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
TypePS/2PS3SFX*TFX
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.

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32 comments
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  • AndrewJacksonZA
    "the power supply is actually one of the more important parts of a build"
    Thank you!!

    Or rather, let's do what the cool kids are doing and rather post a GIF:
    https://giphy.com/gifs/the-office-thank-you-michael-scott-1Z02vuppxP1Pa
  • Eggz
    Great piece for a lot of first-time builders. This should have a sticky somewhere on the site so it doesn't get buried :-)
  • jkhoward
    Quote:
    "the power supply is actually one of the more important parts of a build" Thank you!! Or rather, let's do what the cool kids are doing and rather post a GIF: https://giphy.com/gifs/the-office-thank-you-michael-scott-1Z02vuppxP1Pa


    Seriously, not enough people realize how import a good PSU is. I am working with someone is heavily overclocking an i7 and two 970 in SLI as well as 4 SSD and a few hard drives, a bunch of fans, with a 750W PSU.
  • jkhoward
    Also... I am digging the age of some of these images.
  • blackmagnum
    This article brings back embarrassing memories. https://giphy.com/gifs/the-office-thank-you-michael-scott-1Z02vuppxP1Pa
  • alidan
    Quote:
    Quote:
    "the power supply is actually one of the more important parts of a build" Thank you!! Or rather, let's do what the cool kids are doing and rather post a GIF: https://giphy.com/gifs/the-office-thank-you-michael-scott-1Z02vuppxP1Pa
    Seriously, not enough people realize how import a good PSU is. I am working with someone is heavily overclocking an i7 and two 970 in SLI as well as 4 SSD and a few hard drives, a bunch of fans, with a 750W PSU.


    unless i'm thinking wrong, isn't that within the power limits of a 750? im even assuming that each gpu is 300 watts and i know they shouldn't hit that even with the most aggressive of ocs

    granted there is a distinction between a good psu and a bad one, but im just assuming its a good one.
  • chimera201
    Motherboard slots haven't evolved much. Wished every slot was like a USB slot
  • turkey3_scratch
    612443 said:
    Seriously, not enough people realize how import a good PSU is. I am working with someone is heavily overclocking an i7 and two 970 in SLI as well as 4 SSD and a few hard drives, a bunch of fans, with a 750W PSU.


    Your point being... ?
  • renosablast
    Steps 1 and 3 should be combined, and step 2 comes after 1 and 3. You better worry about the CPU and motherboard combo compatibility before you worry about a graphics card.
  • renosablast
    Sorry, meant steps 2 and 4 before 3.
  • Dark Lord of Tech
    I love when you see a $1500.00 build with top quality components and then they have a $40.00 PSU listed with it.
  • Outlander_04
    IMO the very first component selection for a gaming build should always be the .... MONITOR.
    Decisions on where and how to spend the rest of the budget can only be made once you know the resolution , and whether its 60 Hz, 144 Hz or whatever else is available
  • MasterMace
    Gonna throw in my disagreement on the priority, mentioned nice and early in the article. The first thing you pick is never your case. There's 3 things you can decide to be your starting point when building a pc to make it a smooth ride; either, 1. Budget. 2. CPU 3. Graphics. By picking 1 of these 3 things as your starting point, you can have a very smooth build process. Does that mean you buy your case last? No, I've seen plenty of builds where the case arrives first as a way of storing the items, but when you want a solid build, your case is last priority, as it has no impact on your performance and restricts the size of your items.

    Even if you wanted to build an odd form factor, like an itx, you would still pick the cpu or the budget before the case.
  • Thank you for explaining ESD correctly. I have been annoyed with articles over exaggerating about ESD a lot. So just touching something metal can help? Well, next time I think I'll set a PC on my wooden desk instead of the carpet.
  • kunstderfugue
    Quote:
    I love when you see a $1500.00 build with top quality components and then they have a $40.00 PSU listed with it.


    The XFX TS Bronze 550 comes down to $43 ish from time to time and that's a mighty fine PSU to power a single graphics card build.
  • nitrium
    Quote:
    "the power supply is actually one of the more important parts of a build" Thank you!!

