What is a PSU (Power Supply Unit)? Explained!

The power supply unit, or PSU, is an important hardware component that powers your computer system. For users building their own PCs, they tend to place a lot of focus on flashy and visible components while overlooking the PSU despite its importance. Picking the right power supply for your PC is a crucial task. With so many terminologies surrounding it, buyers probably find it difficult to understand the different kinds of power supplies available in the market, their form factors, including the power connectors for various system components, and the confusing 80 Plus PSU quality ratings. In this guide, we will explain everything related to a PC’s power supply unit, so read on.

What is PSU: Overview & History

As the name implies, your PC’s power supply unit (PSU) is responsible for providing electricity to the various system components. Until the ATX standard of power supplies and motherboards came out in 1995, power supplies did not have a standardized design or form factor. As a result, there were many different kinds of power supplies with no universal design, like AT, Baby AT, and more. Intel patented the ATX standard for motherboards and power supplies in 1995. By 1999, the older Baby AT/AT power supplies became obsolete.

The ATX standard is what is being used today in the form of new revisions like the brand new ATX 3.0 power supplies, which include the 16-pin PCIe Gen 5 12VHPWR connector for graphics cards like Nvidia’s RTX 4090 and RTX 3090 Ti. There are also derivatives based on the ATX standard, like the SFX power supplies, which are popular for use with mini ITX motherboards.

Older computers used to have linear power supplies, but every PC today employs a switched-mode power supply, also referred to as SMPS. Why did the design shift happen? Linear power supplies were considerably inefficient, usually under 40% efficiency, while being much larger and heavier. Switched mode power supplies (SMPS), on the other hand, were more complex but still preferable for computer systems moving forward. The overall footprint was much smaller and the efficiency was more than double; two factors that quickly helped SMPS become the standard for computer PSUs.

internals of an old computer power supply
An old computer’s PSU (IBM XT), Image Courtesy: Wikimedia Commons

With the history out of the way, you might have questions like — why do we even need a PSU (power supply unit) instead of plugging in the PC directly to the wall socket? Maybe, you are curious to learn about power supply efficiency ratings. Well, we have detailed that and more, so keep reading.

How Does a Power Supply (PSU) Work

First, let’s understand why we need a power supply. Desktops, laptops, phones, and other electronics simply can’t function using the kind of electricity that we usually get from the wall socket, which is AC power. Electronics require Direct Current (DC) instead of Alternating Current (AC) to function properly. AC power is what you get from wall sockets around the world, so you need a go-between to convert AC power to DC power. Why DC power, though?

DC offers the advantage of outputting a constant voltage. AC doesn’t do this, so electronics can’t operate or charge without DC current. PC power supplies are responsible for this conversion of AC power from the wall socket to DC power, which can then be used to run your computer. This DC power is distributed to the various system components of your computer. This includes your CPU, graphics card, motherboard, RAM, etc.

dc vs ac current
Image Courtesy: Matsusada Precision

Essentially, AC/DC power adapters, like the ones used in laptops & phone adapters, are all technically power supplies. So essentially, the hardware inside these adapters is similar to a computer PSU’s internal circuitry, but in a very different design and size footprint. We will be focusing on desktop power supplies in this guide. To understand how the power supply unit of a PC works, we will take it from the top. Let’s see what happens when you turn on your PC:

  • You press the power button on your PC, which sends a signal to the motherboard of your computer.
  • The motherboard then triggers the power supply to turn itself on.
  • The PSU starts to pull AC power from the wall socket. It is connected with a power cord, provided with the PSU itself, or included with the computer system.
    • Most power supplies today have an Active PFC feature, with which the input AC voltage can be detected automatically. There is a standardized input AC voltage in the range of 100V to 240V used by countries around the world.
    • Older power supplies did not have an Active PFC feature. They used to have a switch for the PSU’s input voltage, commonly set in either 110-115V or 220-230V mode.
  • The inside of the power supply contains various components such as transformers, capacitors, and rectifiers. All of these work together to convert the AC power to a stable output of DC power with a constant voltage, required for computers to work properly.
    • Transformers are used for converting the AC power to a lower voltage that is suitable for the computer.
    • Rectifiers are for converting the AC power to DC power.
    • Capacitors are used for filtering & smoothening out the converted DC voltage to remove any ‘AC Ripple’, which is the residual AC current & voltage referred to as noise, which is still lingering in the DC output, previously rectified.
  • The clean DC power which is regulated & filtered is then sent to the output rail, which is made up of electrically conductive material that stores the DC voltage for the purposes of distribution.
    • The output rail could be a multi-rail or single-rail system. The simple difference between single-rail and multi-rail power supplies is that various power cables of your PSU can pull electricity from more than one-rail, if it is a multi-rail power supply. Otherwise, all the power will be provided by a single dedicated rail.
    • Multi-rail power supplies are not by many anymore. Single-rail power supplies are widely used in consumer-grade PCs, including many high-end PCs too. Multi-rail PSUs are usually intended for workstations and servers today.
  • Finally, the power supply component cables, such as your 24-pin connector, 8-pin PCIe cable, MOLEX connector, SATA power cable, and more, pull the electricity from the PSU’s rail, which can distribute the power in 12V/ 5V/ 3.3V form, as needed by your computer components.
  • This entire process happens instantaneously, after which you can see your PC turn on and boot into Windows.

There you have it; a complete explanation of how the power supply (PSU) of your PC works to deliver power to your system components. PSUs today also support features such as overvoltage protection, over-temperature protection, short-circuit protection, and more. Every power supply has a different feature set, efficiency rating, and max wattage rating. Power supplies available in the market today cater to users of different budgets & wattage requirements. That said, let’s talk about some common PSU features, including the connector types, PSU types, and more.

What Do Different Power Supply Connectors Do?

The power supply unit supports several different connectors, which are plugged into various locations on your PC’s motherboard. Other hardware components like the processor, graphics card, and hard drive also require dedicated power connectors. We now explain what each PSU connector is used for:

(20+4)-Pin Motherboard Connector

The 24-pin connector on the PSU plugs into your motherboard, supplying power to components, including the PCIe slots, M.2 slots, RAM, and other places. It is one of the main connectors for supplying power to your PC’s motherboard. Still, your PC requires several dedicated connectors for other hardware. This cable is the biggest and easily identifiable. It is actually designed in a 20+4 pin configuration, so it can be split if the motherboard only requires a 20-pin connection. Although, most PCs today require the entire 24-pin connector to be plugged in.

24 pin motherboard connector
24-pin ATX power connector for motherboard

(4+4)-Pin CPU Power Connector

The CPU power connector directly provides power to your processor. It is designed in a 4+4 pin configuration, which can be split. Most motherboards today require a full 8-pin connection. Still, many older or low-end motherboards with entry-level VRMs require only a 4-pin connector. Also, there are boards that will need a single 8-pin and another 4-pin for additional power.

8 pin cpu power connector
Dual 8-pin (4+4) CPU power connectors

Moreover, certain processors with an extremely high TDP, like the Intel Core i9-13900K or the Ryzen 9 CPUs in AMD’s latest 7000 Series lineup, require multiple 8-pin CPU power connectors.

(6+2)-Pin PCIe Power Connector

PCIe Power Connectors are connected to the graphics card and directly supply power to the GPU of your PC. The PCIe slot also supplies up to 75W to the GPU, but a majority of the power is supplied through this connector. Many high-end GPUs these days use multiple 8-pin PCIe power connectors. This cable is designed with a 6+2 pin configuration, so it can be split depending on your GPU’s needs.

8 pin pcie power connector
Three 8-pin PCIe power connectors installed on RTX 3070 Ti

Also, PCIe power connectors look similar to the CPU power connectors that we discussed earlier. But, most PSUs come with a manual, and they also label the power connectors for your ease. It’s not that hard to differentiate the two if you look closely.

PCIe Gen 5 Power Cable (12+4 Pin)

This is a new power connector and is only found on the new ATX 3.0 power supplies. This 16-pin power connector is also installed onto the graphics card (GPU) of your PC. Another name for this connector is PCIe Gen 5 Power Cable. It is designed with a 12+4 pin configuration, which is split or combined, depending on the power requirements of the graphics card.

atx 3.0 power supply pcie gen 5 16 pin power connector
16 Pin Power Connector, Image Courtesy: NVIDIA

For now, it is only found on certain Nvidia RTX 30-series and 40-series Founders Edition GPUs. Nvidia also includes an adapter with these cards, so that users who don’t get this cable with their PSU can still use the GPU on their PC.

