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.

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
Recommended PSU	350W+	400W+	450W+	500W+	550W+	600W+	650W+	700W+	750W+	800W+	850W+

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.

ATX

SFX

TFX

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.

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.

BROWSE THE RANGE OF BRAIDED POWER SUPPLY CABLES

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 each power supply compares using two independent standards, Cybenetics and 80 PLUS, that different manufacturers quote in their product descriptions.

While Cybenetics and 80 PLUS test power supplies slightly differently, and arguably the Cybenetics standard is more thorough, both standards report their results in a similar way. They rate power supplies as Bronze, Silver, Gold, Platinum, Titanium or Diamond, in order of increasing efficiency. You can see how the different Cybenetics and 80 PLUS standards compare in the tables below.

Cybenetics Power Supply Ratings

	Bronze	Silver	Gold	Platinum	Titanium	Diamond

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