8 Technical Specifications of PSU Explained – The Power Supply of PC

By | October 29, 2021

The Power Supply Unit or the PSU is un-arguably the most important part of any PC build.

It is the component that supplies power to all of your other hardware and can greatly damage your components if it does not work properly.

Casual PC buyers and builders will often cheap out on their PSU to have a bigger budget for other components.

This is because they do not fully understand the importance of the Power Supply. Common problems can be less output power than what is required by the pc components, or unstable power or lack of protection. All these problems can cause hardware damage which can be quite expensive.

We are breaking down the features of a power supply to help you better understand the PSU. This article shall explain the key specs about power supply units and how they affect the PC performance.

  1. Form Factor
  2. Power Output or Wattage
  3. Efficiency Rating
  4. Rails
  5. Protection
  6. Modularity
  7. Variable RPM Fan / Intelligent Fan / Zero RPM Mode / Eco Mode
  8. Fan Size
Thermaltake Toughpower Grand RGB 750W PSU

Thermaltake Toughpower Grand RGB 750W PSU

1. Form Factor

The form factor of a power supply is simply referring to its size. Most Power Supply Units nowadays are ATX12V aside from a few special PSUs. The older models are harder to find and have been dropped in favor of the ATX12V.

The most common sizes are ATX and SFX. The ATX are the full sized PSUs that come in all wattage capacities are made for mid tower and full tower pc builds.

Whereas the SFX sized PSUs have lower power supply (around 650W) and are targetted more towards smaller and compact sized pc cases for mini-ITX builds with lower power requirements.

Here is an image showing the comparison of the size difference of the sff and atx form factors.



More details about PSU form factors on the wikipedia page:

1.1 AT/ATX

The AT and ATX form factor are both predecessors of the current ATX12V. Both of these form factors are still available for purchase at a few stores but are definitely falling out of the market since newer hardware does not need the AT and ATX form factor anymore.

AT stands for “Advanced Technology” and is connected directly to the 230-volt line.

The main disadvantage of this type of PSU is that there is no standby mode and you can either turn it on or off. Due to this, the unit doesn’t consume any power when it is turned off even if you leave it plugged in the socket.

The ATX makes use of the older 20-pin cable for its main power connector. Newer motherboards have a 24-pin connection rather than the old 20-pin one, making ATX not usable with the newer standards. It just simply cannot provide enough power to run new motherboards.

Initially ATX form factor was available in 2 sizes namely ATX/PS2 and ATX/PS3. ATX/PS3 was smaller in size with dimensions around: 150mm (W) x 86mm (H) x 100mm (D). The depth for this form factor may vary from 100mm up to 139mm.

However ATX PS/2 established itself as the most popular standard and is commonly known as ATX PSU.

ATX PS/2 Power Supplies (or ATX PSU) are around 140mm long (depth), 150mm wide, and 86mm tall, though some models have more depth, especially those that offer more wattage.

The Corsair HX850i PSU has a depth of 180mm for instance. The larger depth allows it to integrate a big size low-noise fan easily, since high wattage psus also need more cooling.

Corsair HX850i PSU DImenstions

Corsair HX850i PSU DImenstions

Information about the dimensions can be found here:

1.2 ATX12V

The ATX12V is a standard based on the ATX form factor.

There are a few different versions of ATX12V, though all versions still maintain the same shape and size. Each newer version has enhanced cable connectors to make them work with newer motherboards.

ATX12V Power Supply

ATX12V Power Supply

ATX12V v1.0

The first version added a 4-pin 12V connector to deliver power for the processor since processors were becoming more power-hungry as the generations passed.

A 6-pin auxiliary power was also added, though it is no longer used today. This version was first introduced in the year 2000.

ATX12V v1.3

The v1.3 added the SATA cable as a replacement to the 4-pin peripheral cable and has become the standard for current HDDs and SSDs. This version was introduced in 2003.


The ATX12V Power Supply Units that were version 2.0 and above introduced the 24-pin main power connector, which can also be used with older 20-pin motherboards.

The ATX12V was updated with the release of ATX12V v2.4 in 2013 and has been in use ever since.

For more details about various ATX12V versions check this article here:

1.3 EPS12V

The EPS12V PSU was designed for servers, mainly adapted to power servers that can provide a more powerful and stable connection for important server applications and processes.

