The Basics of Computer Power Supply Units (PSU)

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When people talk of computers, conversations tend to revolve around Central Processing Units (CPUs), memory, and video cards. Along with these components comes a stampede of tech talk about these and those gigahertz or gigabytes. The power supply unit (PSU), an often overlooked component, has been funneled down to just one aspect, its wattage. However, power supply units supply electrical power to all of the components in a computer. Choosing the right one with the right attributes is key to stable computing and upgradeability in the future.

Power Rating

Most power supply units are designed to cover a wide range of applications from the 100-120 volt standard in North America and Japan to the 220-240 volt standard in Europe, Asia, Africa, and Australia. A PSU’s wattage rating is the most common specification cited by manufacturers and is based on the total maximum power of which a PSU is capable.

Typically, a desktop PSU will range from under 400 watts at the low end to over 1000 watts at the high end. Some PSUs are capable of supplying power in the 2000 watt range but these units are very expensive and used only for servers, workstations, and very extreme home computers with multiple processors and video cards.

The problem with power ratings is that there is no standard industry practice for reporting a PSUs wattage. Whereas one manufacturer may report a wattage of 800, another may report a wattage of 750 for the same unit.

Certainly, when competing on wattage output as a measure of power supply quality, manufacturers have an interest in reporting the highest wattage possible. Sometimes they get a little, shall we say, creative.

PSUs are usually made up of multiple rails or pathways through which power flows at different voltages the computer components need. This means that the total wattage delivered by a PSU can not be delivered on just one rail but must be split across several. This is an important consideration when, for example, powering multiple video cards, which require a lot of power specifically from the 12 volt rails. Luckily, most standard computers will not run into these sorts of problems.


There are a slew of connectors that come standard on PSUs, some of which are not used in all computers. When buying a PSU, one must be certain that the connectors included conform to the needs of the computer, or at least that a reliable adapter is available for your components. The most common connectors are:

  • 20 or 24 -pin Main Power – for main power to the motherboard
  • 4 or 8 -pin ATX12V – for dedicated power to the CPU
  • 4-pin Peripheral – also called a Molex for optical drives, hard drives, etc.
  • 4-pin Berg – for floppy drives
  • 15-pin SATA – for SATA hard drives, optical drives, etc.
  • 6 or 8 -pin – usually used to power PCI-e video cards
  • Auxiliary – for various custom components

Note that a connector listed as 20+4 will work in both a 20 or 24 -pin application. Similarily, a 4+4 ATX12V will work as 4 or 8, and a 6+2 PDI-e will work as an 8 or 6. The connectors of a PSU largely determine what capabilities a computer has. Two or more 6-pin PCI-e connectors allow multiple video cards to be installed, and very high-end, cutting-edge cards use the newer 8-pin connector. Each SATA device obviously requires a power connection. Sometimes an inexpensive device called a splitter can turn one power connector into two. However, an analysis of how much power will be drawn must be evaluated so that the power supply is not overloaded.


Laptops and other portable computers are powered by PSUs similar to a desktop computer. However, they are much smaller, and due to the low power requirements of a portable computer, only supply about 75 to 150 watts. Replacing a laptop power supply can be costly since some laptop power supplies are designed specifically for the manufacturer’s computers.


Installation of a PSU (pictures and instructions here) is fairly easy but should only be attempted by someone comfortable with computer hardware. At no time should a PSU be opened as there are no user-serviceable parts inside. More importantly, PSUs have capacitors, thes are components used to temporarily store charge. Even with the PSU unplugged, a dangerous, potentially lethal shock is still possible.


Choosing a power supply is more than just choosing one with a lot of wattage. The connectors, the wattage, and the application now and in the future must be considered. When building a computer, choosing the right power supply can save money since unlike other computer components, PSUs do not quickly become obsolete and can be used in future computer-building projects.