Computer Hardware:
     • Tools, Static & Cleaning
     • Form Factor
     • PC Case & Fans
     • Motherboard
     • Processor (CPU)
     • Power Supply Unit
     • RAM
     • Hard Drive
     • Solid State Drive
     • Optical Drive
     • Floppy Disk Drive
     • Graphics Card
     • Sound Card
     • Network Card
     • Computer Monitor
     • Keyboard & Mouse
     • Laptop / Netbook
     • Building a Computer
     • Overclocking

Operating System & Backup:
     • Operating System
     • Drivers
     • Windows Tools
     • User Accounts
     • Backup
     • Windows 10

Internet & Network:
     • Internet
     • Wi-fi or Cable
     • Improve Broadband Speed
     • Network Computers

Computer Peripherals:
     • Printer
     • Scanner
     • External Hard Drive
     • USB Flash Drive

Computer Security:
     • Anti-virus
     • Anti-Spyware
     • Phishing
     • Firewall

Common PC Problems:
     • Slow Computer
     • Hardware Failure
     • Software Failure
     • Printing Problems

     • Windows Shortcuts
     • Glossary of Terms
     • HTML Colour Picker
     • Number Base Converter

Power Supply Unit

The Power Supply Unit (PSU) converts mains A.C. (Alternating Current) electricity into low voltage D.C. (Direct Current) to power the motherboard and components in your computer.

Most modern desktop computers use a standard ATX power supply unit which consists of a square metal box with the various wires and power connectors attached. On the back is the on/off switch, mains socket, and sometimes a voltage selector (230v or 115v) which must be set to your region.

PSU - ATX Power Supply Unit

PSU - ATX Power Supply Unit.

The PSU is one of the most likely computer components to fail although they are quite easy to replace. It should be stressed that you should never open a PSU and try to fix it as it contains capacitors which hold a charge even when the power is off which can cause serious injury. You should just replace the failed PSU with a new one. It is advisable to buy a good quality brand with sufficient wattage for any future upgrades as a cheap low-wattage PSU can fail and might also destroy the motherboard or other components. You will also need to buy one with all the correct connectors to power all the drives, motherboard, and possibly the graphics card. PSU connectors are covered later in this article.

You should buy a PSU from a reputable manufacturer which comes with a warranty and support. It is also a good idea to do your research by looking on-line for power supply unit reviews. Good quality brands include Corsair, Seasonic, and Antec. Other things to look out for is the weight, as a heavier PSU generally means bigger and better components and larger heatsinks to dissipate the heat. A PSU usually includes a fan and a larger fan will be preferable as it will be quieter than a smaller fan. Also look out for the efficiency rating of a PSU and look for the 80+ certificate which means it is over 80% efficient and loses less than 20% to heat.

A basic desktop computer might have a PSU with a wattage of 300W or 350W and a high-powered games computer might have a wattage of 500W to 750W or more. Even though you may replace a PSU with one with a higher wattage, the computer will not draw any more power than before but it will be able to support further upgrades like more drives or a more powerful graphics card etc. To give you an idea of the PSU wattage required for your computer than you can find a Power Supply Calculator at or at

The PSU will provide three primary voltage rails of +3.3V, +5V, and +12V. The 3.3V and 5V supply are for various digital circuits, and the 12V supply is generally for motors, found on hard drives or DVD drives, and also cooling fans. Modern computers now also use the 12V supply for many more components including the CPU and the power hungry modern graphics card.

As well as selecting a PSU with a sufficient overall wattage you should also understand that the overall wattage is divided between all the voltage rails and with a modern computer the wattage of the +12V rail(s) is the most important. There should be a label on the PSU which tells you the amperage of each of the voltage rails as well as the overall wattage of the PSU. You can work out the wattage of a rail by the formula - Watts = Amps x Volts, which would give you 300W for a +12V rail at 25A (300 = 25 x 12). Some Power Supply Units also have several +12V rails (multi-rail) rather than just one single +12V rail and will divide its output (wattage) between all the +12V rails. There is no significant benefit between selecting a PSU with multi-rail or single rail and both types should work equally well.

PSU - Label displaying power output

PSU - Label displaying power output.

A Power Supply Unit should display a label detailing the power output of each rail as well as the overall output. The PSU label in the photo above is on a Corsair CX750M which has one +12V rail of 62A which can handle 744 Watts of power (744 = 62 x 12). Along with the other voltage rails, the overall wattage is 750W.

Having so many wires and connectors leading from the PSU means that you will need to bunch up all the unused cables and tie them out of the way with cable ties. Also keep all the wires neat and tidy so they do not interfere with other components such as CPU and case fans and do not restrict the airflow in the computer. You can buy a modular PSU which allows you to plug only the power cables that are required into the actual Power Supply Unit.