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TekSpek Power Supplies
Power Supply units

Power Supply units

Date issued:

Power-supply units, or PSUs, are an integral part of any computer build. Often overlooked, it is the component that is likely to last the longest, and it can be moved between systems when you upgrade to the latest and greatest in the future.

Though just a boring black box to the uneducated, there is far more to the humble PSU than meets the eye, and in TekSpek we discuss the key attributes you should look out for when choosing one.


The defining factor of any PSU is its wattage capacity. This headline figure represents the amount of sustained power a PSU can deliver to the components within a PC. This is different from the amount of power coming from the wall socket, however, and the reason is a PSU needs to convert 230V/110V alternating current (AC) from the mains to the direct current (DC) required by computers and all household electrical items. More on this later.

Coming back to point, common PC supplies are available in capacities usually between 300W and 1,500W, encompassing both ends of the spectrum. As a rule of thumb, entry-level PCs require no more than 300W, mainstream PCs often make do with 500W, performance PCs use 700W PSUS, whilst power users, who often add a second graphics cards and overclock their PCs, can pull as much as 1,000W. This is especially true for high-end desktop (HEDT) systems housing the Intel Core X-series and AMD Ryzen Threadripper CPUs.

Delivering higher capacities increases the price because better components are needed to convert the AC wattage to DC. Extra capacitors, filtering, and rectifiers all need to be beefed-up, so as another rule of thumb, expect to pay around £10-£15 for every 100W of additional capacity.


Capacity is certainly not only the metric by which to evaluate a PSU. Efficiency, too, is a key consideration, and it is defined as the ratio of delivered DC power compared to the required AC power at the wall. Let us give you an example. A PSU that pulls 1,000W at the wall yet delivers 900W to the components is defined as being 90 per cent efficient. Similarly, a better PSU, delivering 950W when consuming the same 1,000W at the wall, is rated as 95 per cent efficient.

Efficiency is key because the higher the number, the less heat is generated as a by-product, meaning a supply can run cooler and quieter, Of course, higher efficiency also means lower running costs over the long term because less electricity is wasted in the transition between AC and DC.

As the efficiency of a supply is a basic proxy for its quality - higher-efficiency PSUs are more difficult and more expensive to manufacture from an engineering perspective - it is handy that the 80 PLUS organisation offers a rough and ready reckoner by bracketing supplies based on their efficiency ratings... when evaluated in their labs.

There are six levels of certification ranging from a standard 80 Plus to, at the very top, 80 Plus Titanium. Based on testing with a 230V EU internal non-redundant mains supply, a PSU needs to hit the following efficiencies, at the following loads, for the manufacturer to be awarded a certain level of certification.

percentage load
20% 50% 100%

80 Plus

82% 85% 82%

80 Plus Bronze

85% 88% 85%

80 Plus Silver

87% 90% 87%

80 Plus Gold

90% 92% 89%

80 Plus Platinum

92% 94% 90%

80 Plus Titanium

94% 96% 94%

As you can see, achieving ever-higher efficiencies is demanded as we scale the 80 PLUS ladder. Most manufacturers are keen to achieve at least 80 PLUS Bronze for the mainstream PSU, Gold for their performance models, and Platinum and even Titanium for their premier offerings. Only a handful are able to muster the electrical performance required by Titanium, however, and these are usually supplies derived from the server market, where every watt saved is hugely important across an entire datacentre.

SCAN believes that anyone looking for a PSU to last more than one build ought to seek out supplies that conform to 80 PLUS Gold, at the least, and popular models from the likes of EVGA, Corsair, be quiet!, FSP and SilverStone are all able to hit this price-to-performance sweet spot.

Form Factors

Another set of considerations to think about is form factor. Most PC supplies conform to the ATX form factor, meaning they are 150mm wide, 86mm high and between 140mm and 180mm deep. Adhering to these form factors enables them to slot into a wide range of tower and mid-tower cases. However, the recent trend of building powerful PCs into smaller chassis, helped by the energy-efficient nature of the latest slew of CPUs and graphics cards, has paved the way for other form factors to come to the fore.

