What Are Power Supply Unit (PSU) Standards? Features, Sizes, and Key Points to Consider When Choosing

Power Supply Unit standards, how to choose them, and important points to consider are explained here.

Depending on the standard, the body size differs, and even within the same standard, the depth length varies.

Therefore, it is necessary to select a power supply unit (PSU) that fits the PC case.

This article explains various PSU standards such as ATX, EPS, and SFX, as well as key points to consider when selecting one.

First, basic knowledge about PSU standards will be summarized, followed by detailed explanations of different standards and their differences.

The article also covers basic knowledge about PSU standards, cables, performance, and compatibility, providing guidance on how to choose the right unit.

≫ Related Article: How to Choose a Power Supply Unit for a Custom PC [Performance / Features / Compatibility]

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About Power Supply Unit Standards

First, here is a summary of what should be known about PSU standards.

Compatibility with the PC Case Power Standard

PSU standards vary, and the physical size differs depending on the standard.

Therefore, when selecting a PC case, it is important to check which PSU standards are supported.

For example, an ATX power supply unit is commonly used in desktop PCs. Its standard size is 150mm in width, 140–180mm in depth, and 86mm in height (specific sizes by standard are explained later).

On the other hand, an SFX power supply unit suitable for small PC cases is more compact at 125mm width, 100mm depth, and 63mm height.

Thus, when selecting a PSU, it is essential to choose one that matches the PSU standard supported by the PC case.

If the wrong standard is selected, the PSU may not fit into the case, or the screw holes may not align. Therefore, it is important to carefully check compatibility when choosing both the PSU and the PC case.

ATX Standard as the Mainstream Choice

The ATX standard PSU is the most common in custom-built PCs.

For a typical mid-tower case, the ATX PSU is almost always the standard option.

SFX Standard for Compact or Slim PC Cases

SFX and SFX-L standards are smaller than ATX and are designed for compact PC cases.

These are often used in slim cases for everyday or business PCs without a dedicated GPU, or in generally smaller PC cases.

SFX PSUs, while compact, can provide sufficient power. They are designed with careful fan size and layout considerations to maintain efficient airflow in limited space.

However, SFX PSUs have fewer options compared to ATX units. Therefore, when selecting one, it is necessary to carefully check output capacity, connector types, and dimensions.

PSU Depth Must Fit in the PC Case

While matching the PSU standard with the case is necessary, even within the same standard, PSU depth varies.

For example, ATX PSUs have fixed width and height, but the depth ranges from 140mm to 180mm.

In particular, high-capacity or high-function models tend to be deeper due to additional internal components.

Therefore, when choosing a PSU, it is important to confirm not only the case’s supported standard but also the actual PSU depth to ensure proper fit.

Many modern PC case specifications now include not only supported PSU standards but also the maximum depth allowed. Be sure to select a PSU that is shorter than the specified limit.

PSU depth refers to the horizontal length of the unit, as shown in the image.

It can be understood as the length extending inward when viewed from the back of the PC.

Also, even if the depth is fine, you’ll need to store the excess cables coming out of the power supply unit, so you’ll want the depth to allow for that space as well.

Some PC cases also place storage brackets near the PSU, so it is important to check whether there is sufficient space to manage the cables.

*Note: The model in the image does not include a bracket, but in some cases, the left half of the red-marked area could be occupied by one. Brackets may be removable to create more space, but not always.*

If space is insufficient or such considerations are undesirable, a modular power supply unit that allows unnecessary cables to be removed is recommended.

Types of Power Supply Unit Standards

Here is an explanation of the types, sizes, and differences between PSU standards.

Among these, the ATX standard is the mainstream choice. For smaller cases, SFX and SFX-L standards are common.

Other standards are not widely used today. Knowing these three is generally sufficient.

ATX Standard

The ATX standard is the mainstream PSU standard for consumer PCs.

Therefore, it is the primary choice for most custom PC builders.

Introduced by Intel in 1995, this standard defines PSU size, shape, and power delivery specifications, ensuring broad compatibility with many motherboards and cases.

