Sure, it’s what turns your PC on and keeps it running through all those spreadsheet marathons and fantasy fragfests. But beyond startup and shutdown time, your desktop’s power supply unit (PSU) doesn’t attract a lot of attention. For most shoppers, it’s seen as a generic component, a supporting player, versus the PC’s glamorous silicon stars like the processor or the graphics card.
For a workaday PC tower that isn’t pushed to the limits of its hardware, that perception is fine. Having a “good enough” PSU will suffice. But there are important differences among PC power supplies. And the more tied you are to extracting maximum performance from your PC, the more you should treat the PSU not as a checklist item, but as a component among equals.
PSU shopping is laden with its own language, though. This guide will give you a brief rundown of desktop power-supply lingo and basics here in 2021, and get you up to speed on what to look for.
What Fits? Power-Supply Form-Factor Basics
Power supplies, as we know them in desktop PCs, go all the way back to the original IBM PC. But a brief history of today’s PSU designs really begins a little later, in a time before the now-familiar ATX form factor existed, to the IBM PC AT and PS/2 of the 1980s. From these, we got the AT form-factor motherboard with dual six-pin power connectors, and the PS/2 form factor for power-supply casings (not to mention, a miniature keyboard connector).
From there, Intel developed the AT Extended (ATX) motherboard form factor, which added more room around the processor and put an extended port panel behind that space. With the latter came a new 20-pin power connector that would support electronic switching. Power-supply manufacturers responded by putting ATX PSU internals in their PS/2-form-factor casings. Intel took over the standards organization and relabeled the form factor “ATX.”
That stands to this day, though there is some motion in the industry (specifically, spurred by Intel) toward a newer 12VXO update to the ATX standard, The 12VXO update would streamline the internals of the PSU and deliver only 12-volt power. (Current mainstream PSUs deliver 12-volt as well as legacy 3.3-volt and 5-volt lines.) The 12VXO is not a factor for shoppers quite yet, but that ties into the duality of ATX: Beyond the actual sizes of PSU enclosures, ATX also remains a defining power standard.
As for those sizes, though: Today’s most common form factors are PS/2 (better known as “full ATX”) and SFX, along with their derivatives. Full ATX is the full-size desktop power supply that most of us know well from upgrading or building PCs over the years. SFX, though, is a more modern development designed for smaller desktop PCs.
The original PS/2 form factor had a mounting plate measuring 150mm wide by 86mm tall, a depth of 140mm, and an optional two-slot support tab protruding from the front (with the power plug at the rear). The original SFX specification, meanwhile, was 125mm by 63.5mm by 100mm, but many OEMs used a sideways-mounted variation measuring 100mm by 63.5mm by 125mm.
Other (less common) form factors are outlined on pages 47 through 67 of Intel’s desktop power supply design guide. Note that there is no “MicroATX” power supply form factor, though some sellers designate SFX as such. Most MicroATX PC cases use either full ATX or SFX mounting patterns for the PSU area, and other compact PC cases (such as Mini-ITX models) that might use rarer sizes (such as TFX or custom, proprietary form factors) usually come with any such uncommon kind of power supply pre-installed.
We mentioned full ATX and SFX derivatives a couple of paragraphs back. While the original PS/2 form factor’s 150mm by 86mm mounting plate is common in full-size power supplies, most of today’s high-capacity full ATX models exceed its specified 140mm mounting depth.
Plain SFX PSUs stick to their rated depth, but they can come in long versions, too. PSU and case maker SilverStone, for instance, offers extended-length SFX power supplies under the label “SFX-L,” with an extra 30mm providing room for its designers to specify a larger 120mm fan and more component hardware inside.
These extended-length PSUs intrude upon space normally reserved for cables, but many modern PC cases have some space to spare. So, key point number one is to match the type of PSU (full ATX versus SFX versus SFX-L) to the PC case you have or are considering. Point two, you should note the depth of any PSU you are considering buying, and look at the spec sheet for your PC case to make sure that the PSU depth measure is below the limit. (In addition, many PC-case reviews will describe how far a power supply can intrude before it’s blocked.)
Also know that some prebuilt desktop systems from major OEMs (notably, Dell and HP), as well as some highly compact desktops, may use proprietary PSUs that can only be replaced by the same specific proprietary models, usually sourced from the OEM itself. A red flag is a nonstandard main power connector to the motherboard that doesn’t match the standard 24-pin (more about that in a moment). If in doubt, contact the PC maker’s support line or online chat to discuss the details of what is in your particular system.
Got Good Leads? Getting to Know PSU Cables
The individual cables coming off a PC power supply are often referred to as “leads.” Intel’s original ATX power spec required only a 20-pin motherboard connector, later adding a separate square four-pin “P4” connector to provide a 12-volt lead to power the CPU independently. (This latter development showed up in the specification update called “ATX12V.”) The later EPS12V standard extended the main ATX lead to 24 pins to provide extra power to PCI Express (PCIe) slots and doubled the dedicated CPU power connector to eight pins.
