What a PSU does

The PSU converts the 230V AC power from the mains wall socket in your home or office to the various DC voltages the different components within your system require. The most common is 12V, while some legacy components still require 5V or 3.3V. Most modern PSUs have a single 12V rail, but some cheaper models have multiple 12V rails. However, multiple 12V rail PSUs are not without problems, first in that they are less power efficient; and secondly, they require careful planning when building your system to ensure component compatibility.

Whether you choose a PSU with a single, or multiple 12V rails, a high-quality PSU will supply the correct voltages to components, prolonging their life and getting the most from your system. This guide takes you through the main considerations when choosing a PSU.

PSU Size

The first and most important decision when picking a PSU is its size. We don’t mean its physical size, but its maximum power rating, which is measured in watts (W). This is critical because you need to pick a PSU that is sufficiently powerful for all the components inside your PC. For instance, a high-end gaming PC could need as much as a 1,000W PSU whereas a 300W PSU will suffice for an entry-level home office PC.

To understand what size you are likely to need, you need to know the power draw requirements of each component of your PC or workstation. These figures can be found by looking at the technical specifications of each component. Once you have these, add them up and leave an overhead of around 30% - this is so the PSU will run with a medium load on the PSU. A PSU constantly running at higher loads will make it less efficient - translating to wasted power and excessive noise (more on this later).

The GPU or graphics card is likely to be the most power-hungry component in your system, so that is a good starting place. For example, an NVIDIA RTX 5090 GPU has a power draw of 575W, so a PSU of 1,000W or more is recommended. The tables below show the recommended minimum PSU for the most popular GPUs.

Recommended PSU Wattage by GPU

NVIDIA GeForce RTX

1000W+
RTX 5090
850W+
RTX 5080, RTX 4090
750W+
RTX 5070 Ti, RTX 4080 SUPER, RTX 4080
700W+
RTX 4070 Ti SUPER, RTX 4070 Ti
650W+
RTX 5070, RTX 4070 SUPER, RTX 4070
600W+
RTX 5060 Ti
550W+
RTX 5060, RTX 5050, RTX 4060 Ti, RTX 4060

AMD Radeon RX

800W+
RX 6900 XT, RX 6950 XT, RX 7900 XTX, RX 7900 GRE
750W+
RX 9070 XT, RX 9070 GRE, RX 6800 XT, RX 7900 XT
700W+
RX 7700 XT, RX 7800 XT
650W+
RX 9070, RX 6700 XT, RX 6750 XT, RX 6800
600W+
RX 7600 XT
550W+
RX 7600
500W+
RX 6600 XT, RX 6650 XT
450W+
RX 9060 XT, RX 6600
400W+
RX 6500 XT
350W+
RX 6400

Example PC Builds

The tables above show the minimum recommended PSU for the most popular GPUs in a typical system configuration. However, it’s worth noting that different system configurations may need a more powerful PSU. Below you can see an example of what impact such choices can have on two systems, each configured with an NVIDIA GeForce RTX 5080 GPU. Note, we haven’t included every component in this table, just the ones that differ between the two configurations.

Config A (Gamer) Config B (Content Creator)
Component Power Consumption Component Power Consumption
NVIDIA RTX 5080 GPU 360W NVIDIA RTX 5080 GPU 360W
AMD Ryzen 9 9950X3D CPU 120W AMD Threadripper 7980X CPU 350W
64GB DDR5 RAM 20W 256GB DDR5 RAM 80W
1x SSD 15W 3x SSDs 45W
Cooling 36W Cooling 48W
Total 551W Total 883W
Recommended Minimum PSU 850W Recommended Minimum PSU 1,200W

PSU Form Factor

The next decision you need to make is selecting the form factor, to ensure that your PSU and PC case are compatible with one another. PSUs follow an industry standard known as the ATX spec that defines the shape and size of the PSU as well as its functionality. This means that the vast majority of ATX PSUs will fit in most ATX and micro-ATX cases. The main exception to this is some compact cases, which require a smaller PSU based on the SFX or TFX standards. To be on the safe side you should check what size PSU your case supports before buying anything. The diagrams below show the physical differences between the three most common types of PSU.

ATX power supply dimensions

ATX PSU

SFX power supply dimensions

SFX PSU

TFX power supply dimensions

TFX PSU

The ATX spec has continued to evolve over the last 30 years, adding new capabilities and features. For instance, ATX 3.0 added a new 16-pin 12VHPWR connector, delivering up to 600W for graphics cards such as the NVIDIA GeForce RTX 4090. It also required PSUs to be at least 60% efficient at extremely light loads. The later ATX 3.1 standard replaces 12VHPWR with an improved version, 12V-2x6. This has longer terminals for improved conductivity and shorter sense pins that enable GPUs to power down immediately if the connection is compromised, providing better safety. ATX 3.1 PSUs also have slightly higher efficiency than ATX 3.0 PSUs thanks to having a shorter hold-up time requirement.

