November 23, 2021

Computer PSU

Choosing a power supply for a gaming PC or high-end desktop (HEDT) computer may seem straightforward enough, but modern power supplies today have multiple specifications that need to be carefully checked to ensure compatibility with the computer system under construction. In this article, we examine the important characteristics of a power supply unit (PSU) and demonstrate how to go about choosing the best power supply for your personal computer (PC) building project.


PSU specifications

ATX form factor

The form factor of the PC will determine the type or format of the power supply that you will need for it. The vast majority of PC builders will choose to put together a machine that conforms to the ATX specification, which means the power supply for the PC build will also have to be an 'ATX power supply'. Since the ATX form factor is the most common format used today to build a computer, this will be the format that will be focused on in this article.


The next characteristic to look at when choosing a computer power supply is the power or wattage needed for your system. To decide how much power you need, you can use freely-available online calculators to add up the power requirements of all of the components in your proposed build. Three PC power supply calculators that are often recommended are :

In our example PC build, the proposed PC components that will be used will consist of the following: 

PC Component


Intel 6-Core i5 11600K 


Gigabyte Aorus Elite AX

Graphics Card

Nvidia RTX 3070

Memory (RAM)

2 x 8GB Crucial Ballistix 3200MHz modules


1 x 3.5" SATA HDD 7200rpm


1 x SATA 512GB SSD

AIO cooler

Corsair Hydro H100i V2


3 x 120mm (front); 1 x 140 mm (back)

Therefore, entering this information into the three popular online power supply calculators results in the following wattage suggestions for the PSU:

(OuterVision calculator)

OuterVision Calculation

(Newegg calculator)

Newegg Calculation

(Cooler Master calculator)

Cooler Master Calculation

The PC components used in this calculation will make up the initial computer build, however, one also has to take into account any future changes or upgrades to the system. In our case, the most likely future upgrades will include the doubling of the total amount of RAM memory to 32 GB (which adds on another 6W approx.), the addition of more hard drives (+20W per HDD approx.) and / or SSD drives (+10W per SSD approx.)

In addition to these calculations, other tasks to carry out when determining the maximum wattage the PSU should be capable of include:

  • Consulting the motherboard manual to find out what wattage the motherboard recommends the power supply should have. For instance, in our example, the minimum power supply wattage recommended for the Gigabyte Aorus Elite AX should 500W.
  • Consulting the specifications page of the graphics card should also indicate the required system power supply wattage to operate it successfully. In the case of the RTX 3070, this should be 650W.
  • Determining any extra power requirements needed if you intend to overclock your system.

In our example, taking into account all of the above elements, a 650W power supply appears to provide the perfect balance between power needed now and any future expansion or upgrades to the computer system.



Although we have narrowed down the power supply wattage needed and thus the number of PSUs that fit this requirement, there is still a wide variety of power supplies to choose from as a multitude of manufacturers produce power supplies at this power level. Even within a single brand, there are multiple models of 650W ATX power supplies that can be used in our PC build. Consequently, we have to look at other power supply characterisitics to narrow the field down further.

From one reputable brand to another, equivalent models of PSU usually have similar features and prices, so one of the ways to move forward from this point in our PSU search is to settle on a single reputable brand and investigate all the models of the required wattage (in our case, 650W) that a brand has to offer. Today, some of the top brands with good reputations when it comes to PSUs include:

  • Corsair
  • EVGA
  • Seasonic
  • be quiet!
  • Silverstone
  • Thermaltake
  • NZXT

(this is a non-exhaustive list, and other brands could surely be included here, however, this is more than enough for our purposes)

For our PC build example, we will settle on the Corsair brand for the PSU, which has one of the strongest reputations for making reliable power supplies. All of the 650W ATX PSU models that Corsair currently has to offer are shown in the following table below:

Corsair 650W Power Supplies

Model No.80 Plus
Fan SizeFan
Zero RPM
White3-yrNov2.31127120 mmSleeve
Bronze3-yrNov2.31227120 mmSleeve
Bronze5-yrNov2.4126140 mmRifle
Bronze5-yrSemiv2.4126120 mmSleeve
Bronze5-yrSemiv2.4226120 mmRifle
Bronze5-yrFullv2.4247120 mmRifle
Gold7-yrSemiv2.4146120 mmRifle
Gold7-yrSemiv2.4246140 mmRifle
Gold10-yrFullv2.4148140 mmFDB
Gold10-yrFullv2.4149140 mmRifle
Gold10-yrFullv2.4247135 mmMagnetic
Gold10-yrFullv2.52246135 mmRifle
Gold10-yrFullv2.4247140 mmRifle
FDB = Fluid Dynamic Bearing

Now that we have narrowed down the wattage and the brand we require our power supply to have, we should look at the specifics of each model to determine which one is the most appropriate for our particular setup.

80 PLUS efficiency

Any reputable computer power supply today will conform to the 80 PLUS efficiency standard which gives an indication of how well a PSU converts mains electricity into power that can be used by the PC. The more efficient the power transformation the less waste heat will be produced. This means that electricity bills will be lower and PSU components will not be exposed to as much heat over time, prolonging their lifespans.

In addition, since enabling a PSU to conform to a higher power transformation efficiency requires the use of higher quality components, this means that PSUs with higher 80 PLUS certifications will inevitably be higher quality power supplies and will tend to last longer.

Consequently, when choosing a PSU, the objective should be to pick one that conforms to as high a standard of power transformation efficiency that one can afford. The 80 PLUS power transformation efficiencies are shown in the table below with the most efficient power transformation occurring in 80 PLUS Platinum-certified power supplies and the least efficient PSUs adhering to the 80 PLUS or 80 PLUS White standard.

The efficiencies of power conversion of different power supply certifications

% of Rated Load





80 PLUS (White)





80 PLUS Bronze





80 PLUS Silver





80 PLUS Gold





80 PLUS Platinum





80 PLUS Titanium






Different PSUs often come with different lengths of product warranties. Although the warranty itself is useful for peace of mind when buying a power supply, as with the 80 PLUS efficiency standard, it also gives an indication of the quality of the device. Since no PSU manufacturer is going to build a product that it might reasonably expect to break down within the warranty period, one can assume that the longer the warranty, the better the quality of the components used to construct the PSU. Unsurprisingly, the 80 PLUS efficiency standard will tend to correlate with the length of the PSU warranty period and so, here too, one should aim for a PSU with as long a warranty as one can afford.


When it comes to the modularity of the PSU, there are 3 different types:




This is the least expensive type of PSU as cables are soldered permanently into the power supply. However, air flow can be partially impeded by any unused power cables which will have to be left tucked away somewhere within the case.


This usually means that only the motherboard  and CPU cables from the PSU are permanently attached while other power cables are plug & socket types. Since every computer has a motherboard & CPU, it is unlikely there will be any redundant unused cables.


All PSU cables are plug & socket types. As with semi-modular, there will never be any redundant power cables as any unused cables will be left out of the PC build altogether. Modular PSUs can also make full use of higher-quality / better-looking third party cables. However, modular PSUs are the most expensive type.

When choosing the modularity type of your PSU, the preferred option is always going to be either modular or semi-modular. However, if you are on a tight budget, a non-modular power supply will work just fine and will always be the least expensive option amongst equivalent PSUs.

ATX version

When choosing a PSU, you will also come across the ATX version the power supply conforms to. Modern PSUs will all adhere to one of the latest ATX versions, all of which will be compatible with your PC build, so choosing the ‘correct’ version is not as critical as it might seem. Later ATX versions will incorporate some changes or new features into the standard but these changes are relatively minor and are only critical if you are looking for a particular feature. The main differences between the ATX versions that are currently in today's crop of Corsair 650W PSUs are shown in the table below:

ATX Version



Published February 2008. It specified a maximum ripple/noise value, and country-specific input line harmonic content & electromagnetic compatibility requirements. It also updated some of the power supply configuration & cross-regulation information.