When graphics cards started requiring even more power than PCIe slots could provide by themselves, PSU manufacturers added six-pin supplemental PCIe power leads to power supplies. In turn, some top-end video cards eventually needed even more power than a single six-pin connector could provide, leading to PSU designs with eight-pin PCIe leads, twin six-pin leads, and even combo eight- and six-pin leads that plug into either socket (sometimes dubbed “6+2” leads).
Until recently, and the rise of M.2 SSDs, most PCs had at least a few bay-mounted hard drives or 2.5-inch SSDs (and before that, internal optical drives) that followed the Serial ATA (SATA) standard. Separate from the SATA data cable, SATA drives employ their own discrete SATA power connectors, a distinctive thin “L” blade of a connection that is keyed to insert only one way.
Other internal peripheral devices, such as liquid-cooling water pumps and fan hubs, may still use classic four-pin ATA power connectors. These are commonly called “Molex connectors” (but, contrary to popular belief, usually are not made by Molex). Some sound cards and front-bay controller panels until recently even used the ancient four-pin floppy drive power connector. But that old-school connector is fading away in modern PSUs.
Most PSUs will have enough physical leads for all the gear you would want to power off of a PSU of its wattage. But you want to be especially sure to double-check if you are installing your PSU in a system with legacy hardware or building a PC with a monster video card.
All Mod Cons: Understanding Modular PSU Cables
As more and more cables started poking out of power supplies, it became increasingly obvious to PC upgraders and builders that stashing the unused ones in a big wad between the PSU body and the case wasn’t a good option. That’s why most top-quality power supplies today make use of modular cable connectors: that is, cables you can plug in as you need them, leaving out the unused ones to reduce clutter.
Power supplies that have only removable cables are called “fully modular” PSUs, and those with a few permanently attached cables are called “semi-modular.” Why not make every cable modular, in every design? The added socket connections add cost, impart some resistance, and reduce efficiency, which is why so many high-end PSUs include at least a soldered-in main (24-pin) motherboard lead. (After all, everyone will need to use at least that cable in any given PC.) Fully modular, 100%-removable cable designs make sense only for PC builders and modders who use custom-length cables and might want to replace the 24-pin main lead with something shorter.
Be aware that even though some power supplies from different PSU makers use the same style of modular jack (and one brand’s cables might fit another’s PSU chassis), not all of them are wired the same way. Users should only ever connect the modular cables that are specified to work with their exact PSU model or series. Don’t grab leftover, mystery modular cables from your parts box and plug them into a different modular PSU, in the hopes they’ll work—unless you like fireworks and want to buy new PC parts!
As noted at the end of the previous section, when assessing a power supply, you need to look at the components and peripherals you have installed that require a dedicated power connection. Most modern PSUs will provide more than enough connectors to supply any reasonable amount of SATA devices or ancillary Molex-powered peripherals. The key “question mark” connectors will be the PCIe ones—specifically, how many you get on a given power supply. You’ll want to make sure you have the leads you need for any graphics card or cards you are installing. The “6+2” connectors we mentioned in passing earlier can plug into either a six-pin or an eight-pin video card power socket. A PCIe power lead that has only six pins, though, won’t suffice for an eight-pin receptacle on your video card.
Note that some very high-end video cards these days actually demand three six- or eight-pin PCIe power leads, and only some high-wattage PSUs will provide you with that many. (Some may give you only two.)
Also note that some recent Nvidia GeForce RTX 3000-series Founders Edition cards make use of a special proprietary 12-pin power connector on the card end that connects to your PSU’s leads via an adapter or splitter (provided by Nvidia with the card).
In that case, don’t be deceived to look for a PSU that has a native 12-pin GPU PCIe connector; that’s not a thing.
Watts the Deal: How Much Power Do You Need?
When motherboard and graphics-card manufacturers started powering CPUs and GPUs from separate 12-volt connectors, many older power supplies were still designed to put out a substantial portion of their amperage to 5-volt and even 3.3-volt leads. That led to widespread advice and articles recommending wildly exaggerated power-supply ratings to cover your needs.
A given PSU’s output is expressed in watts. To be sure, buying a bit more wattage than you need at the moment to cover future upgrades or system changes is never a bad idea. Indeed, it is money well spent especially if you’re buying a premium power supply that you expect to carry over into future PC rebuilds, or if you plan to upgrade to a much more powerful CPU or GPU down the road.
