A Practical Guide to Computer Power Supplies
Ratings, Connectors, and Form Factors
1 Purpose
This document provides a practical guide to the Power Supply Unit (PSU), the component responsible for providing all the power to a computer system. You will learn how to interpret PSU specifications, identify its various connectors, and understand the differences between the types of power supplies you will encounter in the field.
2 What You’ll Learn
By the end of this reading, you will be able to:
- Interpret PSU ratings like wattage and the 80 Plus efficiency scale.
- Identify the main ATX power connectors and state their purpose.
- Differentiate between modular, semi-modular, and non-modular PSUs.
- Recognize the difference between common PSU form factors like ATX and SFX.
This reading maps to the following program and course learning outcomes:
- Program Learning Outcomes (PLOs):
- 3. Apply terminology and numeric or system concepts: This guide covers the specific terminology of PSUs, including wattage, efficiency ratings, and connector types.
- Course Learning Outcomes (CLOs):
- 1. Identify hardware and basic network components: This document is focused on the identification and selection of a critical system component.
- 4. Configure hardware and basic network components: Choosing and correctly connecting a PSU is a primary step in system assembly and configuration.
This exercise will help you develop the following skills and knowledge, which align with the O*NET SOC Code 15-1232.00 for Computer User Support Specialists.
| Learning Objective | O*NET KSAs | Technologies Used |
|---|---|---|
| Interpret PSU specifications like wattage and efficiency. | Knowledge: Computers & Electronics Skills: Reading Comprehension, Critical Thinking |
PC Hardware |
| Identify common PSU connectors and their uses. | Knowledge: Computers & Electronics Abilities: Information Ordering |
PC Hardware |
3 The Role of the Power Supply Unit (PSU)
The Power Supply Unit (PSU) is the heart of a PC’s electrical system. It takes the high-voltage AC power from the wall and converts it into the various low-voltage DC outputs required by the computer’s components. Choosing the right PSU and connecting it correctly is a critical skill for any system builder or technician.
3.1 Understanding PSU Ratings
Two of the most important specifications on any PSU are its wattage and efficiency rating.
Wattage: This is the maximum amount of power the PSU can deliver to the computer’s components. A simple office PC might only need a 300W PSU, while a high-end gaming PC with a powerful graphics card could require 850W, 1000W, or even more.
Efficiency (80 Plus): A PSU’s efficiency rating tells you how much AC power from the wall is successfully converted into DC power for the components. The rest is lost as heat. The 80 Plus certification is a common standard, with several levels:
80 Plus Rating Efficiency at 50% Load 80 Plus 80% 80 Plus Bronze 85% 80 Plus Silver 88% 80 Plus Gold 90% 80 Plus Platinum 92% 80 Plus Titanium 94%
A more efficient PSU wastes less energy as heat, which can lead to a cooler, quieter, and more reliable system with a lower electricity bill.
3.2 PSU Connectors
Modern PSUs have a variety of connectors to power different components.
| Connector | Description |
|---|---|
| 24-Pin Motherboard | The main connector that provides the motherboard with power. |
| 4/8-Pin CPU | Provides dedicated power to the CPU. High-end CPUs may require the full 8 pins. |
| 6/8-Pin PCIe | Provides power to graphics cards. Often designed as a 6+2 pin for compatibility. |
| SATA Power | A thin, L-shaped connector used to power SSDs and hard drives. |
| Molex | An older, 4-pin connector still used for some case fans and accessories. |
3.3 Modularity
- Non-Modular: All cables are permanently attached to the PSU. This is the cheapest option, but can lead to a messy case with many unused cables.
- Semi-Modular: The essential cables (like the 24-pin and CPU connectors) are attached, but other cables for peripherals are detachable.
- Fully-Modular: All cables are detachable. This provides the cleanest look and easiest cable management, but is the most expensive option.
3.4 Power Supply Form Factors
Not all PSUs share the same size, connector layout, or cooling strategy. Manufacturers adhere to standardized form factors so builders can match the right supply to the chassis and workload.
Desktop and Small Form Factor Units
- ATX: The mainstream choice for full-size desktops, typically rated from 450–1000 W with a 120 mm fan for quiet cooling.
- SFX/SFX-L: Shorter housings for mini-ITX builds. They often ship with adapter brackets so they can mount in ATX cases.
- TFX/Flex ATX: Slim, elongated supplies used in compact business desktops and point-of-sale systems.
Rack-Mount Server Supplies and Redundancy
Rack-mount systems prioritize uptime over acoustics. Server PSUs are packaged as narrow, high-density modules that slide into the chassis from the rear or front.
- Form factor examples: 1U “blade” modules, 2U high-capacity cartridges, and proprietary sleds tailored to specific server lines.
- Hot-swappable design: Each module connects to a shared power backplane, allowing technicians to replace a failed unit without shutting down the server.
- Redundant operation: Enterprise chassis typically operate in dual (1+1) or N+1 configurations. If one module fails, the remaining unit(s) instantly absorb the load to keep services online.
- Shared monitoring: Management controllers expose telemetry (temperature, fan speed, output current) so administrators can pre-empt failures.
Comparing Common PSU Options
| Form Factor | Typical Power Range | Cooling Style | Common Use Cases |
|---|---|---|---|
| ATX | 450–1000 W | Large axial fan | Gaming and workstation towers |
| SFX / SFX-L | 450–850 W | Compact axial fan | Small form factor desktops |
| TFX / Flex ATX | 200–400 W | High-speed blower | Ultra-compact OEM systems |
| 1U/2U Server Module | 750–2000 W | High-pressure blower | Rack servers with redundant PSUs |
4 Next Generation Power Supplies
Next-generation power supplies are more efficient, reliable, and powerful than their predecessors, thanks to advanced materials and smarter digital controls. While modern power supplies provide fundamental stability, next-gen units introduce key advancements to handle the demanding, variable power needs of modern electronics and smart infrastructure.
Next-generation power supply features:
Improved energy efficiency: They waste less electricity and turn more power into usable energy, which lowers electricity bills and reduces environmental impact.
Adaptable and stable power: These units can intelligently adjust their power delivery to match what your devices need, preventing damage from power surges or inconsistent flow.
Better safety features: They come with advanced protections that automatically shut down the unit in case of issues like too much heat or overloads, keeping your connected devices safe.
Quieter operation: A more efficient design means less heat is produced, so cooling fans don’t have to work as hard. Many models have special fan modes for silent performance during light use.
Smarter monitoring: Some units allow you to connect to software that monitors power usage, temperature, and performance in real-time, giving you better insight and control.
5 Reflect and Review
Now that you have reviewed this document, take a moment to reflect on your learning. In your Microsoft Teams Student Notebook, create a new page for this topic and write down the following:
- 3 different PSU connectors and what they power.
- 2 benefits of choosing a PSU with a high efficiency rating (e.g., 80 Plus Gold).
- 1 question you still have about choosing the right PSU for a computer.
This reflection is for your instructor to review and helps solidify your understanding of the concepts.
Test your understanding with the following questions. These questions provide retrieval practice and reinforce key concepts covered in this reading. In your Microsoft Teams Student Notebook, answer the following:
- What are the two most important ratings to consider when choosing a PSU?
- A user wants to build a PC with the cleanest possible cable management. What type of PSU modularity would you recommend?
- You are installing a new graphics card and it has an 8-pin power socket. What PSU connector should you use?
- What is the main difference between an ATX and an SFX power supply?