These supplies shrink the internal components down to fit into an industry-accepted size. The SFX form factor typically measures 125mm x 63.5mm x 100mm (w x h x d) and houses a 60mm fan, though derivatives can use a larger 80mm fan by increasing the height.

The TFX form factor, meanwhile, is generally accepted to measure 85mm x 64mm x 175mm and will therefore fit into specific chassis alone. Smaller chassis, often home to mini-ITX-sized motherboards, often use either SFX or TFX PSUs. Most PSU manufacturers have decent-specification supplies in these two form factors, but we recommend you only consider them if having a tiny PC is absolutely paramount. For everyone else, the standard ATX supply ought to be the first choice.

Shoehorning the same capacity and 80 PLUS rating into smaller supplies is more difficult from an engineering perspective, of course, and looking at like-for-like when comparing one firm's ATX vs. SFX PSUs, the user can expect to pay a 30 per cent premium for the smaller supply.

Modular cabling, hybrid fan mode, and other niceties Stepping up the quality of a PSU usually means that more features become available. Most premium supplies tend to use modular cabling. In a nutshell, this means that all of the various cables - main 24-pin, 8-pin CPU, PCIe for graphics, Sata, and so on - are connected to ports on the supply, rather than being hardwired to the chassis. Such an approach has an obvious advantage insofar it enables tidier builds, which can be an important consideration for users who like to show their hardware off.

Modular cabling also means you only connect what is absolutely necessary, reducing cable clutter considerably and potentially aiding better airflow through the computer case. Premium supplies tend to have six ports on to which six individual cable-runs are attached, though each run may in turn have multiple connectors on it. SCAN would advise modular cabling if your budget permits.

Remember we said that higher-efficiency supplies produce less heat than entry-level models? A lack of heat is the principal reason that many 80 PLUS Gold and Platinum supplies have what is known as a hybrid-fan mode. Put simply, due to a relative lack of heat being produced, the PSU's fan only turns on once a certain percentage of load has been reached.

It is common for this figure to be 40 per cent of the total capacity on really good supplies, meaning an 800W PSU's fan will only activate once the required load is above 320W or so. Pragmatically, this means that the fan will remain off in most instances, which is clearly a boon for users who enjoy a quiet computing experience.

Going hand in hand with efficiency, another hallmark of a decent supply is the ability to run at its maximum continuous wattage when the ambient temperature is high. Entry-level supplies don't tend to have a temperature rating, mainstream ones are rated at their maximum output an ambient 30-40°C, whilst the very best can do so at a toasty 50°C ambient. Should you see such a figure, you can assume the supply is high quality.

There are other factors to consider for the PSU aficionado, as well. Ripple suppression, which can only be measured by specialist equipment, defines how well a PSU is able to transform AC current to DC. The better the suppression, the more likely the PSU will be stable under intense load. Features such as voltage regulation and what is known as hold-up time also separate the basic supplies from excellent PSUs.

In 2019 Intel released a new standard based on an all 12V design. Called ATX12VO it's not expected to replace current standards but to exist alongside it. Such supplies will only convert AC to 12V DC, while the 5V and 3.3 lines, used for driving peripherals and storage, will be derived by convertors on the motherboard. The purpose of the move is to increase the PSU efficiency further, and the ATX12VO platform is expected in the OEM market first, with the likes of Dell, HP and Lenovo's pre-configured systems.


A PSU is more than just the black box you plug your PC components into. Platform-level advances in efficiency have given rise to near-silent PSUs whilst modular cabling now enables super-clean builds.

SCAN's advice would be to look for a 400W PSU with at least an 80 PLUS Bronze rating if you are building an entry-level computer, a 600W 80 PLUS Silver for a quality mainstream PC, leaving higher capacities the domain of users who like to overclock their systems or, for that matter, run multiple graphics cards in situ.

With due notice of all of the above, it is difficult to purchase a bad PSU in 2020. Established manufacturers such as Corsair, be quiet!, FSP, EVGA, et al, all have multiple PSUs that span capacities and features. SCAN encourages you to make a list of your preferred features for a PSU and choose a supply appropriately. As always, SCAN Computers has a wide selection of power supplies on the website.