Published April 2013. This specified the dimensions for a power supply chassis in Revision 1.31 of the 'Design Guide for Desktop Platform Form Factors'.


Published September 2017. This introduced support for Alternative Sleep Mode (ASM). Windows 10 implements this through its Modern Standby functionality.


Published June 2018. This requires PSU manufacturers to ensure that power supplies with Alternative Sleep Mode (ASM) can withstand power cycles every 180 seconds. In addition, it recommends that PSU fans turn on after at least a 2-second delay for improved user experience.

Motherboard power

A representation of the 24-pin ATX motherboard power cable

Most ATX motherboards today are powered by a 24-pin power connector from the PSU. This connection powers most of the components on the motherboard other than the CPU and some PCIe cards (eg. high-end graphics cards) which have their own specific power connections. As most modern ATX PSUs have this 24-pin connection as standard, it is sufficient to just confirm that it is present when choosing a power supply.

CPU power

A representation of an EPS12V power cable

The connections that power the CPU directly are often referred to as the EPS or EPS12V connections (in some motherboard manuals, they are also sometimes referred to as the 8-pin or 4-pin “ATX 12V power connector”). These are power connections that run from the PSU to the motherboard that are separate from the main 24-pin ATX motherboard power cable discussed previously.

Today’s power supplies for ATX motherboards come with either 1 or 2 of these EPS12V connections. One of these will always be a primary 8-pin connection (sometimes in the form of 2 x 4-pin connectors that are stuck together) and is the minimum power connection requirement for all CPUs on ATX motherboards.

A second EPS12V connection from some PSUs will consist of either a 4-pin connection or another 8-pin connection (usually 2 x 4-pin connectors that can be separated). This second CPU power connection is only necessary if you are using a high core-count (10+ cores) CPU and/or you plan on overclocking it. In any case, the motherboard will dictate the number of EPS12V connections you can physically make with your PSU (for example, the Gigabyte Aorus Elite AX has two EPS12V power receptacles, a primary 8-pin connector and a supplementary 4-pin connector) so consulting your motherboard manual is a good place to help you decide how many EPS12V connections you will want in your PSU.

PCIe power

A representation of an 8-pin PCIe power cable

The PCIe power connections from the PSU provide PCIe cards that need it with supplementary power. This supplementary power is in addition to the 75W of power that comes through the motherboard PCIe interface itself (PCIe x16). Today, the prototypical PCIe card that requires the extra power connections from the PSU is the mid-to-high-end graphics card which often requires at least one direct 6 or 8-pin (6 +2 pin) power connection from the PSU (higher-end graphics cards sometimes require two or even three 8-pin connections). As a consequence, most PSUs made for gaming PCs come with the ability to make at least two PCIe power connections, with PSUs designed for higher-end builds able to make 4 or more PCIe connections. Notably, each PCIe power cable is often split into two so each PCIe socket on the PSU can make two PCIe power connections.

Power provided by each PCIe connection type

Connection Type

Power (max)

PCIe x16








To decide how many PCIe power connections you will need for your PC build, you will need to determine the number and types of PCIe cards (almost always graphics cards) that will require supplementary power in your machine. In addition, future PCIe card additions or upgrades should also be considered to ensure that your PSU has enough supplementary PCIe power connections to serve these needs. Obviously, the more supplementary PCIe connections that a power supply is capable of making, the merrier, but higher numbers usually means a higher-priced PSU.

SATA power

A representation of a SATA power cable

SATA power connections from the PSU are primarily needed to power hard disk drives (HDDs) and SATA-powered solid-state drives (SSDs). They can also be used to power other specialised PC components such as RGB controllers, all-in-one (AIO) coolers or liquid cooling systems.

The vast majority of ATX PSUs come with the ability to make multiple SATA connections, however, these are usually facilitated by a smaller number of actual SATA power ports on the PSU. Instead, to connect multiple SATA devices to the PSU, a single SATA cable usually has multiple SATA power connectors along its length allowing it to power a series of SATA devices.