The EPS12V had 8-pin CPU connector that was originally used to power multiple CPUs within a single unit/server. However, as time passed, processors needed more power than what the original 4-pin could provide, making the 8-pin 12V cable the current standard in ATX12V.

So it is common to see PSUs that are marked as both ATX12V and EPS12V because of identical spefications.

1.4 Small Form Factor (SFF)

There are a few variations within the SFF Power Supplies. These PSUs were often used in mini-ITX based builds with smaller cases where the standard ATX PSU simply wouldn't physically fit.

Most small form factor Power Supply Units used to only be able to support lower voltage systems but in recent years, smaller form factor PSUs were released that could support up to 1000W.


SFX is a Small Form Factor PSU is the most commonly used small PSU and its power specifications are identical to ATX. This is the best PSU for modern builds that make use of smaller cases and use mini-ITX motherboards.

FSP Dagger Pro 650W SFF PSU

FSP Dagger Pro 650W SFF PSU

The smaller form factor cases have been growing in popularity in recent years not only for enthusiasts but also for casual builders.

The regular size of an SFX Power Supply is around 100mm long, 125mm wide, and 63.5mm tall.

Due to lower power capacity SFX power supplies are not suited for high power graphics cards like the RTX 3080/3090, as they need a lot of power under peak load. However SFX psus can be used to power RTX 3060/3070 based systems though the user needs to be careful when building such a system.

Most SFX power supplies have power capacity in the range of 650-700 watts, but there are newer SFX psus that can provide upto 1000 Watts of power.

A 650 watt sfx psu can easily power a modern 125 watt cpu like the i9-11900K along with a 220 watt gpu like the rtx 3070. However you should not attempt to overclock your system with such limited power supplies.


The TFX or Thin Form Factor is another small form factor design that is smaller but longer and can fit inside slim and long systems. This is an older form factor though that has a low power output of only around 300w – 400w, aside from a few special cases.

The TFX form factor is rarely used nowadays and has been replaced with newer SFF PSUs like SFX.


The Compact Form Factor or CFX is an L-shaped PSU that looks similar to the SATA cable connection. This was usually used for small pre-built PCs and special cases. The CFX has been outdated throughout the years and can rarely be bought today.

2. Power Output or Wattage

The Power Output of a Power Supply is usually measured in Watts. Modern units usually range from as low as 450W to as high as 1600W.

  • 550W
  • 650W
  • 750W
  • 850W
  • 1000W

Power needs will vary from PC to PC so the PSU Wattage will purely depend on what components and parts your PC has. Most mid-range PCs will require around 500W-650W of power while 1000W and above are usually reserved for extremely high-end and enthusiast builds.

Typically, you need a higher wattage power supply than the power your system actually consumes. This is to ensure that there is enough power at all times, especially since the power required by your system spikes from time to time.

The power requirements of your system increases if you have dedicated graphics card, more powerful processor, or a larger high-spec ATX motherboard. The more hardware components there are, the more power is needed.

Though modern hardware is designed to be more and more power efficient.

3. Efficiency Rating

The Efficiency Rating of a Power Supply is extremely important for the safety of your components and to ensure that everything will run smoothly. Efficiency refers to the power the PSU sends to your PC and how much of that power is lost to heat.

The lower the efficiency of your PSU, the higher it will cost to run your machine since it will be drawing more power from the outlet. Aside from that, a more efficient PSU will also allow your PC to run at lower temperatures since it isn’t adding too much heat to the system.

The main way to ensure that you have an efficient and great performing Power Supply is to base it on its 80 PLUS certification rating.

80 Plus Efficiency Rating

80 Plus Efficiency Rating

There are a variety of ratings and, as a rule of thumb, the higher the 80 PLUS rating, the better the quality of Power Supply. This is why most knowledgeable PC builders will always tell you to stay away from Power Supplies that don’t have an 80 PLUS rating of any kind.


This is also referred to as “80 PLUS White” and is the lowest efficiency rating that a PSU can have aside from no rating at all. This is a better PSU than those fishy Chinese or Korean “True Rated” ones and they are often only a bit more expensive than the “True Rated” Power Supplies.

