PC power supply units (PSUs) are often overlooked when people put a parts list together. It might seem like an easy decision to save some money on your PSU and spend that cash on the more high-profile components, like CPUs and graphics cards. However, the PSU is a key part of any system and getting it right will increase your system's long term reliability and performance.

So what do you need to consider when choosing a PSU?

How much power do you need?

You need to understand how much power your system need to operate at its full capacity. Once you have this figure (in Watts) you should buy a PSU which covers this, plus at least 25% more, but preferably 50% if your budget allows. This extra leave room for overclocking (see CPU guide) and potential future upgrades, say, to a more powerful graphics card. It also means your PSU is operating at 50% load, which is when they are most energy efficient.

There are plenty of tools out there to caclulcate your power needs, but my personal favourite is included in the PC Part Picker system builder. Simply add all your components and it will tell you a very precise power draw figure.

Using the figure given, add 25%, then round up to the nearest 50. For example:

Estimated Wattage of 433W + 50% (216.5W) = 649.5W.

For this, I would choose at least a 650W power supply.​​

Check your case dimensions.

For the majority of people building a PC, the PSU needed will be a standard ATX size PS2. These will fit in all standard desktop PC towers.

If you are building a small form factor system, a PS2 power supply will be too chunky. For this, you will need a SFX unit. These are smaller in all dimensions, leaving room in your tiny case for other vital components.

Efficiency is a key factor.

The general rule is, the more efficient your PSU the better. It signifies a few things that give you peace of mind: higher build quality, better components, and quieter operation due as they generate less heat, thus reducing the fan noise.

The efficiency of a PSU is shown by an "80 plus" rating. If a PSU is certified 80 plus, it means that 80% of the power it generates is available to your system whilst the other 20% is wasted as heat.

There are several grades within the 80 Plus certification:

  • 80 Plus
  • 80 Plus Bronze
  • 80 Plus Silver
  • 80 Plus Gold
  • 80 Plus Platinum
  • 80 Plus Titanium

The higher up this scale you go, the better the efficiency, which comes at a premium. For most users, 80 Plus Bronze is perfectly acceptable.


So we've narrowed it down, but there are still more considerations.

Firstly, does the power supply have all the right cables and fittings to power all the components in your system? Here are the cables to look out for:

ATX (20+4 or 20) - This cable provides the power to your motherboard. Some older PSUs only have a 20 pin connector, which would be unsuitable for all modern motherboards which use the 20+4 pin connection. A 20+4 pin can be use in a 20 pin motherboard though, as the +4 can be detached from the larger grouping of 20.

EPS (8 pin or 4 pin) - These are used to power your CPU. Low end motherboard only need the 4 pin, whereas high end motherboards can utilise an 8 pin and a 4 pin at the same time. This is for additional power when overclocking. Check your motherboard spec to find out what you need.

PCI-E (6 or 6+2) - These cable were introduced to provide extra power to graphics cards, when they became too big to be supported by the motherboard alone, which provides 75W. A 6 pin PCI-E provides a further 75W, whilst a 6+2 pin provides 150W. Check how many connections are needed by your graphics card.

SATA - Sata connectors provide power to HDDs, SSDs and disk drives. Most power supplies come with plenty of these, so unless you plan on having a crazy number of storage devices, you should be fine.

MOLEX - Usually on the same power lead as SATA, Molex cables are old tech. They used to do all the things that SATA connections do, but are now reduced to powering cheap case fans. These won't really be a consideration in any modern PC build.

Fully wired?

You'll also need to decide if you want all the cables already attached to the PSU, or if you want to be able to choose which cables you need. You have three choices:

Fully wired - as the name suggests, all the power cables are hard wired into the unit. The advantage of this is there is less resistance as the connections are soldered directly to the circuit board, but the benefit is minimal. They are also cheaper than the other options. It does mean that you will have unused cables in your system, which can make cable management tricky.

Semi-modular - this is the middle ground. Only the essential cables are hard-wired into the PSU (ATX and EPS). All other cables need to be connected by the builder. In my opinion, this is the sweet spot for pricing and ease of cable management.

Fully-modular - the most expensive of the three options. Every cable has to be manually connected to the PSU. The main advantage of this, over a semi-modular version, is it makes fitting the ATX and EPS connections very easy.

Don't buy the cheapest PSU you can find.

Last, but certainly not least; DO NOT BUY A CHEAP PSU!

A cheap power supply is cheap for a reason; poor build quality, cheaper internal components and fewer feature like heat sinks and large fans.

A PSU should feel heavy. If you buy one and it is light, send it back. Stick to reputable, big name manufacturers (Corsair, SeaSonic, BeQuiet, to name a few).

I have repaired many a PC that has been let down buy a cheap PSU. At best, you'll need to replace the unit alone, but at worst, they can fry your system.

Still unsure?

If, after reading all that, you are still unsure about what is best, get in touch with us here at MC, and we will do our best to point you in the right direction.