What Does a PSU Do and Why You Should Not Overlook It

What does a PSU do and why you should not overlook it

Power Supply Units (PSU) are probably one of the most overlooked computer parts. But you should not overlook it at all, because it is actually very important for your other components.
Power Supply Unit delivers power to all the components in your computer, which makes it the key component for your computer to even turn on. PSU is connected to the electrical outlet in your wall and it then converts the current to the voltages used by the different components. The Power Supply has different cables that output different voltages. Cheap PSU might end up frying all the other components in your computer.

Power supplies come in different price ranges, which is because they have different output wattages, cable managing properties, efficiency levels, sizes and some of them even come with different connectors. In this post, I will cover all the basics you need to know when selecting your power supply unit. If you are building your first computer, I suggest that you read all the other component overviews from this site. This ensures you have enough knowledge to understand all the functions of different components. I hope I can offer some valuable knowledge I have learned while working with computers.


First off, you will need a power supply that offers enough wattages for your computer. The wattages a PSU offers are usually marked very clearly in the product itself, in the packaging and in the PSU advertisements. That is because it’s one of the most important things to know about the power supply you are going to buy. This means that you need to know how much your other components need power.

The easiest way to do this is to use an online calculator like Cooler Master’s power supply calculator. There are many other PSU wattage calculators on the web for you to use, but in the end, they are all basically the same. These calculators will give you an idea what your computer’s load wattage is, and what is the recommended power supply wattage and they often give you even a power supply recommendation depending on what you filled in as your components.

PSU wattage calculator results

Power supply wattage calculator results


It is a good practice to pick power supply with more wattages than the actual need for a few reasons. If there are any power spikes, your computer won’t just shut down because there isn’t enough power. Also, if you are going to overclock, your power consumption will be higher and also future upgrades will be cheaper since you don’t have to buy new PSU every time you change an old component to a new one.

However, most new parts consume less power than old ones, but we tend to buy components with higher performance to upgrade our computers. Higher performance means higher power consumption. All in all, it’s a good idea to leave some room for changes and possible power spikes, especially if you are planning to overclock.

Input Voltage

Some power supplies, mostly the older ones, might have a switch on the back. The switch has two positions, 115V or 230V. This is because the power outputs are different in different countries. For example, in the US the outlet delivers 115V and in EU countries it delivers 230V. If you don’t know what the power output in your country is, you can check it from this website or ask a neighbor.

What this means, is that if you live in the US (or anywhere the output is near 115V), you have to switch the PSU to 115V and if you live in EU (or anywhere the output is near 230V), you have to switch it to 230V.

Now the important part, if the power supply doesn’t have a switch on the back, it doesn’t mean that it can automatically use any voltage. You have to actually make sure that the specific PSU is capable of using the output you happen to have. You can check it from the manual or other documentation about the PSU. It might be around 115V or 230V and can only use voltages near that.

However, most of the new power supplies can use any voltages from ~90V to ~260V. Check what the advertisers say about the voltages it can use before buying the new power supply. Otherwise, you might end up burning your computer or even your house.


Power Supply Units come with a few different cables. The different cables might first seem very complicated, especially if you haven’t built a computer before. Don’t worry, as confusing it first might be, they are actually very simple and easy to understand.

We won’t go in depth of the wattages or anything specific like that in this post, I will cover only what cable goes into what slot and why. After you know that, you can dig deeper and learn more about the cables.

Fully modular PSU

Fully modular PSU connector slots


PC Main or ATX Connector

The main cable is a 24-pin power cable for the motherboard. This cable simply gives power to the motherboard, it is the biggest cable and it is plugged into the biggest connector in the motherboard. It is easy to locate, it’s on the side of the motherboard. Most motherboards have 24-pin connector in them, but some might have a 20-pin connector. Therefore, the motherboard power cable that comes with the PSU is usually 20+4-pin connector cable, so it will fit both 24- and 20-pin connectors.

P4 or EPS Connector

The next cable provides the processor with the power it needs since the motherboard can’t provide the CPU enough power anymore, it is done with an external cable straight from the PSU. However, this cable is plugged into the motherboard, near the CPU socket. You can usually see a mark like “CPU” on the motherboard pointing out that it’s the slot for the CPU power cable.

In high-end motherboards that allow overclocking, the cable is usually 8-pin, so that it’s capable of supplying enough power. Normal, cheaper motherboards have commonly just a 4-pin connector. Power supply units might have both 8-pin and 4-pin cables or more commonly just an 8-pin cable that can be split if needed.

PCI-e Connectors

PCI-e cables provide power to devices connected to the motherboards PCI-e expansion slots. This is because the PCI-e interface can’t supply enough power to some devices, mostly graphics cards. These cables are connected straight into the device, not in the motherboard as the cables talked before. The number of pins needed in the PCI-e cable varies on the device used. High-end graphics cards need more power, more power can be supplied with more pins, simple.

PCI-e cables used are 6-pin, 6+2-pin, and 8-pin cables. Low-end graphics cards only need the 6-pin connector and some high-end, enthusiast level graphics cards might need two 8-pin connectors to have enough power. Most commonly the middle-class and graphics cards need 8-pin power cable to have enough power. Usually, it is done with a 6+2-pin cable. Make sure what your graphics card needs before plugging any cables in, just to be sure.

SATA Power Connector

SATA Power Connectors supplies power to the storage drives, SSD and HDD. New Hard Disk Drives use SATA cables now, which makes power supply cabling a lot simpler since you probably only need to use SATA power cables. SATA power cable is L-shaped and can only be connected one way, so you don’t need to worry about plugging it the wrong way. It is similar to SATA data cable, the data cables L-shaped head is just shorter. SATA power cable is plugged straight into the storage device.