PSU Connectors

Your third decision is to make sure the PSU has the right connectors for the components inside your PC. The design of these connectors is determined by the ATX spec, so all you need to do is make sure that the power supply has sufficient numbers of the right connectors for your PC. Every PSU will have a motherboard connector, but the number and type of other connectors varies. Here’s our guide to the most important connectors and what they are used for.

24-pin ATX

All power supplies have a large and bulky 24-pin ATX connector. This provides 12V, 5V and 3.3V to the motherboard and PCIe add-in cards such as entry-level graphics cards, sound cards, network cards and RAID controllers.

24-pin ATX motherboard power connector

8-pin EPS 12V

The 8-pin EPS12V connector provides 12V to the CPU via the motherboard. The larger 8-pin version is used by mid-range and high-end CPUs and is essentially two 4-pin ATX12V connectors positioned side by side. On many power supplies the EPS12V can be split in half if your motherboard only has a 4-pin ATX12V socket. Dual CPU motherboards and some ultra high-end motherboards require a PSU with two 8-pin EPS12V connectors.

8-pin EPS12V CPU power connector

8-pin / 6+2-pin PCIe

The larger 8-pin PCIe connector provides 12V to high-end graphics cards. On many power supplies two pins can be detached from the side so you plug the remaining 6-pin connector into mid-range graphics cards. Make sure you choose a power supply with sufficient 8-pin PCIe connectors for your graphics card(s). Beware, despite having the same number of pins, the 8-pin PCIe for graphics cards and 8-pin EPS12V for CPUs are not compatible with one another and have different pin-outs.

8-pin and 6+2-pin PCIe graphics card power connector

16-pin PCIe 5.0 ATX 3.0 / ATX 3.1

The large 16-pin PCIe 5.0 connector provides 12V to select high-end graphics cards. There are two versions of the 16-pin connector, the original 12VHPWR connector which is found on ATX 3.0 PSUs and the improved 12V-2x6 connector which is found on ATX 3.1 PSUs. You can get adaptors that convert two or more 8-pin PCIe cables from an ATX 2.0 power supply to a single 16-pin PCIe 5.0 cable. Just make sure to use at least two separate 8-pin PCIe cables to ensure too much current isn’t drawn from the cables which could cause damage to your PC.

16-pin PCIe 5.0 graphics card power connector

SATA

The SATA connector has largely replaced the older Molex connector in most modern PCs. It provides 12V, 5V and 3.3V to components such as HDDs and SSDs. Make sure you choose a power supply with enough SATA connectors for these components.

SATA power connector

Molex

What most people refer to as the Molex connector is technically the AMP MATE-N-LOK 1-480424-0. No wonder nobody calls it that anymore outside of datasheets!

The Molex connector dates all the way back to 1963 and provides 12V and 5V to older models of components such as HDDs and SSDs, fans and watercooling pumps. Make sure you choose a power supply with enough Molex connectors for these components.

Molex peripheral power connector

PSU Cabling

We advise avoiding budget PSUs where the cables of all possible connected components are soldered in place. This makes them difficult to install and you need to find a place for any unused cables. PSUs from mid-range upwards use a modular approach with numerous cable ports, so you only connect the cables required, making for less clutter and better airflow inside your PC.

Fully modular Corsair PSU with detachable cable sockets Detachable cable set supplied with a fully modular PSU

Fully Modular

Fully modular PSUs allow you to connect only the cables you need, resulting in a cleaner build and better airflow.

Semi-modular Corsair PSU with fixed and detachable cables

Semi Modular

Semi-modular PSUs have some fixed cables, like the motherboard power cable, but allow you to add other cables as needed.

Non-modular PSU with all power cables permanently attached

Non Modular

Non-modular PSUs have all cables permanently attached, which can lead to cable clutter and potentially impede airflow.

Comparison Fully Modular Semi Modular Non Modular
Pros Easy to install.
Best airflow and cooling inside your PC.
Customisable with different colour cables.		 Improved airflow and cooling inside your PC. Lowest cost.
Cons Highest cost. The captive cables are harder to install than detachable cables.
Medium cost.		 Captive cables make it harder to install.
Restricted airflow and reduced cooling inside your PC.

Modular PSU Cables

Cables for modular PSUs are sold in braided designs of many colours so you can personalise your system to match any case colours or RGB lighting effects you may want. It’s critical to only buy compatible cables, as each brand and model can use different connectors, pin-outs and cable resistance.

Black modular PSU cable kit
Black
Black and white modular PSU cable kit
Black and White
Red modular PSU cable kit
Red
White modular PSU cable kit
White

PSU Efficiency

We mentioned earlier that PSUs are more efficient when they run at medium load. The graph shows the efficiency curves of some typical PSUs. As you can see the majority of PSUs are most efficient at around 50% load, but less efficient at high loads and least efficient at very low loads. Low efficiency is a bad thing because it means the PSU is wasting power, making it both hotter and noisier, plus pushing up your electricity bill unnecessarily. The grade of PSU also has a bearing on how efficient it will be and this is further explored below.

