Power Supply Buyers Guide

The humble Power Supply Unit (PSU) might not be the most frequently discussed component in your PC, but it's a critical element as provides power to every component. So perhaps, unsurprisingly, using the wrong one could be costly - as your system could crash or not power up at all in the first place.

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. Supplying the correct voltages to components will prolong their life and get 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 am overhead of about 40-50% - 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 below tables shows the recommended minimum PSU for the most popular GPUs.

Recommended PSU Wattage by GPU

NVIDIA GeForce RTX

1000W+ RTX 5090

850W+ RTX 4090, 5080

750W+ RTX 3070 Ti, 3080, 4080, 4080 SUPER, 5070 Ti

700W+ RTX 4070 Ti, 4070 Ti SUPER

650W+ RTX 3070, 4070, 4070 SUPER, 5070

600W+ RTX 3060 Ti

550W+ RTX 3050, 3060, 4060 Ti

AMD Radeon RX

800W+ RX 6900 XT, 6950 XT, 7900 XTX, 7900 GRE

750W+ RX 6800 XT, 7900 XT

700W+ RX 7700 XT, 7800 XT

650W+ RX 6700 XT, 6750 XT, 6800

600W+ RX 7600 XT

550W+ RX 7600

500W+ RX 6600 XT, 6650 XT

450W+ RX 6600

400W+ RX 6500 XT

350W+ RX 6400

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 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. The below diagrams show 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 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.

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

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

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

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

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.

PSU Efficiency

We mentioned earlier that PSUs are more efficient when they run at medium load. The graph to the right 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 how efficient it will be and this is further explored below.

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


Overall Efficiency	≥82 to 85%	≥85 to 87%	≥87 to 89%	≥89 to 91%	≥91 to 93%	≥93%
Overall Power Factor	≥0.950	≥0.960	≥0.970	≥0.975	≥0.980	≥0.985
Overall 5VSB Efficiency	71%	73%	75%	76%	77%	79%
Vampire Power	0.25W	0.22W	0.19W	0.16W	0.13W	0.10W