Power Supply Buyers Guide

Why the PSU is important

The humble power supply unit (PSU) is the unsung hero of your PC. It won’t make your PC run any faster, but choosing the wrong PSU could make your PC crash or not power up at all in the first place. The PSU converts the 230V AC from the mains wall socket in your home or office to the various DC voltages the different components in your PC requires. The most important of these voltages is 12V, while some legacy components still require 5V or 3.3V.

You can break down picking a PSU into five principle decisions:

1. How big a PSU do I need? >
2. Form factor >
3. Connectors >
4. Cables >
5. Efficiency >

Read on to discover how to answer those questions when choosing a PSU for your PC.

How big a PSU do I need?

The first and most important decision to get right 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.

As a general rule its best to choose a PSU with some power to spare. This is because PSUs work more efficiently at medium load levels. The image below shows the efficiency curve of a typical PSU. As you can see the PSU is 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 graphics card is by far and away the most power-hungry components in a PC, so you can use the table below as a guide to what size PSU is right for you. Simply read across the columns till you find the graphics card you own or intend to buy. Don’t forget you also need to leave plenty of headroom for the CPU and other components in the PC too.

Recommended PSU 350W+ 400W+ 450W+ 500W+ 550W+ 600W+ 650W+ 700W+ 750W+ 800W+ 850W+
NVIDIA Geforce Graphics Card 3050, 3060, 4060 Ti 3060 Ti 3070, 4070 4070 Ti 3070 Ti, 3080, 4080 4090
AMD Radeon Graphics Card 6400 6500 XT 6600 6600 XT, 6650 XT 7600 6700 XT, 6750 XT, 6800 6800 XT, 7900 XT 6900 XT, 6950 XT, 7900 XT

PSU Form Factor

The second decision you need to make is picking the form factor, to ensure that your PSU and 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 its functionality. This means that the vast majority of ATX PSUs will fit in most ATX and micro-ATX cases. The only anomaly to this are 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.

Below you can see the physical differences between the three most common types of PSU.




PSU Connectors

Your third decision is to make sure the PSU has the right connectors for the components inside your PCC. 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.

4-pin ATX12V

The 4-pin EPS12V connector provides 12V to the CPU via the motherboard. The smaller 4-pin version is used by entry-level CPUs.

8-pin EPS12V

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.

6-pin PCIe

The 6-pin PCIe connector provides 12V to mid-range graphics cards. Make sure you choose a power supply with sufficient 6-pin PCIe connectors for your graphics card(s).

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.

16-pin PCIe 5.0 12VHPWR

The special 16-pin PCIe 5.0 12VHPWR connector provides 12V to select high-end graphics cards. The main bank of 12 pins provides power while the four smaller pins in the row above are used by the GPU to communicate with the PSU about how much power it needs. Despite having 16-pins the new connector is approximately the same size as the old 8-pin PCIe, but can transmit more current, enabling more powerful graphics cards.

Only ATX 3.0 power supplies have this new 16-pin PCIe 5.0 cable, but you can get adaptors that convert two or more 8-pin PCIe cables from an ATX 2.0 power supply to a single 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.


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.


The SATA connector has largely replaced the older Molex connector (see left) 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.

Power supply cables explained

While the cables of budget power supplies are soldered in place, many mid-range and high-end power supplies feature modular cables. These modular cables have connectors at both ends, one for the power supply itself and the other for the component it will power, such as the CPU, graphics card or HDD/SSD.

This is beneficial because with a non-modular power supply (pictured) you need to find space inside your PC for the mess of unused cables.

In contrast with a modular power supply insert, you only need to plug in the cables your PC actually needs. This frees up space inside your PC, making it run cooler.

Speaking of visual appeal some power supply manufacturers produce replacement modular cables with individually braided wires in different colours. You can also purchase cable extenders if you have a particularly large case

Why settle for plain black cables when there is a wide range of different colour cables to choose from to complement your PC’s colour scheme.


Power supply efficiency explained

As already explained, it’s worth choosing a power supply with a higher wattage than your PC actually needs as this can make the power supply run more efficiently. You can see how the efficiency of different power supplies compare from their 80 PLUS rating which sets minimum efficiency standards at three different loads, 20%, 50% and 100%. There are currently six different rating levels from the least efficient 80 PLUS rating to the most efficient 80 PLUS Titanium rating as detailed below.

Power supply load 80 PLUS 80 PLUS Bronze 80 PLUS Silver 80 PLUS Gold 80 PLUS Platinum 80 PLUS Titanium
20% load 82% 85% 87% 90% 92% 94%
50% load 85% 88% 90% 92% 94% 96%
100% load 82% 85% 87% 89% 90% 91%

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