensure that the width, length, and height of the power supply unit you are purchasing will fit those of your case enclosure.  The best bet here would be to purchase a unit which is nearly identical to the one you are looking to replace or, if setting up a new system, looking at your case manual for help with specifications.  Standard ATX power supply dimensions will usually be in the range of 3.25" x 6" x 5.5" (H x W x D) while typical mATX (Micro ATX) power supply dimensions are in the range of 2.5" x 5" x 4" (H x W x D).  While these dimensions may shift slightly based on the unit and manufacturer, mATX units will always be smaller than ATX units.

In regard to form factor, it is important that you purchase a unit that correctly matches your case size.  A mATX size PSU will more than likely be too small for a full-sized ATX case, whereas an ATX supply would be too large for a mATX case.  It is important to understand the internal layout of your case/motherboard and to select the correct form-factor power supply for the job.  This is particularly important if you are looking to replace a Dell power supply, HP / Compaq power supply, Sony Power Supply, Gateway Power Supply, etc.  Computer manufacturers have traditionally broken away from industry standards to release custom fitted form factors for their system lines.  The reasons for this are twofold: 1) it allows them to configure the shapes of their cases to meet their own personal requirements; 2) it ensures that much of the 'replacement parts' revenue will channeled back through their suppliers.  The most common mistake in power supply dimension considerations is related directly to this and it should not be taken lightly.  If you are unsure whether or not your power supply requirement fits a standard ATX or mATX mold or if your manufacturer took on a new design, it is best to look it up before you start searching for your replacement unit.

2) Brand & Reliability
Nobody wants a power supply that will die in a few months so it is important to know and understand the history of the manufacturer you select.  With so much misinformation (or lack of information in some cases) out on the internet this could be an increasingly difficult task.  A good way to obtain reviews of a particular power supply brand/model is to search on Google and see if other people have had problems with the unit.  Such searches may lead to Power Supply Forums or review sites where you can obtain information on what other users have experienced.  Take every human testimonial with a grain of salt as most people will only discuss their power supply when it fails.  Still, however, it is important to look for warning signs and red-flags that may arise - especially problems that come up time and time again.

Another good way to look at the reliability of a unit is to look for certification from your processor company.  AMD and Intel are careful to certify power supplies that will work correctly with their processors so these lists are a great reference for any Power Supply shopper.  To obtain an AMD Approved or Intel Approved label a manufacturer must not only produce a unit that meets their output / quality requirements, but authorized units must also adhere to strict restrictions placed on unit design (such as airflow requirements, etc) and functionality minimums.  The tradeoff here is that once a unit receives an AMD or Intel Certification, the item manufacturer can raise their asking price.  So if you are willing to pay more for the assurance, this could be the best way to go.  For more information please visit the AMD Power Supply Page or the Intel Power Supply Page.

A quick and easy way to obtain very useful brand/model information may also come directly from a reseller / retailer that you can trust.  Contact the store you are shopping at and ask them about the number of DOA (Dead On Arrival) complaints they get and request information on returns, replacements, etc.  The more you know about the company making your power supply, the better you will be able to determine what's right for you.

The purpose of this article isn't to pump a certain brand or to influence you one way or the other, so personal preferences with manufacturers will be left for your own research.  The following list contains power supply companies that have released at least one product that was certified by AMD:

3) Output Connectors / Specifications
Any complete description of a power supply will include the number and types of connectors present on the power supply unit along with specifications of the min/max power ratings to be found on each rail.  For someone who isn't familiar with all of the technical aspects associated with the specifications, the easiest way to gauge the output performance quality is to look at the current specification on the +12V rail. This rail serves as the primary source of power to the processor and is crucial for system stability and performance.  While there isn't an exact specification on these numbers, generally it is best to purchase a unit with a 15 Amp or greater (the higher the better) max current specification on the +12V rail.  For higher-end units this number will usually be in the range of 20A or more.

The requirements you place on the number of connectors and the different types available will be largely a function of your individual needs.  Here you must pay attention to any devices you will be using that may have special power requirements - most notably, the Pentium 4 (P4) processor.  For P4 processor applications, the standard ATX power connector can not deliver enough power to the motherboard and CPU.  Rather than changing the ATX standard, power supply manufacturers created an additional 4-pin power connector running from the power supply to the motherboard.  This connector injects additional power into the +12V rail and makes up for the needed difference.  This new cable is referred to as the ATX12V connector and has slowly become a standard for most new PSUs (it has also been adopted for use by some AMD Processors).  Power supplies that have the ATX12V cable are usually referred to as 'P4 Ready'.

Other factors in selecting for your connector requirements may include looking for 24-pin versus 20-pin ATX connectors (for newer motherboards), the need for SATA or PCI Express power connectors, the number of components you will need connectors for, etc. 