80 PLUS PSUs are 80% efficient at 20% load, 50% load, and 100% load for 115V powered systems. It is a great PSU for low-end PCs that don’t require much power to run.

80 PLUS Bronze

The 80 PLUS Bronze PSU category is probably the best option for budget builders since it doesn’t break the bank that much with it costing only around $50 - $80 while still performing well.

These Power Supplies are great for entry-level gaming machines and are the go-to PSU for budget builds. 80 PLUS Bronze Power Supplies are 82% efficient at 20% load, 85% efficient at 50% load, and 82% efficient at 100% load for 115V systems.

80 PLUS Silver

80 PLUS Silver is a dying PSU efficiency rating category. You can rarely see any 80 PLUS Silver Power Supplies nowadays since 80 PLUS Gold is inexpensive and has been the go-to option for a good mid-range PSU.

The price difference between 80 PLUS Silver and Gold rated PSU is also very negligible so manufacturers just choose to manufacture more 80 Plus Gold Power Supplies instead.

80 PLUS Silver PSUs are 85% efficient at 20% load, 88% efficient at 50% load and 85% efficient at 100% load for 115V powered systems.

80 PLUS Gold

These Power Supplies are hands-down the best ones to buy for mid-range builds. They are great PSUs for most builds and are cheaper compared to higher-rated PSUs, making it a good middle ground between price and performance.

80 PLUS Gold PSUs are 87% efficient at 20% load, 90% efficient at 50% load, and 87% efficient at 100% load for 115V powered systems.

80 PLUS Platinum

80 PLUS Platinum rated Power Supplies are the ones you want if you are building a top-of-the-line system with the best and latest hardware.

The 80 PLUS Platinum rating is usually reserved for high Wattage PSUs starting at around 750W and above. These PSUs are very efficient with 90% efficiency at 20% load, 92% at 50% load, and 89% efficiency at 100% load.

80 PLUS Titanium

The 80 PLUS Titanium rating is the best rating a PSU can get. These are quite honestly considered overkill for most systems unless you are running extreme builds with multiple GPUs or CPUs.

For the most part, 80 PLUS Platinum rated Power Supplies are enough but of course, there are the ones that truly want the best, and you can’t get any better than Titanium.

These units are rated for 90% efficiency at 10% load, 92% efficiency at 20% load, 94% efficient at 50% load and 90% efficient at 100% load for 115V powered systems.

4. Rails

Rails are essentially the source of voltage within the Power Supply. It is composed of wires and circuits that transfer a specific voltage from the outlet to your PC.

Generally, PSUs have 3 standard rails, the +3.3V rail, +5V rail, and the +12V rail. All three rails serve a different purpose and power a different component within your PC.

Usually most ATX12V PSUs will have all the necessary rails required for specific connectors and this is something that you need not think about when purchasing a new psu.

+3.3V Rail

The +3.3V rail has been a standard design for years now, and they usually power things such as the M.2 slots, SATA, RAM slots, and some microcontrollers on the motherboard. Though this can vary depending on the motherboard.

+5V Rail

The +5V rail usually powers connectors and things like USB ports, PS/2 ports (if available), PCI headers, some RGB headers, and storage devices.

+12V Rail

The +12V rail powers most of the important and power-hungry components in your PC which includes both the CPU and the GPU. Some PSUs make use of multiple +12V rails that are considered safer when handling high powered components.


These are PSUs that only have a single +12V rail to supply power to the components. These are considered to be the best option for overclockers since it is said that single rails offer cleaner power as you aren’t running it through other rails.


Multi-rail PSUs are those that have more than one +12V rail that is said to be the safer option, especially for areas with power fluctuations. Having multiple rails allows each rail to have a separate amp rating that handles lower loads compared to PSUs that only have a single rail.

5. Protection - OVP/UVP/OPP/SCP/OCP/OTP

Power Supplies nowadays have a great built-in over-voltage protection and under-voltage protection that eliminates the need for having a separate Automatic Voltage Regulator (AVR) when running your machine.

The voltage protection works when the electric company is supplying too little or too much voltage, the PSU will automatically shut down. This also works for things such as thunderstorms or lightning storms where power spikes can occur.

The extra voltage protection protects not only your PSU but also your components from any serious damage by simply shutting down the system.

