How To Build A PC: From Component Selection To Installation

Step 7: Select A Power Supply

Although it doesn't get its fair share of recognition, the power supply is the single most critical component for system stability and longevity. We've seen cheap models literally go up in flames, taking out several key pieces of hardware in the process. Picking an underpowered model might get you crashes or even boot failures. Since low-quality parts often fall short of their specifications, we'll start off with a link to our power supply reviews and a list of reputable units that have surpassed the expectations of our forum experts. You’ll notice that power supplies don’t get updated as often as other parts, because that technology doesn’t progress as quickly. Quality units have “staying power”.

How much capacity your system needs depends on its hardware configuration. Graphics cards are the most power-hungry components in gaming systems, while CPUs take priority if you're using integrated graphics. Several power supply calculators are available on the Web, though some are more up-to-date than others. The good news is that oversized power units can easily sustain undersized systems without damage, though efficiency sometimes drops when the unit is loaded by less than 20% of its rating.

Power supplies are divided into multiple primary (12 V, 5 V, 3.3 V) and secondary (-12 V, -5 V, 5 V standby) voltage outputs. Better-quality power supplies provide separate over-current protection on each of these output levels, called "rails". Additionally, Intel specified that each rail could provide no more than 18 amps, to reduce the risk of connector meltdown/cable fire.

As the need for more than 18 A of 12 V power became obvious, most manufacturers started dividing their 12 V output into multiple 18 A rails. That created load-balancing trouble as, for example, a two-rail unit could have two highly-loaded cables on one rail and two relatively unloaded cables on the other. This would trip the amperage protection circuitry, even though the internal transformer had power to spare. So-called single-rail power supplies were then devised that violated Intel's mandate, but allowed these systems to at least function. And "smart" power protection circuits have since been employed to reduce the risk of a fire from a single connector (which was the reason for the mandate in the first place).

Simple calculators might do the job for basic configurations, but the highest-end graphics cards place higher load bias on +12 V rails (so much so, in fact, that AMD's Radeon R9 295X2 even has a very specific +12 V rail requirement). Most of today's highest-performance power supplies are correspondingly designed to serve up lots of current on the +12 V rail, though cheaper parts occasionally skimp in that specification. Be on the lookout for this as you shop. AMD and Nvidia originally guided customers to the PSUs with enough 12 V amperage through their lists of CrossFire- and SLI-certified supplies. However, 80 PLUS and its efficiency ratings are also popular sources for determining higher-quality products.

Power supplies are rated in output, and one benefit from 80 PLUS reports in that they contain efficiency data from 20% to 100% load. This enables Tom’s Hardware readers to find a similar configuration in one of our builds, read the input power that we report, and calculate the required output power using 80 PLUS efficiency ratings. For example, a complete machine that draws 647 W through our meter at 85% efficiency needs a 550 W-rated unit (647 x 0.85). Even if you add a little over-capacity for USB-powered peripherals and future drive upgrades, that same machine can run comfortably on a high-quality 600 W unit.

Power supply form factors are not named after motherboard standards, in spite of the way they’re often sold. The ATX motherboard form factor does specify how they’re wired however, and an ATX-compliant power unit could follow one of several sizing standards. These include PS/2, PS3, SFX, or TFX, plus propriety parts.

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 called “ATX”, the PS/2 power supply form factor is a carry-over from the 1980s, long before ATX even existed. Its mounting pattern continues to be used in most mid- and full-tower ATX systems, but large-capacity units are often far longer (deeper into the case) than required by the original specifications. The odd-appearing metric dimensions are artifacts from an original design based on fractional inches. But the inch-based screw threads aren’t as friendly to metric conversion.

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. Conflation with SFX can also be blamed on Intel’s placement of its physical dimensions within SFX design guidelines.

One might say that SFX is two form factors, one that’s 5” by 4” and the other 4” by 5”. As a potential third candidate for SFX naming, Intel also specifies a 50 mm-tall version as “SFX, 40 mm Profile” in reference to its fan size. The three (sub-standards) can be differentiated by visual inspection as being wider, deeper, or thinner than the other two. The wider one is more common in consumer-level cases, and the one that’s coincidentally (and mistakenly) most often referred to as microATX. This form factor also allows up to 17 mm of fan housing to extend from one side of the lid, into the computer case.

The narrow TFX form factor allows some companies to make their slim cases even slimmer, though it also intrudes farther into the case. Because PS3, SFX, and TFX are often sold side-by-side under the microATX banner, buyers must often look at the pictures to determine what the seller is actually selling.

EPS supersedes ATX as the electrical standard for high-amperage power supplies, with a 24-pin “EPS” main connector powering most on-board devices and an 8-pin EPS 12 V connector delivering power to the CPU. Most manufacturers make these connectors divisible, with 4-pin sections breaking away to allow fitment in 20-pin ATX and 4-pin CPU power headers.

Also shown is an 8-pin PCIe supplemental power cable for high-end graphics cards, from which two pins can be split away to make it work with 6-pin headers. The plastic insulator surrounding these pins is shaped differently from the 8-pin CPU power connector, preventing accidental misuse.

There’s also some cross-compatibility between wider and narrower cables. Many systems with 8-pin CPU power connectors will operate sufficiently from a 4-pin cable, lacking the extra current needed to support a high overclock. And it’s often possible to hang the end of a non-divisible cable over the end of a narrower connector.

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, because some accessories use it to power other things, you 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 multi-drive backplanes.

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.