How To Build A PC

Step 7: Select A Power Supply

Even though it’s an afterthought for most builders, the power supply is actually one of the more important parts of a build. Picking a quality power supply can mean the difference between a well running system and one that suffers from crashes and boot failures. Worse yet, cheap generic models can literally explode into flames, taking the rest of your computer with it. For that reason, we recommend you take a look at our power supply reviews, as well as our Best PSUs column, and our list of the top reputable PSU brands.

Each build is going to have its own unique power needs, but generally it all boils down to two things: overall wattage, and rail specific power. Overall wattage deals with how much overall power a system needs in order to function. There are several power supply calculators available on the web that can help in determining your needs, though some of them are updated more often than others. The most important thing to remember if you’re questioning how much power you need is that oversized units can easily power undersized systems, but not the other way around, so it’s best to aim high and overshoot. Oversize power supplies can also be useful down the road if you plan to upgrade your system, as they can avoid the need to purchase a newer, larger unit.

Rail specific power is the measure of how much power certain components in a build draw from the power supply. More often than not, this usually applies to how much power large components like graphics cards pull from the power supply’s main +12V rail. For most mainstream builds with one or two mid-range graphics cards, this usually isn’t much of a concern. However, builders with high-end graphics cards or who plan on overclocking should refer to the manufacturer’s specifications for how much power a particular card needs, and then double check that the power supply they’re interested in is up to the task.

Power supplies are also often rated in terms of their efficiency, with higher quality units certified at higher 80 PLUS rating levels. These efficiency ratings enable Tom’s Hardware readers to take a similar system configuration from one of our builds, read the wall socket power draw that we report, and then calculate the required output power by applying one of the 80 PLUS efficiency ratings. For example, a build that pulls 647 W from the wall socket and that is rated at 85% efficiency will need a 550 W-rated unit (647W x 0.85). From there, if you add a little extra capacity for USB-powered peripherals and future drive upgrades, a high quality 600W unit should do the job.

Despite common misconceptions, power supply and motherboard factors have almost nothing to do with each other. The ATX motherboard form factor refers to its size rather than how it’s wired, and the ATX standard for power supplies refers to what connections are offered and how much power they can handle. ATX-compliant power supplies come in several sizing standards that include PS/2, PS3, SFX, TFX, and other less common proprietary formats.

Power Supply Form Factors
Height5.875"5.875"2.50"70 mm
Width3.375"3.375"5.00"85 mm
Depth5.625"4.00"4.00"175 mm

Often incorrectly referred to as “ATX,” the PS/2 power supply form factor is a carry over from the 1980s, long before the ATX standard came about. Its height and width, along with its mounting pattern, continues to be used today in almost all full-tower and mid-tower ATX systems, as well as many microATX and even some mini-ITX systems. One thing to keep in mind is that many of today’s high capacity units often exceed the PS/2 standard in terms of depth, and may not fit in every case designed for PS/2 power supplies. Therefore, it’s usually worth referring to the size restrictions listed on a particular case manufacturer's website, so there are no surprises.

Using the same mounting holes as standard PS/2 units, PS3 allowed Hewlett Packard to shorten the overall depth of its 1990s full ATX mini-tower cases. Confusion over PS3’s age can be attributed to the extensive time it took for Intel to add the existing standard to its power supply guidelines. Further confusion with SFX can also be blamed on Intel’s placement of its physical dimensions within SFX design guidelines.

The SFX form factor for power supplies actually refers to two different sizing standards, one that’s 5” by 4” as well as one that’s 4” by 5”. There’s also another standard defined by Intel, which is 50mm tall, although it’s much less common. Overall, the 5” by 4” size is the most common version found in most stores and is generally only used in mini-ITX cases, although there are a handful of HTPC cases that require an SFX power supply as well. SFX form factor power supplies can also be used in larger cases that are designed for PS/2 power supplies, through the use of an adapter bracket.

Even less common than its PS/2 and SFX cousins, the TFX form factor is a special format that enables manufactures to make narrower cases by trading width for depth. TFX power supplies aren’t all that common and are usually relegated to being used in small HTPC or other proprietary form factor cases.

In modern computers, the newer EPS electrical standard replaces the older ATX standard, with an 8-pin 12-volt connector delivering power directly to the CPU and a 24-pin main connector powering the rest of the board. The newer EPS connectors are backwards compatible with the older ATX standard and many manufacturers make it so the extra four pins can be separated from the main connector for an easier fit into the old 20-pin ATX and 4-pin CPU headers.

Since the PCIe slots are limited to a maximum power output of 75W, nearly all power supplies include either a 6-pin or an 8-pin connector to provide supplemental power for mid to high-end graphics cards. The 8-pin power connector is compatible with the 6-pin connector, with two pins that split away, to enable its use on less demanding cards. The PCIe connector itself is also shaped differently than the 8-pin CPU power connector in order to prevent accidental misuse.

Drive power cables include the old-fashioned 4-pin “ATA” style, a smaller “floppy” style, and the more modern “SATA.” Increasingly, power supplies lack the floppy power cable, but since some accessories still make use of it, you’ll often get an adapter for one of the ATA-style connectors. In this day of SATA-based storage, the four-pin ATA leads rarely hook up to drives, but rather power cheap fans, fan controllers, and other accessories.

In total, builders must find a power supply that’s quality-made, fits their case, has enough capacity, and has all the required cable ends. If that last measure isn’t met, adapters are usually available.

MORE: Best Power Supplies
MORE: Power Supplies 101
MORE: How We Test Power Supplies
MORE: All Power Supply Content

Create a new thread in the UK Article comments forum about this subject
This thread is closed for comments
1 comment
Comment from the forums
    Your comment
  • James_39
    Selecting the case is a bizarre thing to have in Step 1.Surely that would be best placed once all other components, including accessories, have been factored in? Otherwise you end up selecting components purely on the basis they fit in your case, not forgetting that some (most) cases don't support water cooling kits/pipes etc mentioned in Step 8.