PSUs: Understanding Computer Power Supply Units

A Complete Guide to PC Power Supplies: ATX, SFX, TFX, & EPS Types

When building or upgrading a PC, one of the most overlooked yet critical components is the Power Supply Unit (PSU). While flashy GPUs and lightning-fast CPUs tend to steal the spotlight, it’s the PSU that quietly powers every component in your system. A reliable power supply ensures stable performance, protects your hardware, and contributes to energy efficiency.

In this comprehensive guide, we’ll break down everything you need to know about computer power supplies — from their internal components and form factors to how to choose the right one for your needs.

What Is a PC Power Supply?

A power supply unit takes the high-voltage AC electricity provided by a mains outlet and transforms it into lower-voltage DC electricity that your computer’s motherboard, processor, graphics card, storage devices, and other peripherals require to function. This conversion must be precise to ensure system stability and to protect the components from power irregularities. 

Modern PSUs are usually switched-mode units, meaning they efficiently transform power while adapting to different input voltages worldwide.

PC Power Supply Components 

To understand how a PSU works, it helps to know its key internal components:

• Transformer: Steps down high-voltage AC from the wall to lower voltage levels suitable for conversion to DC.
• Rectifier: Converts the stepped-down AC into pulsating DC.
• Capacitors: Smooth out the pulsating DC into a steady flow of direct current and help filter noise.
• Voltage Regulator: Ensures consistent output voltages (such as +12V, +5V, +3.3V) despite fluctuations in input voltage or load.
• Cooling System: Most PSUs use an internal fan to dissipate heat generated during operation. High-end models may feature semi-passive or fanless modes under low loads for quieter performance.
• Protection Circuitry: Includes safeguards like Over Voltage Protection (OVP), Under Voltage Protection (UVP), Over Power Protection (OPP), Short Circuit Protection (SCP), and Over Temperature Protection (OTP) to prevent damage to your system.

These components work together to deliver clean, stable power — a necessity for both everyday computing and high-performance gaming or workstation builds.

Types of Computer Power Supplies

The form factor of a power supply determines its size, shape, and connector compatibility. Let’s explore the most common types:

ATX: 

The ATX (Advanced Technology eXtended) power supply is the most common form factor and is designed for standard mid-tower and full-tower desktop builds. It typically ranges from 300W to over 1000W.

SFX: 

SFX (Small Form Factor Extended) PSUs are more compact, perfect for mini-ITX builds or smaller cases. Despite their size, many offer high wattage and modular cabling.

TFX:

TFX (Thin Form Factor Extended) PSUs are designed for slim desktops and HTPCs. They are longer and thinner than ATX, typically with limited power output around 300W to 350W.

EPS:

EPS (Entry-Level Power Supply) is commonly used in servers and high-performance workstations. It typically supports dual CPU setups and has an 8-pin connector instead of the 4-pin found in ATX units.

What To Consider When Choosing a PC Power Supply?

Here are some key factors to help you choose the right PSU: 

Wattage Requirements:

The first step is to calculate your system’s power needs. Use online PSU calculators or add up the TDP (Thermal Design Power) of your components. It's wise to choose a PSU with at least 20–30% headroom.

Efficiency Rating:

Look for 80 PLUS certification, which indicates how efficiently the PSU converts power. Ratings range from 80 PLUS White to Titanium. Gold or Platinum is ideal for most users. 

Form Factor:

Make sure the PSU physically fits your case. A compact build needs an SFX or TFX PSU, while a full-tower setup will take ATX or EPS models.  

Voltage Regulation:

A high-quality PSU maintains tight voltage regulation (within ±5% or better), which protects your components from power fluctuations.  

Protection Features:

Safety should never be compromised. Choose units with OVP, UVP, SCP, OTP, and Over Power Protection (OPP). 

Choose the Best Power Supplies for Your Needs

Here are some top-tier power supplies across different wattage ranges and use cases:

220-P6-1000-X1 EVGA SuperNOVA 1000 P6 1000W Power Supply: A high-efficiency, fully modular ATX PSU with 80 PLUS Platinum rating. Ideal for high-end gaming rigs and workstations with multiple GPUs.

SPI-300G-B Sparkle Power 300-Watts Switching Power Supply: A compact, reliable option for basic office PCs or older systems. Offers solid performance at an affordable price.

120G21600X1 EVGA SuperNOVA 1600-Watts 80 Plus Gold Power Supply: One of the most powerful PSUs on the market, perfect for extreme builds, server setups, or future-proofed systems. Features full modularity and excellent thermal management.

HG2-5600V Emacs 600-Watts Power Supply: A mid-range ATX PSU offering dependable power delivery for mainstream gaming PCs. Great balance of performance and value.

FSP350-60GLC Sparkle Power 350-Watts ATX Power Supply: A no-frills, entry-level PSU ideal for budget builds or HTPCs where power demands are low.

Each of these units serves a specific purpose — from energy-efficient office machines to powerhouse gaming rigs.

Frequently Asked Questions

Let’s clear up some confusion with these frequently asked questions.

What’s the Difference Between ATX and TFX Power Supplies?

ATX power supplies are standard-sized and offer higher wattages, while TFX power supplies are more compact and usually meant for low-profile systems.

How About ATX vs EPS?

EPS power supplies are like upgraded ATX units—more connectors and higher wattage. Ideal for workstations and servers.

What’s the Golden Rule for Power Supply Units?

Always calculate your system’s power needs and buy a PSU with headroom and protection features. Don’t cut corners.

How Do I Know What PSU I Need?

Use an online calculator or check wattage requirements for each component. Add a 20–30% buffer and consider your upgrade plans.

What’s a Good Wattage for a Power Supply Unit?

• Basic builds: 300–500W
• Gaming: 650–850W
• Workstations: 1000W+

What Efficiency Rating Should I Look For?

80 PLUS Gold is a sweet spot for most people. Bronze for budget builds, Platinum if you want top-tier efficiency.

How Much Does a Power Supply Cost?

Anywhere from $40 for budget units to $200+ for high-end, fully modular models.

Final Thoughts

Choosing the right Power Supply Unit is crucial for a stable, safe, and long-lasting PC. With so many form factors and features, it’s worth doing your research to find a PSU that fits your system’s needs and your long-term upgrade path.

Whether you're building a powerful gaming PC, a quiet home theater setup, or a mission-critical workstation, you’ll find dependable power supply options at Compu Devices. We offer a wide range of energy-efficient PSUs —from top brands like EVGA, Sparkle Power, Emacs, and FSP—at competitive prices. 

Visit Compu Devices today to power up your next build with the best in class. Your system deserves it!

Also Read:

The Rise of Energy-Efficient PC Power Supplies: What You Need to Know