Power Supply Buyers Guide

Why the right power supply is important for your PC

The humble power supply is the unsung hero of your PC. It won’t make your PC run any faster, but the choosing the wrong power supply could make your PC crash or not power up at all in the first place. Read on to discover what to look out for when choosing a power supply for your PC.

What a power supply does

The power supply, also known as the PSU (power supply unit), 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 5V or 3.3V.

How big a power supply does my PC need?

Power supplies are rated by how much power, measured in watts, they output. This does not mean that the power supply will always its maximum power output, but can produce up to its maximum depending on the configuration of your PC and how hard it is working. For instance, your PC might only need 150W when you’re watching YouTube but could need 500W when you fire up a game.

As a general rule its best to choose a power supply with some power to spare. This is because power supplies work more efficiently at medium load levels. While every power supply has slightly different characteristics, the image below is representative of the typical efficiency curve of most power supplies. As you can see this power supply is most efficient at around 50% load, but less efficient at high loads and least efficient at very low loads. Lower efficiency is a bad thing because it means the power supply will be wasting power, making the power supply both hotter and noisier, plus pushing up your electricity bill.

As every PC component draws a different amount of power, it is important to choose a power supply that has sufficient power output for your PC. Many power supply manufacturers provide calculators on their websites, which will recommend a power supply, based on your PC configuration.

We recommend the OuterVision power supply calculator as it has both expert and basic modes and isn’t tied to a particular manufacturer.

You can also use this guide to point you in the right direction.

My PC has
1 CPU amd-ryzen 1 CPU amd-ryzen 1 CPU amd-ryzen 1 CPU amd-ryzen 1 CPU amd-ryzen
Integrated graphics 1 x Entry-level to Mid-range Graphics Card 1 x High-end Graphics Card 2 x Graphics Cards 4 x Graphics Cards
Recommended PSU wattage
Under 450W 450W or more 550W or more 850W or more 1600W or more

What a power supply does

Apart from the maximum power output there are several other characteristics to look out when choosing a new power supply.

Power supply form factor explained

PC power supplies follow an industry standard known as the ATX spec that defines the shape and size of the power supply as well its functionality. This means that the vast majority of ATX power supplies will fit in most ATX and micro-ATX cases. The only anomaly to this are some compact cases, which require a smaller power supply based on the SFX or TFX standards. To be on the safe side you should checking what size power supply your case supports before buying anything.<>
Below you can see the physical differences between the three most common types of power supply.




Power supply connectors explained

Once you know what form factor power supply you need the most important decision is making sure the power supply has the right connectors for 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. 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 PCI-E 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 PCI-E

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

8-pin / 6+2-pin PCI-E

The larger 8-pin PCI-E 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 PCI-E connectors for your graphics card(s).

Beware, despite having the same number of pins, the 8-pin PCI-E for graphics cards and 8-pin EPS12V for CPUs are not compatible with one another and have different pin-outs.

12-pin PCI-E

The special 12-pin connector provides 12V to select high-end graphics cards. Despite having 12-pins this new connector is approximately the same size as the 8-pin PCI-E, but can transmit more current, enabling more powerful graphics cards. As not all power supplies have this new 12-pin cable, you can get adaptors that convert two 8-pin PCI-E cables from your power supply to a single 12-pin PCI-E cable. Just make sure to use two separate 8-pin PCI-E 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, 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%
20% load 82% 85% 87% 89% 90% 94%

Power supply cooling explained

As power supplies are not 100% efficient they require some sort of cooling system to dissipate excess heat. While there are a handful of passively cooled power supplies available, the vast majority of power supplies are cooled by an embedded fan. This fan sucks cool air into the power supply, which then is forced out of a grille at the back.

As with all types of cooling, the larger the fan the better, as the fan doesn’t have to spin as fast as a smaller noisier fan to move the same amount of air. It’s also worth keeping an eye out for power supplies with intelligent fan control. These models adjust the speed of the cooling fan depending how hot the power supply is, keeping noise to a minimum.

The best PC power supplies

There you have it, everything you need to know when choosing a new PC power supply. We hope you've found this buyers guide helpful. Don't hesitate to contact one of our friendly advisors for more advice if you still have questions on how to select the perfect power supply. If you’re all set to go we recommend checking out these power supplies from these top brands..