Molex is an old cable type, it is now pretty much obsolete and you don’t probably need to know about it since SATA cables have replaced them. Power supply units usually still have space for them since some people could still be using Molex.

Adapter cables

There are a number of different adapters when it comes to power supply units. Some of them might help you if you need to plug, for example, new HDD which uses SATA Power cable to a non-modular old Power Supply Unit which uses Molex power cable for storage drives. Just google the adapter you need and you probably will find one.


Power supply units can be modular, semi-modular or non-modular. What this means is simply that the power supply units have cables that can be removed or not. Fully modular power supplies are usually referred to just as modular power supplies. The benefit of having a fully modular PSU is that all the cables can be removed. If you don’t need all of them, your computer will have fewer cables laying around and all the cables are easier to manage.

Semi-modular means that some of the cables, commonly peripheral ones are removable. This offers basically the same benefit as the fully modular PSU. Semi-modular power supplies can be harder to find, but they are usually cheaper than fully modular. Non-modular power supplies have all the cables connected and you are not able to remove any of them without breaking the PSU.

Semi-modular PSU

Semi-modular power supply unit


The non-modular power supplies very easy to use since you don’t need to worry about connecting any of the cables yourself into the PSU. However, non-modular power supplies are not that pretty to look at, especially if you have a tempered glass side on your computer case and it shows the PSU and some cables.

With fully or semi-modular power supply units you are able to remove the excess cables and change some or all of them to some custom colored ones, this is something to think keep in mind if you want to have some specific color theme or custom style in your PC.


Power efficiency might not sound something you care much about when it comes to building your computer, but if you do pay your electric bills yourself, this gets from boring to interesting very quickly. If you don’t pay your bills yourself, it’s still a good practice to save electricity wherever possible. There is actually a Climate Savers Computing Initiative whom the goal is to reduce greenhouse gas and save energy.

80 Plus certification levels

80 Plus certification levels


The efficiency of power supplies is advertised as 80 Plus certification program, it’s a voluntary program, so not all power supplies might have any certification label on them, but many do. The program promotes efficient energy use and it has different levels of certifications.

The power supplies are tested under different workloads and the efficiency is then recorded. The different levels of certifications mean different levels of power efficiency. See the chart below for more details about the different certifications and efficiency levels under different workloads.


To fully understand the chart, you need to know a few things first. Basically, the higher the percentage in the chart, the better the result. If you have 115V outlets and you have 80 Plus Gold certified PSU, it means that under 20% workload it will be able to use at least 87% of the power fed to it and only waste at most 13% as heat. Under 50% workload it can use at least 90% of the power and under 100% workload, it can use at least 87%.

Depending on what the outlet voltage in your country is you can check the chart results of different 80 Plus certification levels when comparing power supplies. The most common 80 Plus certification is Gold and it offers good efficiency levels with both 115V and 230V.

There have been cases where the product wasn’t actually certified but advertised as a certified product, and some of the products didn’t even meet the requirements for the certification. If you want to make sure the power supply unit you are going to buy is actually certified and thus also fills the requirements, you can do so. Just go on the 80 Plus programs website and check that the power supply is listed there, with the correct certification level. This is basically the only way you can be sure the PSU is as efficient as in the advertisements.


Power supplies come in different sizes, just like computer cases. It is pretty easy to understand that the PSU needs to fit inside the computer case. The PSU form-factors and sizes are listed in the chart below. The names are usually displayed without the 12V at the end.


The most common sizes for consumer power supplies are ATX and ATX Large. They share the same height and length which makes them both fit most of the cases, but it’s a good idea to play it safe and check what is the depth reserved for the PSU in your computer case, you can find this information from the case’s manual.

The other power supply sizes aren’t that common for consumers and you don’t need to worry about them. Just make sure the power supply you are buying is ATX or ATX Large and the depth of the case is enough for it and you are good to go.


The common way of putting the PSU in the slot it belongs is to put it the fan pointing up. This way the fan will blow the warmer air from inside the case outside of the case, which is a good thing. It helps your other components keep lower temperatures and you don’t need to invest in additional fans to have enough air flowing through the computer.

There is one scenario you might want to place PSU so that the fan is facing the bottom of the case. It’s not a problem since often the bottoms of the cases have a lot of holes in them. So the power supply fan would be able to get air from outside of the case and blow it out as well. Why would you want this because the fan up -option cools your case at the same time.

Well, if you are using water-cooling you might want to have the PSU fan facing the floor. This is simply because if there happen to be a leak in your water loop, it might get into the PSU and fry all your components. If the fan is facing down the water can’t get in the PSU at all and you in worse scenario lose only one or two components.


There is often a list of abbreviations of safety methods used in the power supplies, but no explanation what they actually mean and protect us from. I will open here the meaning and explain quickly what they are for. I will go through SCP, OPP, OCP, OVP, and UVP.


  • SCP (Short-Circuit Protection)
    In the case of a short-circuit, this feature prevents damage to the core components of the PSU and its system components.
  • OPP (Overload Protection)
    If the system is oversized and requires more power from the PSU than it can perform, this protection function is activated.
  • OCP (Overcurrent Protection)
    If the load on a single line is higher than indicated, the PSU automatically switches off.
  • OVP (Overvoltage Protection)
    If the voltages increase above a certain tolerance value on the single lines, the PSU automatically switches off.
  • UVP (Undervoltage Protection)
    If the voltages fall below a certain tolerance value on the single lines, the PSU automatically switches off.


Now that they are explained, it seems like a reasonable idea to invest in some PSU that can actually save all the other components when something goes wrong with the power. This can save you a lot of money and time depending on your build.



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PC Rookies is a project to share information related to mostly security related topics.

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