PSU: The beginners guide to building. part 1

The Power Supply Unit, or PSU for short.

a computer power supply from inside a desktop or tower PC case.

Most beginner guides will start with “AMD or Intel?” or “AMD or Nvidia?” or maybe “How much RAM?” But not us, we are going to start with possibly the most overlooked part of the PC. The humble Power Supply Unit or PSU for short.

It is often overlooked as most people tend to budget in the processor, RAM, Graphics card etc. The PSU tends to be the last thing most people look for and by this point the budget has either run out or not looking very good any more.

The downside to this is, alot of people are buying cheap, generic, no brand or unknown brand PSU’s. Honestly, don’t do that. Set aside a budget for the PSU for reasons I will get to, but most experienced system builders will agree, “Don’t cheap out on your PSU.”

Why shouldnt people go cheap on the PSU?

Consider this, you have just built your brand new PC. You have have just spent lots and lots of money on the various bits. You have more RGB that the local airport is concerned you are rerouting aircraft and all of this is powered by the PSU.

It is the job of the PSU to supply and maintain power to all your components, everything in your new shiny box is connected to the PSU somehow or another. If the PSU fails it could spell disaster for the rest of the system, a simple surge could fry parts of your new build and render them useless. If it isnt properly regulated it could be sending too much or not enough current to the various components inside your system and if something shorts out, the PSU may not have any short or surge protection to project itself.

What PSU should I go for then?

First thing you are going to need to know is what wattage you are going to need. PSU’s can start say way down at 200w and go all the to 1000w range. There is a handy website that can calculate what PSU wattage your system will need:

Power Supply Calculator

Click the link above and fill in the form as best you can with all the parts that is going to be inside your lovely new system. It will then figure out what PSU you need and will also recommend a couple for you to take a look at.

After you have worked out what wattage you need, the next thing most people will look at is: Certification.


What is certification? It was originally founded in 2004 by Ecos Consulting (Now Ecova). It is voluntary for PSU manufacturers to get their units certified but is a serious recommendation when buying one.

In a nutshell, its how well a unit can hold at least 80%+ output, this chart might give you an idea:

For more indepth reading on the subject try here on wikipedia.

Modular or Non-Modular?

If you look at any regular PSU you will notice all the cables coming out the box, these are to connect your parts up inside the system.

Non-Modular will have all the cables hard wired into the box. The upside to this is that they tend to be cheaper and generally all the cables you need will all be there (we’ll cover the cables in a bit). The downside however is that you might end up with a lot of un-used cables that you need to hide and could become a cable mangement nightmare, unless you don’t mind having a load of cables dangling free inside the system of course.

Modular is where you connect the cables you need to the unit. You have full modular where there are no cables connected and semi-modular which only has the Motherboard and CPU power cables hardwired, which does make sense as you will always need those two regardless of the system you are building.

Instead what you have is a lot of loose cables in the box and there will be various 3 / 6 and 8 pin plugs on the side of the PSU. The upside to this is you only plug in what you use, so if you only have 1 SATA SSD for example, you need one SATA power cable, you wont have say another 4 just hanging around not doing anything, so if your after a tidy case and easy cable management, Modular is a good step. The downside however is they tend to cost more than a non-modular.

modular PSU

The picture above is from a Corsair fully modular PSU. You can see that you have different sizes of plugs, this is quite important. You can see the “Peripheral & SATA” are all 6 pin and the 6+2 PCIe and 4+4 CPU are all 8 pin and so on. Basically don’t try and plug a 6 pin into the 8 pin. The end result might not be pleasant.

They are all working on different outputs, currents and ground might be different, you could damage your PC if you plug the wrong cable into the wrong plug, althought the 8 pin will never fit into the 6 pin, the 6 could slip into the 8 if your not paying attention.

You mentioned cables?

Yes, yes I did. Modular or Non-Modular, there are a bunch of cables you are going to have to deal with.

  • 24 Pin ATX This is the “big one” (as in size) this provides power directly to the motherboard. This and the CPU one are required to even power on.
  • 4+4 CPU Provides a direct current for the processor, like the 24 Pin is required to just power up. The reason its 4+4 is the plug itself can be split in half from 8 pin to 4 pin. The reason is because of the power requirements that the processor needs, budget boards and CPU’s on the lower end, only really need the 4 pin, mid range and higher require more oompth so need the 8 pin.
  • 6+2 PCIe This is for the Graphics Card (or GPU for short) mostly mid to high end GPU’s draw more power than the Motherboard can give so you’re to provide your GPU with more power. Like the CPU one, it can be split from 8 pin to a 6 and a 2 pin (most GPU’s will be 8 or 6). Some will require 2 connections if its a high end beefy card, you might have to provide either 2 8pin or some have 1 8pin and 1 6pin, if your card requires two plugs, it is recommended you don’t use a “pony tail” (where one cable has two connections) use 1 per cable.
  • Peripheral and SATA / 6 Pin This is for everything else, mostly your harddrives (be it either HDD or SSD), Optical Drives (if for some reason your still using one) and anything else that requires a SATA power plug (for example, a Corsair H100 will use one for extra power).
  • Universal Power Lead The big black cable, this is the simplest and possibly most important. You need this to plug your PC into your power outlet at home.

Wrapping up.

So you worked out your output, figured what 80+ certificate you think is best (to be honest, aim for Gold or higher), you have figured out what needs power and if modular or non modular is good for you, now you have to buy it.

Shop around, read reviews, use Google or another search engine and look into your possible choosen PSU.

Branded PSU are usually more reliable than unbranded, there are a few names that a generally a safe bet (EVGA, Corsair, SeaSonic to name but a few).

Don’t be tempted by something that might look a better deal, for example; you have just seen a SeaSonic 650w on Amazon for about £100 but then you see a X-Series 700w for £22. Telling you now and I am sure the experienced system builder is behind me on this one, go for the SeaSonic, it will be worth the price and your system will be thankful for it.

Don’t cheap out.