Something new for consideration has also come with the recent advent of Dual Rail units.  Dual Rail power supplies provide two independent +12V rails for use with supporting motherboards.  The goal of this technology is to dedicate power to the processor/cpu on one of the rails and power all of the other components (video cards, drives, etc.) on another idependent rail - cutting down on electrical noise issues and ensuring adequate power delivery to the CPU.  While this may not be the most important consideration now, purchasing a Dual Rail capable power supply may ensure that you will be able to upgrade your motherboard in the near future.

4) Total Deliverable Power
The total deliverable power rating of a power supply is another important factor for consideration.  While this metric should never be mistaken as the end-all power supply measurement, it is generally one of the more accepted tools used to compare different power supplies.  For mATX units the max power rating is usually in the range of 200W - 350W or higher.  For ATX-sized units typical ratings go from 300W - 680W or higher.   In selecting a power supply you should always shoot for a maximum power rating that greatly exceeds the total drain from all of your devices.  The recommended cushion between what your devices draw and the maximum output rating of the power supply is usually set at around 30%.  This will not only ensure proper operation, but will also allow room for growth in case a device is added or a card is plugged in. 

How much power your system will draw will be a function of the processor you are using, the number of devices you will have connected, and every direct or indirect component that will be powered by the PSU.  In most cases, for typical desktop applications a 350W or higher unit is recommended.  For a typical server with many drives (multiple hard drives, optical drives, etc.) a 450W-550W or greater unit should be used.  If too many devices are connected to a supply and the power drain requirements cannot be met, the devices will fail to operate correctly.  Running a power supply beyond its means may also do irreparable harm to the power supply and could even damage the attached components.

As with any other computer component, marketing plays a role in power supply sales and development.  Just as chip makers have always been looking to tout higher and higher clock rates (operating frequencies) for increased sales, power supply companies push the deliverable power specifications to influence potential buyers.  Unlike what AMD or Intel have faced, however, difficulties related to benchmarking power supplies and little interest in doing so has created a tremendous potential for fraud.  Many companies maintain strict quality control standards with open testing policies presented to the consumer.  Some, however, have been known to inflate values and patch old units with newer stickers to increase margins and sales.  Purchasing a 350W power supply from a trustworthy manufacturer that will fit your need may be better than purchasing a 450W unit that could have been rebadged.  It's important to read up about a unit you are interested in and to do your homework on the manufacturer.

5) Noise
In the older days of computing, power supplies were built with little regard to noise or airflow design.  Back then power supplies were often cooled by an enormous and clunky cooling fan with noise levels that could drown out an entire room.  Recently, however, the movement toward more acceptable noise output levels has caught on.  This has culminated to fan-less and noiseless units that have been released over the past few years - replacing traditional air-based fan with a Peltier, Water Cooler, or similar cooling device.  Reduced noise does not come cheap, however, so many of these units have become far too expensive for the average consume.  Units that are completely silent will sell for approximately 2x-4x more than their performance-equivalent counterparts. 

The good news here is that air-cooled (fan based) power supplies have gotten quite a bit better about noise also.  In fact, many manufacturers have dropped plans to develop alternative cooling systems to pursue quiet fans that can be made cheaper and more easily.  Now manufacturers are also incorporating dual-fan systems to help with both aspects of cooling.  First, by incorporating two fans the individual cooling units don't have to be as powerful (and thus output as much noise) as a single fan unit would have to be.  Secondly, by incorporating an intake / output fan system you can greatly reduce the power supply operating temperature and improve the internal temperature inside your case.  Further reducing this noise are power supply dampeners and shock absorbers.  These devices help by reducing vibration-related noise associated with the unit shaking its attachment.

For people looking for absolute silence from their PSU there are also many products and reference websites available.

6) Price
The general rule-of-thumb when it comes to power supply purchasing is to ignore price and go for the best performance.  Most power supply enthusiasts will push the importance of stacking features that you will use 'one-day' or those that will allow room for growth.  Some of these features can be important, but a lot of that movement is an attempt to justify spending such a large amount of money on a PSU.  While these can be important considerations, most would be happy with a power supply that they would never have to deal with again; one that will live out its life in the case that it is housed in.  For these people, when it comes time to upgrade they would rather purchase a new power supply unit with the features needed at that time or, if nothing has changed, revert back to their old unit.

If you fit that classification, then cost is indeed an important consideration.  Why pay for functionality that may be outdated by the time you would ever utilize it?  My recommendation has always been to find a range of units that will satisfy your need and purchase the one that you can best afford.  This doesn't mean seeking out the lowest priced unit without regard to quality.  Nor does this mean buying the best and most expensive unit that you can find.  It simply says to make your best educated decision after considering all the factors that come into the purchase - including how much you will be spending - and choosing the best unit for your budget.

Typically a good power supply will last 2-3 years in a standard working environment.  At the lower end of the spectrum power supplies run about $15-$25 and mid-range units can cost up to $50-$60.  High-end power supply units can cost anywhere from $120+ depending on the quality, deliverable power, noise, and cooling features.  For a complete lineup of power supply items please visit the OutletPC power supplies section.