But now that vendors have caught up to the way newer hardware is designed, power-output recommendations can be far more realistic. Plenty of factors come into how much power your PC will consume under ordinary, light-duty, and peak loads. It varies by the core components of CPU, GPU, and motherboard, as well as supplemental items such as drives (mechanical versus solid-state), LEDs, and PC case accessories. With all those possible parts in their near-infinite combinations and varieties, the best way to get a ballpark idea of your power needs is to use one of the several power-supply calculators available online.
The old standard is the much-used OuterVision Power Supply Calculator, though some PSU makers and resellers also offer their own home-baked versions. See also the Newegg calculator, as well as versions from PSU makers, which include the following (a non-comprehensive list):
We recommend calculating your needs, based on your PC’s components, on several of these sites and averaging the results. They should kick back very close recommendations, though, if you are careful and consistent with your inputs. Thankfully, in this writer’s case, Google’s top three power-supply calculator search results gave good estimates of what his system uses.
Power supplies are rated by output, not input, so you shouldn’t be too surprised if a calculated 415-watt load measures over 500 watts from your wall jack if you were to gauge it with a power meter. In fact, that would fall within 80 Plus Bronze specifications for a 500-watt model. But what does that mean?
PSU Certifications: Understanding the 80 Plus Program
80 Plus is a power-supply certification program that guarantees a minimum of 80% efficiency across a wide variety of loads, with different levels offering increased energy savings via reduced PSU waste heat. (The more efficient the PSU, the less of the wall power that it draws is dissipated as waste heat before reaching your PC components.) The less waste heat emitted to begin with, the less hard the PSU has to work on cooling (via fans, with their attendant noise), the less thermal hardware has to be included, and the less thermal wear and tear on PSU components. (On that front, heat is always the enemy.)
The 80 Plus program doesn’t just test every PSU on the market willy-nilly as a public service. Manufacturers pay to have their PSU products certified. They’ll do that as part of a marketing effort, as well as (perhaps) to comply with potential corporate buyers’ or even governmental requirements around power consumption and conservation.
It makes sense that any manufacturer that paid to have its power supply tested would stick the corresponding 80 Plus label on its packaging. But buyers who suspect that a label may have been applied fraudulently can find a corresponding list of 80 Plus power supplies at the program’s website.
As you can see, you have 80 Plus Standard, Bronze, Silver, Gold, Platinum, and Titanium certifications. Each indicates the level of efficiency at certain fixed degrees of load: 20%, 50%, and 100%. Plain 80 Plus is not so common, nor is Silver, in products on the market. You’ll tend to see the best values in 80 Plus Gold models.
Titanium, the most recent level added, measures in a more fine-grained way than the others the efficiency at very low load levels (adding a 10% level), so PSU vendors have to retain efficiency all the way down the chart. Looking at Titanium may make sense for PCs that spend lots of their time in low-usage states, especially modern PCs that tend to spend most of their time in those states due to ramp-up/ramp-down advances in the design of the CPU and GPU.
Assessing PSU Quality: The Homework Stage
Now, efficiency is one thing to pay a premium for, but the more nebulous “quality” of manufacture matters more. And quality beats quantity in power supplies, as a generic 800-watt unit is far more likely to fail under a 400-watt load than a 450-watt unit from a top-quality supplier.
Assessing quality, though, is a hazy business with PSUs, unless you look at formal, professional reviews, and interpreting those can require a measure of experience or understanding of the principles of physics. Reviewers also can’t feasibly test long-term durability. Even so, thousands of professional reviews are available online, some incorporating power testing under load using specialized equipment. (Sites such as Anandtech, TechPowerUp, and Tom’s Hardware are good sources for hardcore PSU testing info; the longtime, former PSU-authority site, Jonny Guru, recently went offline.)
One thing worth knowing is that you’ll see many power-supply brands out there, but there are fewer actual power-supply manufacturers. Oftentimes, a brand may wish to sell PSUs and will contract with a original device manufacturer (ODM) to produce a power supply line with its own branding on it, and made to its (or, sometimes, the ODM’s) specifications.
Some user groups even maintain lists of additional details such as which ODM produced which branded models. This is ever-shifting, though the lists can help you correlate models from branded vendor to actual ODM, and are generally a more accurate assessment of what you are looking at in a given PSU than just relying on user reviews. (Google the name of the PSU in question and “ODM” for leads on this.)
Still, don’t outright pooh-pooh user reviews when it comes to this particular kind of product. They can be a handy gauge once a PSU model has been on the market a while, especially if it turns out to be a troublesome one. Lots of one-star reviews can indicate a problem, because PSUs aren’t the kind of products that inspire user reviews unless something’s not right.
As a result, user reviews on e-commerce sites like Amazon and Newegg have their place in PSU shopping, given the opaque nature of PSUs and the need for aggregated data to tell you anything meaningful about quality. And even taken with a grain of salt, user reviews looked at with a critical eye are usually smarter advice than making a blind purchase—which, with a little care (and in this point of our guide!) you’re no longer at risk of doing.