SATA Power & MOLEX Connector (4-Pin)

The SATA Power Cable is usually used for providing power to your hard-disk drives (HDDs) or SATA-based solid-state drives (SSDs). The MOLEX 4-pin connector is used for fans and other peripherals but is also commonly used for adapters to convert the power to a different form such as for plugging it into SATA devices, GPUs, and CPUs. However, users are advised against using adapters for their CPU or GPU.

Power Supply Ratings: What Does 80 Plus PSU Rating Mean?

As we had explained before, power supplies convert AC power received from the wall socket to DC power — suitable for PCs to operate. But in the end, the conversion is not entirely 100% efficient. Some of this power is wasted and given off as heat. The 80 Plus Rating system for a power supply indicates how power efficient the PSU is. Power supplies with an 80 Plus Gold or Platinum rating are more efficient than the ones rated at 80 Plus Silver or Bronze.

80 plus certifications
Different 80 Plus Certifications

Generally, the internals are of higher quality in a PSU with a higher 80 Plus rating. Also, the more efficient your PSU is, the better will be your energy savings. This is because less electrical wattage would be wasted in the form of heat produced. So, investing in a good quality power supply is always recommended. You shouldn’t skimp out on this important component, because when you upgrade your PC in the future, you can keep the same high-quality PSU and use the money to get yourself a brand-new graphics card or processor. Moreover, you will continue to have a lower power bill.

Power Supply Types: Non Modular vs Modular PSU Explained

When shopping for your PC build components, you might have noticed a few different types of power supplies available in the market. So now, you must be wondering — what is a modular power supply? Basically, in modular PSUs, you can remove component cables from the power supply unit itself, and install them one by one as needed by your system components. The ones that are left unused can be put away in the box to be used in the future. Along the same lines, we have explained the difference between non-modular, semi-modular, and fully-modular PSUs available in the market. Let’s check them out:

  • Non-Modular Power Supplies are PSUs with every component cable attached to them, and they can’t be removed. After you finish building your PC, the leftover cables have to be cable-managed and set aside inside the PC case.
  • Semi-Modular Power Supplies feature a few removable cables. So, only some of the component cables can be pulled apart and set aside or attached outside the PC before plugging them in.
  • Fully-Modular Power Supplies support the removal of every component cable, including the 24-pin connector. But don’t we need these to run the PC? What is the point of having removable connectors? Well, you can remove all the component cables from the PSU if you ever need to submit your PSU for service under warranty. And you can plug them back in when the new or repaired unit arrives. Also, you can do better cable management with modular power supplies compared to their non-modular counterparts.
fully modular computer power supply
Example of Fully-Modular Power Supply

Power Supply Form Factors: ATX, TFX, SFX PSUs Explained

Power supply units (PSUs) are available in various different form factors, all being a derivative based on the original ATX standard set by Intel in 1995, as we had explained previously. When referring to the power supply’s form factor, we are simply talking about its size, so don’t get confused. Here are different PSU form factors explained:

ATX Power Supply

This is the standard form factor used by most power supply manufacturers in the market. There are two different sizes available for ATX PSUs — ATX PS/2 and ATX PS/3. The former has a slightly longer length and many high-end power supplies with high-wattage ratings use the ATX PS/2 form factor. On the other hand, some power supplies use the ATX PS/3 form factor, sometimes referred to as micro ATX PSU. That term doesn’t officially exist, though.

atx size power supply
ATX-size power supply

PC cases that can support the ATX-sized motherboards (mostly) always support both ATX PSU form factors, but you have to confirm this as high-end PSUs with a longer length don’t always fit in every PC case.

SFX Power Supply

The SFX power supply is much smaller than the ATX design and is popular among system builders looking for a power supply to fit inside their small form factor (SFF) Pcs. As you might know, there are different-sized motherboards available in the market. Accordingly, certain PC cases are designed to fit SFX power supplies and Mini-ITX motherboards. These include PC cases like the Dan-A4, FormD T1, Cooler Master NR200P, and many others.

sfx size power supply
SFX-size power supply

Internally, the SFX PSUs are not that different than ATX-sized PSUs, they are just smaller. So, they are fully compatible with your ATX or micro-ATX motherboards as well. There are also variations of this PSU form factor, such as SFX-L created by SilverStone.