Typical PSU efficiency curves at different power loads

PSU Grades

So how do you tell which is most efficient between two similar 750W PSUs? Well, there are two independent bodies that grade PSUs on their efficiency - Cybenetics and 80 PLUS - manufacturers then quote these standards so you know what you’re purchasing. The Cybenetics and 80 PLUS organisations test power supplies slightly differently - and arguably the Cybenetics approach is more thorough - however both standards report their results in a similar way, offering ratings such as Bronze, Silver, Gold, Platinum, Titanium or Diamond, in order of increasing efficiency. You can see how these compare in the tables below.

Cybenetics Power Supply Ratings

Rating Cybenetics Bronze efficiency rating Cybenetics Silver efficiency rating Cybenetics Gold efficiency rating Cybenetics Platinum efficiency rating Cybenetics Titanium efficiency rating Cybenetics Diamond efficiency rating
Overall Efficiency ≥84 to 87% ≥87 to 89% ≥89 to 91% ≥91 to 93% ≥93 to 95% ≥95%
Overall Power Factor ≥0.910 ≥0.920 ≥0.930 ≥0.935 ≥0.940 ≥0.950
Overall 5VSB Efficiency 70% 72% 74% 75% 76% 78%
Vampire Power 0.25W 0.23W 0.20W 0.18W 0.15W 0.12W

80 PLUS Power Supply Ratings

Load 80 PLUS Standard efficiency rating 80 PLUS Bronze efficiency rating 80 PLUS Silver efficiency rating 80 PLUS Gold efficiency rating 80 PLUS Platinum efficiency rating 80 PLUS Titanium efficiency rating
20% Load 82% 85% 87% 90% 92% 94%
50% Load 85% 88% 90% 92% 94% 96%
100% Load 82% 85% 87% 89% 90% 91%

PSU Noise

If low noise is important to you, it’s worth looking out for a PSU with intelligent fan control. These adjust the speed, and therefore noise, of the fan depending on the internal temperature of the PSU. Some PSUs can even switch off the fan entirely at low temperatures, making your PC even quieter; these are typically described as supporting ‘zero RPM fan mode’.

In addition, while not as widely adopted as its power efficiency rating, some brands do submit their PSUs to Cybenetics to be noise tested. Like the power efficiency rating, the noise ratings are available in several grades, as detailed in the table below.

Rating Cybenetics Standard noise rating Cybenetics Standard Plus noise rating Cybenetics Standard Double Plus noise rating Cybenetics A Minus noise rating Cybenetics A noise rating Cybenetics A Plus noise rating Cybenetics A Double Plus noise rating
Noise Requirement ≥40 – 45db(A) ≥35 – 40db(A) ≥30 – 35db(A) ≥25 – 30db(A) ≥20 – 25db(A) ≥15 – 20db(A) ≥15db(A)

Ready to buy?

SHOP NOW FOR PSUs

Frequently Asked Questions

Here are some common questions and answers to help you find the information you need.

PSU stands for Power Supply Unit, an internal component of every PC that converts the 230V AC power from the mains wall socket in your home or office to the various DC voltages the different components within your system require.

Every PC needs a PSU to power on. PSUs convert the 230V AC power from the mains wall socket in your home or office to the various DC voltages the different components within your system require.

Every PC needs a PSU to power on. PSUs convert the 230V AC power from the mains wall socket in your home or office to the various DC voltages the different components within your system require.

The best PSU for a gaming PC should be high efficiency (Cybenetics/80 PLUS Gold or better), low noise (Cybenetics A or better), both of which will help keep your PC cool, quiet and reduce your electricity bill. You should then choose a PSU with sufficient wattage for your components, most importantly the graphics card and CPU. See the ‘PSU Size’ section of this guide for further details.

Scan (4.8 on Trustpilot) sells reliable PSUs from leading brands such as Corsair, ASUS, be quiet! and Seasonic, all of which have high Cybenetics and 80 PLUS ratings.

To understand what wattage you are likely to need, you need to know the power draw requirements of each component of your PC or workstation. These figures can be found by looking at the technical specifications of each component. Once you have these, add them up and leave an overhead of around 30% - this is so the PSU will run with a medium load. A PSU constantly running at higher loads will make it less efficient - translating to wasted power and excessive noise.

The GPU or graphics card is likely to be the most power-hungry component in your system, so that is a good starting place. For example, an NVIDIA RTX 5090 GPU has a power draw of 575W, so a PSU of 1,000W or more is recommended.

The end of the PSU cable that connects to devices such as the motherboard, graphics card and drive follows defined industry standards (see the ‘PSU Connectors’ section of this guide). However, the other end of the cable that connects to the PSU itself is proprietary, differing not only between brands, but even between models. It’s therefore critical to buy only compatible cables certified for your specific PSU.