The specific dimensions are as follows:

There is variation in depth, with higher capacity and feature-rich models generally being longer.

Therefore, when choosing a case, check not only the PSU standard but also the maximum supported PSU length.

Comparing the minimum depth of 125mm with the maximum of 220mm shows a considerable difference.

For compact sizes, it may not be an issue, but for larger sizes, extra case depth is necessary to manage unused cables.

Many ATX PSUs are also 80 PLUS certified, providing high energy efficiency and helping reduce electricity costs.

Because ATX is the mainstream standard, a wide range of models with different capacities, features, and designs are available, offering many choices.

SFX Standard

The SFX standard is suitable for compact desktops and small PC cases.

Specific dimensions are as follows:

Compared to ATX, it is smaller and saves space while still offering 300W to 800W power capacity, enough for many builds.

It supports entry-level to mid-range graphics cards.

However, modern high-end graphics cards require more power and space. Since SFX is designed for small cases, it is not suitable for large, high-performance GPUs.

In smaller cases, not only the PSU but also CPU coolers and GPU are restricted in size. Therefore, when selecting a compact case and SFX PSU, these limitations must be considered.

SFX-L Standard

The SFX-L standard is slightly larger than SFX, offering higher capacity and cooling performance.

Specific dimensions are as follows:

SFX-L is about 30mm deeper than SFX. This allows for a larger fan, quieter operation, and greater power delivery.

This standard is particularly suitable for users who want to build a compact yet high-performance PC.

It’s ideal for gaming PCs and workstations that require a stable power supply despite their small size.

SFX-L standard power supply units are also more compact than ATX power supplies, so your options for PC cases and components may be limited.

For that reason, it’s an option if you still want to use a small PC case that suits your needs.

EPS Standard

The EPS standard is a type of ATX standard specialized for servers and workstations.

It is designed for business computers requiring high reliability and stable output, so it is rarely used in general consumer PCs.

EPS PSUs tend to be deeper than standard ATX models.

They provide higher capacity and more PCI Express connectors, allowing stable operation of high-performance CPU, memory, and multiple GPU.

TFX Standard

The TFX standard is designed for compact desktop PCs and small servers.

Specific dimensions are as follows:

This standard is optimized for space-conscious systems and is designed to be smaller than the standard ATX standard.

However, TFX-compatible PSUs and cases are rarely seen, making it almost irrelevant for custom PC building.

BTX Standard

The BTX standard was developed as an alternative to ATX.

It was designed to improve cooling efficiency and noise reduction.

BTX comes in multiple sizes, from full-size BTX to compact picoBTX.

However, BTX adoption was low compared to ATX, and it is no longer mainstream.

In practice, BTX-compatible PSUs and cases are almost never seen, so it can be ignored in custom PC building.

FlexATX Standard

The FlexATX standard is for small motherboards.

Specific dimensions are as follows:

Compared to SFX, it is wider but shorter, making it more compact overall.

It is used only for very small cases.

Due to the small size, high-performance PCs cannot be built with this standard. Therefore, it is rarely used.

SSI EPS Standard

The SSI EPS standard is for servers and workstations that handle large data loads or require high performance.

Thus, it is not used in general custom PCs.

Conclusion: Always Check PSU Standards and Sizes

This article explained PSU standards, with the key points summarized below.

When building a custom PC, ATX PSUs will be the most common choice for standard-sized cases.

For those wanting a compact build, SFX or other small form factor PSUs will likely be chosen.

The PC case serves as the “face” of the PC. While custom building offers flexibility, smaller cases may limit high-performance components. Therefore, both case and PSU selection must balance aesthetics, space, and functionality.

This article also covers PSU standards, cables, performance, and compatibility for better selection.

≫ Related Article: How to Choose a Power Supply Unit for a Custom PC [Performance / Features / Compatibility]

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PC Parts Estimation & Compatibility Check Tool

Automatically checks each component you need when selecting parts for a custom-built PC—perfect for beginners.