Peripheral / PATA / Molex power

A representation of a peripheral / PATA or 'Molex' power cable

Peripheral power connections provide power to a range of other PC components such as older non-SATA-powered hard drives, non-RGB fans, and fan hubs. These peripheral power cables terminate at the PC component end in a 4-pin connector, often called a ‘Molex’ connector. Like SATA power cables, there are usually multiple 4-pin Molex connectors on a single cable capable of supplying the power needs of multiple peripheral components. As a consequence, there is usually an ample number of these connections for the majority of PC builders needs.


All power supplies require integral cooling otherwise they would overheat and damage themselves in a very short time. In the vast majority of PSUs, cooling is facilitated by a large integral fan which expels the excess heat from the device. The larger this fan is, the more air it can expel for a given fan speed, so bigger fans tend not to need to spin so fast. This means that the bigger the fan, the more energy-efficient it is and the quieter it will be.

Fans also differentiate themselves by the bearings used in their construction. There are a few different types, the most common of which are listed below:

Sleeve bearing fans: These are the simplest types of fan and the least expensive but they also have the shortest lifespans. Ideally, they should not be mounted horizontally as the bearing lubricant will pool on the lower side of the fan reducing its lifespan due to excessive friction on the less lubricated upper side.

Single and double ball-bearing fans: As the name suggests, these fans have ball-bearings in place of the sleeve bearings which helps reduce friction and perform better at higher temperatures. As consequence, ball-bearing fans will be quieter and last longer than their more basic counterpart.

Rifle bearing fans and Fluid Dynamic Bearing (FDB) fans: These are similar to sleeve bearing bans but have a mechanical mechanism designed into them to lift and distribute the lubricant around the bearings which works even if the fan is mounted horizontally. As a consequence, rifle bearing and FDB fans have longer lifespans than the basic sleeve bearing fan. FDB fans are considered slightly superior to Rifle bearing ones.

Magnetic levitation bearing fans: The latest fan bearings technology involves magnetic levitation. As the name implies, magnets are used to limit the frictional forces experienced by the working parts of a fan, thus significantly increasing its working life and reducing noise. PSUs with this type of fan incorporated are currently considered the most premium type but also, unsurprisingly, the most expensive.

Zero RPM mode

In the most recent generations of power supplies, the PSU fan can be automatically switched off when it is not needed. This occurs at low power consumption when the PSU can radiate away enough waste heat to keep itself sufficiently cool without the need for the extra cooling airflow from the fan. As a result, the fan automatically turns off to save on electricity and help reduce overall noise levels from the PC.

In Corsair power supplies, this feature is known as ‘Zero RPM Mode’, but it might be known under different names in other PSU brands, for example, EVGA calls its version ‘Eco Mode’.

Mean time between failures (MTBF)

Contrary to popular belief, Mean Time Between Failures (MTBF) does NOT provide the amount of time a power supply may be expected to work before failing. Instead, it represents an experimentally-derived value that manufacturers use to assess the reliability of their products.  This makes the MTBF mostly useless to the consumer when choosing a power supply. For instance, in the case of Corsair 650W PSUs, all of them have an MTBF of 100,000 hours which does not help us to differentiate between them.

Protection circuits

To protect themselves and the other components of a PC, modern PSUs come with several different safety protection circuits built-in. These are listed below:

Protection Type



Over-Voltage Protection


Over-Power Protection (Overload Protection)


Short Circuit Protection


Under-Voltage Protection


Over-Current Protection


Over-Temperature Protection

Understanding the role of each of these types of protection circuit is beyond what the average consumer needs to know. Suffice it to say when choosing a PSU, it is usually enough just to make sure that the PSU of choice incorporates all of the protection circuits listed above as part of their design. In any case, reputable power supply manufacturers will likely incorporate all of these protection circuits within their lineup of PSUs anyway.

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