    While not unimportant, it gets far too much attention on the forum's here. PSU's are only relatively rarely the cause of issues, and I'll go out on a limb and say that virtually ANY modern 650W PSU (even ultra-cheap China garbage) will reliably power a single GPU and CPU, regardless of model or how much OCing you do to them.
  • Crashman
    269694 said:
    Quote:
    I am working with someone is heavily overclocking an i7 and two 970 in SLI as well as 4 SSD and a few hard drives, a bunch of fans, with a 750W PSU.
    unless i'm thinking wrong, isn't that within the power limits of a 750? im even assuming that each gpu is 300 watts and i know they shouldn't hit that even with the most aggressive of ocs granted there is a distinction between a good psu and a bad one, but im just assuming its a good one.
    You're exactly right. We've been using high-quality power supplies in most of our System Builder Marathon machines, and dual 970s was in one of the builds. The super-high recommendations you see from other sites are a response to most builders using mediocre-quality units.
  • Crashman
    416912 said:
    Gonna throw in my disagreement on the priority, mentioned nice and early in the article. The first thing you pick is never your case. There's 3 things you can decide to be your starting point when building a pc to make it a smooth ride; either, 1. Budget. 2. CPU 3. Graphics. By picking 1 of these 3 things as your starting point, you can have a very smooth build process. Does that mean you buy your case last? No, I've seen plenty of builds where the case arrives first as a way of storing the items, but when you want a solid build, your case is last priority, as it has no impact on your performance and restricts the size of your items. Even if you wanted to build an odd form factor, like an itx, you would still pick the cpu or the budget before the case.
    Exactly wrong. The first thing people do is say "I want a LAN box" or "I want a media player" or "I want a big gorgeous office PC". They're picking a case SIZE when they make those FIRST statements, so size comes first in the discussion.
  • beoza
    Quote:
    416912 said:
    Gonna throw in my disagreement on the priority, mentioned nice and early in the article. The first thing you pick is never your case. There's 3 things you can decide to be your starting point when building a pc to make it a smooth ride; either, 1. Budget. 2. CPU 3. Graphics. By picking 1 of these 3 things as your starting point, you can have a very smooth build process. Does that mean you buy your case last? No, I've seen plenty of builds where the case arrives first as a way of storing the items, but when you want a solid build, your case is last priority, as it has no impact on your performance and restricts the size of your items. Even if you wanted to build an odd form factor, like an itx, you would still pick the cpu or the budget before the case.
    Exactly wrong. The first thing people do is say "I want a LAN box" or "I want a media player" or "I want a big gorgeous office PC". They're picking a case SIZE when they make those FIRST statements, so size comes first in the discussion.


    I have to agree with you on this Crashman. Whenever I go to build a new system for friends or relatives I always ask what they're going for in terms of use. I like to go with the Form follows function principle which is that the shape of a building or object should be primarily based upon its intended function or purpose.
  • Libero
    Quote:
    Gonna throw in my disagreement on the priority, mentioned nice and early in the article. The first thing you pick is never your case. There's 3 things you can decide to be your starting point when building a pc to make it a smooth ride; either, 1. Budget. 2. CPU 3. Graphics. By picking 1 of these 3 things as your starting point, you can have a very smooth build process. Does that mean you buy your case last? No, I've seen plenty of builds where the case arrives first as a way of storing the items, but when you want a solid build, your case is last priority, as it has no impact on your performance and restricts the size of your items. Even if you wanted to build an odd form factor, like an itx, you would still pick the cpu or the budget before the case.

    Quote:
    Sorry, meant steps 2 and 4 before 3.

    It is same meaning as define a purpose and choose a case. When you buy a computer you must know what the purpose for first example for home/office, web browsing, gaming or multi-tasking. Budget also is depend to each person. So it is not CPU or GPU before case.
  • James Mason
    I wish the pictures of PSU weren't just all Corsairs. That leads people to believe that all Corsair PSUs are "good" PSUs, when we know a vast majority aren't, and the ones a new builder are most likely to be definitely aren't.
  • Crashman
    297651 said:
    I have to agree with you on this Crashman. Whenever I go to build a new system for friends or relatives I always ask what they're going for in terms of use. I like to go with the Form follows function principle which is that the shape of a building or object should be primarily based upon its intended function or purpose.

    It is same meaning as define a purpose and choose a case. When you buy a computer you must know what the purpose for first example for home/office, web browsing, gaming or multi-tasking. Budget also is depend to each person. So it is not CPU or GPU before case.
    Yeh, I should have just kept his last statement and deleted the rest before responding:
    416912 said:
    Even if you wanted to build an odd form factor, like an itx, you would still pick the cpu or the budget before the case.
    What he's saying is even if you choose a form factor first, you're still buying the other parts first. Which is a backwards way of saying that the order of the article, form factor first, makes sense...and he still wants to disagree...

    The problem is that one needs keep a general concept of the case in mind when picking the actual components. One can often pick the exact case to fit that concept at the end, but the "Define a purpose" leads immediately to form factor, and chopping the article off there only to come back to case selection doesn't really make sense.
  • Bjorn_2
    I miss one important piece of advice: do not install anyhting but your primary drive before installing Windows. I have recently gone through the nightmare of trying to aggregate Windows onto my primary drive when replacing my SSD for a bigger one. It seems that if you have multiple drives installed, windows will happily use all for various purposes. So feel free to install everything but please, before installing Windows, disconnect your secondary and other drives.