What is a power supply unit (PSU)?

The power supply unit (PSU) is the element that supplies power to the internal components of the computer. It is connected to the mains by a cable and it converts the 120V or 230V mains AC to the different low-voltages DC used by the computer components (3.3V, +/- 5V and +/- 12V). A laptop PSU is external while it is generally located inside the case of a desktop computer.

Silverstone Strider Titanium 1100W PSU

Main characteristics of a power supply unit:

  • The main characteristic of a PSU is the power expressed in watts (W) it can deliver: it is between 300W and more than 1000W for desktop computers and much less for laptops (from 45W to 90W or even more for powerful laptops). It should be chosen according to the power consumed by the computer components, in particular by the CPU and the graphics card which are the most power-consuming components. A minimum PSU power to ensure a proper functioning is generally recommended by the manufacturers of these components. Be aware that the computer will not work properly if the PSU is not able to deliver the required power.
  • Another important characteristic is the efficiency of the power supply unit, which correspond to the ratio of the power delivered to the computer components to the power supplied by the mains. The greater the efficiency, the less energy is lost in the form of heat. The “80 Plus” certification program was introduced in 2004 to promote the PSU energy efficiency and it requires now the PSU to have at least 80% energy efficiency beyond a 20% load. Additional efficiency level certifications up to “80 Plus Titanium” have been introduced for even more efficient power supply units.
Name Efficiency at 20% load< Efficiency at 50% load Efficiency at 100% load
80 Plus 82% 85% 82%
80 Plus Bronze 85% 88% 85%
80 Plus Silver 87% 90% 87%
80 Plus Gold 90% 92% 89%
80 Plus Platinium 92% 94% 90%
80 Plus Titanium 94% 96% 94%
“80 Plus” Certification levels
  • Another characteristic is the electrical current in amperes (A) that the power supply unit can provide for different voltages. This information is usually indicated on the power supply case and is important for power hungry components such as high end graphics cards for example.
  • Power Supply Dimensions: Laptop power supply sizes vary a lot from model to model and there is no specific size. For desktop computers, there are two main standards: ATX or SFX. The ATX standard defines that the power supply must have a width of 150 mm (5.9 in), a height of 86 mm (3.4 in) but does not specify the depth. This is the most common PSU standard and it is used in medium or large computer cases. The SFX standard is intended for smaller cases, it specifies that the power supply unit must have a width of 125 mm (4.92 in), a height of 63.5 mm (2.5 in) and a depth of 100 mm (3.94 in). These two formats are therefore not compatible in size and you should check the specifications of the computer case before choosing a power supply unit.
  • Another characteristic is the modularity of the power supply unit. Some PSU are modular and allow to choose which cables are connected to the power supply unit, the power cables for hard disks, the motherboard or the graphics card for instance. Other PSU have a set of cables that are all attached to the power supply unit and may therefore require adapters to be used when they do not provide enough cables of a certain type.
  • Another characteristic is noise emitted by the power supply. The fan used to dissipate the heat of the PSU is the main culprit and some can become noisy under a heavy load. Some low power PSU come without a fan, they are called “passive” and are therefore silent.


Price and choice of a power supply unit

Laptop manufacturers usually provide an external PSU to power the laptop. Don’t forget to store it with your laptop or you may have to look for a spare one. Manufacturers often offer the option of acquiring a new one or compatible model for your laptop.

The power supply unit can sometimes be configured when purchasing a desktop computer. A basic PSU is often supplied with low-end computer cases but it is not included by default in high-end computer cases as it should be chosen according to the power requirements of the computer.

The most important factor for choosing a power supply unit is the power it can supply to the computer components, in particular to the most power-hungry ones such as the CPU or the graphics card. A high-end graphics card can consume 300W or more, and some high-end CPUs consume more than 200W when performing heavy tasks. The choice of the power supply unit should therefore consider the sum of the maximum consumption of the computer components and add a margin of at least 20% (remember that the power supply efficiency is never 100%). Therefore a computer with a high-end graphics card and CPU may need a power supply unit able to provide 750W or more. Conversely, a 300W PSU will certainly be enough for an office computer using an integrated graphics processing unit. In other situations, the silence of the computer will be privileged and a passive power supply will be used.

The prices of power supplies units vary from a few tens of euros or dollars for the most basic units to several hundred euros or dollars for high-end ones. It will depend on the quality of the components, the power and the level of the “80 plus” certification.

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