TFX & Flex-ATX Power Supply

TFX & Flex-ATX power supplies are used in certain PCs with limited height or length support. They are usually found in OEM PCs, servers, and mini computers. These PSUs are designed for a certain use case and aren’t sold actively in the consumer market.

What is a PSU (Power Supply Unit)? Explained!
Flex ATX Power Supply

How Many Watts PSU (Power Supply) Should I Get?

Let’s look at the answer to the most-asked question — “what power supply do I need?” For this, we have detailed how you can calculate the total wattage needed by your system components. You need to use this Power Supply Calculator (visit) by OuterVision, an excellent tool for measuring the power consumption of your components. Here, select “Basic” if you are new to PC building. You can select “Advanced” if you want to take more factors into consideration such as your CPU’s voltage & clock speed, adding another graphics card, etc.

Next, input the details for all your system components – CPU, RAM, GPU, storage, etc. – one by one using the drop-down menus. Once you are done, click on “Calculate” to see the load wattage of your PC components and the recommended PSU wattage.

power supply wattage requirement calculator
Outervision’s PSU Calculator Tool

Now, remember to leave a healthy headroom over the recommended wattage for future upgrades. We recommend adding another 100 to 200 watts (at least a 20 percent headroom), as per your requirements and budget constraints. You should buy a power supply unit in that wattage bracket. As per the calculator, a mid-range gaming PC using the Ryzen 7600X CPU & RTX 3070 Ti GPU needs a 550W PSU at the very least.

Do I Need to Connect a UPS To My Computer’s Power Supply?

Most power supplies today come with enough quality features and protection mechanisms to ensure that your PC parts don’t suffer from damage due to electrical instability or power black-out. But a sudden system shutdown can cause problems with your Windows partition. In our experience, using a PC without a UPS or a power backup has never caused any damage to the system components. But, after a sudden system shutdown, the Windows partition can become unstable and the system wouldn’t boot or have BSODs (blue screen of death) in Windows 11.

computer ups
Image Courtesy: Schneider Electric

So yes, you should definitely connect a UPS to your PC’s power supply. As an alternative, simply ensure that there is some other form of power backup. Otherwise, there is always a chance of your Windows partition & data becoming corrupted. As we said, you don’t have to worry much about physical damage to your components. Most power supplies available in the market right now will prevent that catastrophe. But in case your Windows partition does get corrupted, you need to use the System Restore utility feature, as detailed in this guide. And in case that doesn’t work properly, you will have to reinstall Windows 11.

We hope you have understood everything related to your PC’s power supply unit (PSU). In this guide, we explored everything from various power supply connectors to exploring how the power supply works. We also touched upon the history of power supplies, and how the ATX standard came into existence. We also went over the various PSU form factors, modular vs non-modular PSU types, and the recommended wattage requirements in this guide. If you still have any doubts about power supply units in PC builds, let us know in the comments below.

How do I choose the best power supply for my gaming pc or workstation?

To choose the best power supply for your PC, you must calculate the total system power wattage first. Then, make sure the PSU you buy can support that wattage adequately. We suggest you read reviews to ensure it is a good-quality unit with 80 Plus Certification.

How much PSU is enough for a PC?

A PSU with at least 20 percent headroom left (in terms of wattage) is enough for your PC. You should leave more headroom if you want to upgrade your PC later with a high TDP processor or graphics card.

How do I know if my power supply is weak?

If you are experiencing system shutdowns and are unable to overclock or extend power limits, or having instability issues on your PC, your power supply might be too weak for the system.

What is the lifespan of PSU?

The power supply unit (PSU) is one of the most durable, lasting hardware components of a PC. When it is a good quality PSU, it can last up to 5-10 years or more. Many high-end power supplies come with a long warranty too.

Is it OK for PSU to be warm?

Yes. Since your PSU is actively pulling electricity from the wall socket, and converting it into suitable DC power for your PC, there is going to be heat involved and the PSU will be warm.

Is it okay if my PSU Fan is not turning on?

Many new power supplies come with a zero fan RPM feature. This only turns on the cooling fan after a certain wattage threshold has been reached. So, if your PSU has that feature, then it is okay if the PSU fan is not turning on. It will automatically spin when required.

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