Aside from high or low voltages, high currents can also greatly damage your PC. The current protection present in power supplies works by limiting the amount that can be pulled from the outlet.

There is a circuit that protects the PSU by simply shutting down when there is too much current or power being delivered to the unit.

All these protection technologies have names like

  • OVP - Over Voltage Protection
  • UVP - Under Voltage Protection
  • OPP - Over Power Protection
  • SCP - Short Circuit Protection
  • OCP - Over Current Protection
  • OTP - Over Temperature Protection
  • BOP - Brown Out Protection

Here is an article on coolermaster website that explains these terms in details:

A good quality PSU will have many or all of the above mentioned protection mechanism implemented in the circuit to ensure longevity of the PSU and safety of the PC hardware.

6. Modularity

Gone are the days where you had to hide a bunch of excess cables within the PSU shroud or the HDD trays so that they won’t clutter up your clean build.

Nowadays, more and more PSUs have been implementing the ability to fully attach and detach the power cables that come with the PSU.

6.1 Non-Modular

Non-modular PSUs have all the connector cables hardwired into them and you have to manage the un-used excess cables by hiding them somewhere inside the case.

All the cables you need will be coming out of the back of the PSU, and there is pretty much nothing you can do about unused cables except to tie them up and tuck them in.

One upside to non-modular PSUs is that they are usually very cheap, making them a great budget option.

6.2 Semi-Modular

Semi-Modular PSUs can come in a variety of ways. The 24-pin always comes pre-attached, though the 8-pin CPU and PCIe cables can come pre-attached or modular, depending on the Power Supply model, while all the other cables are fully detachable.

semi modular psu

Semi Modular PSU

These are usually cheaper than fully modular ones and still offer a ton of flexibility in terms of cable management.

You can remove the un-used cables making the inside of the pc case more clean and manageable.

6.3 Full-Modular

Full-Modular PSUs are units that come with all cables fully detachable. You can simply plug in all the cables you need and store the ones you don’t need in the box, allowing you to have a clean and sleek build.

thermaltake fully modular psu

Thermaltake Fully Modular PSU

They are a great time and space saver in terms of cable management and you can also choose to buy various colored cables in exchange for the stock ones as long as your PSU supports them.

The biggest downside of fully modular PSUs is that they can be very expensive, most of them costing around $100 or more.

7. Variable RPM Fan / Zero RPM Mode

Also known as Variable RPM Fan / Intelligent Fan / Zero RPM Mode / Eco Mode, this is a new feature in modern PSUs that turn the fan off when not needed and increase its RPM when more cooling is needed. Hence the fan can remain off or run at variable speeds as needed.

This makes the PSU less noisy, since if your system is not drawing too much power, there will be no need to turn the fan on.
psu smart zero fan switch

The PSUs will typically have a button on the backside near the power-in socket, that can be used to turn off the variable rpm feature of the psu fan.

Some may argue that fan running less often will increase its longevity and the bearings in the fan may last longer and less dust will buildup on the heatsink inside the PSU.

8. Fan Size

Fan Size varies across different PSU models. Typical range is 120mm to 140mm. In general larger fans can provide more cooling at lower rpm, therefore they may be quieter. However there are other factors that affect the cooling of the PSU.

For example the Corsair RMx White Series RM750x PSU has a 135mm fan whereas the Cooler Master MASTERWATT 650 has a 120mm fan.
Thermaltake Toughpower Grand RGB 750W Gold PSU has a 140mm fan.

Fans larger than 140mm can cause other issues, so its better to stay within the standard values of 120-140mm.


So if you are planning to buy a new psu for your next build or existing pc, the most important things to consider would be the wattage/power rating, modular structure and support for variable rpm fan.

The wattage shall determine how much power the psu can supply to your system. The module structure makes it easy to manage cables.
Variable rpm fan will stay off when not needed, making the psu quieter.

Most standard PSUs from reputable brands will have features like voltage protection. However there are plenty of cheap PSUs that actually do not mention any of protection features or do not have them.

So if you have any questions or feedback let us know in the comments below.

About Silver Moon

A Tech Enthusiast, Blogger, Linux Fan and a Software Developer. Writes about Computer hardware, Linux and Open Source software and coding in Python, Php and Javascript. He can be reached at [email protected].

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