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What is a Power Supply Unit (PSU)?

David Benyon, ITAD Testing Operative Oct 14, 2019

A Power Supply Unit (PSU) is an internal hardware component that supplies components within a computer with power. Specifically, a power supply converts the alternating high voltage current (AC) into direct current (DC), and they also regulate the DC output voltage to the fine tolerances required for modern computing components. Most power supplies are switched-mode (SMPS), which has both efficiency advantages and makes designing for multiple voltage inputs easier. This means that most PSUs can operate in different countries where the power input might change. For instance, in the USA the voltage is 120V 60Hz, whereas in the UK it is 240V 50Hz.


When do I need a PSU?

The power supply unit is a crucial part of any IT system. Without it, your device would not work. It’s no surprise, then, that most systems include a power supply upon purchase. There is an alternative to a PSU that can be used in some instances, however. By choosing Power over Ethernet (PoE), electrical power can be carried within network cables without being tethered to an electrical outlet. This is ideal for systems that need more flexibility; PoE can provide wireless access points to wherever is most convenient, and less space is taken up by wiring.


How do I choose the right PSU for my system?

Firstly, when choosing a power supply unit, it is important to make sure it is compatible with the form factor of your server case and motherboard. This will ensure it fits within your system. Secondly, wattage is an important factor to consider. The higher the wattage rating, the more power the unit can provide to your system, meaning that you need to evaluate how much power your components require to run effectively. For instance, if the components in your system require 600V, it would be ideal to buy a 1200V power supply unit, as most power supplies have the highest efficiency at ≈50% load. This also allows for room to expand your system with further components if needed.


Lastly, when replacing or upgrading a PC power supply unit, it is important to take brands into consideration. Popular brands for power supplies include Corsair, Antec, EVGA and Seasonic. The choice often comes down to personal preference, compatibility with your system, and what you are using the power supply for (e.g. gaming, a small or large business, or personal use). One piece of advice is to look out for an 80 Plus Platinum rating, as this has great energy efficiency and can minimise power costs.


How efficient should my power supply be?

80 Plus power supplies have a scale of efficiency, going from 80 Plus and 80 Plus Bronze all the way to Titanium. ‘80 Plus’ means that power supplies in this range will always operate at 80% efficiency at a minimum, and as you go up the scale towards 80 Plus Platinum and Titanium, you can experience up to 94% efficiency (when at 50% load). The latest 80 Plus PSUs require high wattage in order to run most effectively, and so the 80 Plus Gold, Platinum and Titanium supplies (up to 94%) are ideal for large data centres. 80 Plus Silver power supplies and below (a maximum efficiency of 88%), however, are more suitable for PCs and desktops. It is important to remember that the difference between a 90% efficiency rating and a 92% efficiency rating will make a massive difference in terms of energy utilised within large scale data centres.


Do I need more than one PSU?

In short, a server will always need two power supplies. There are different modes of operation for this, depending on how much redundancy you need in your system. One option is to have a fully redundant power supply system, which means that one PSU is always switched off and there is an emergency fall back in case of downtime. The other option is to have shared power supplies, where both are on at the same time and share the workload.


For maximum redundancy, it is also a good idea to have an Uninterrupted Power Supply (UPS), which enables your computer to run for a limited time if power is lost. There are three types: online, offline and line-interactive. Online Uninterrupted Power Supplies ensure the quality of power remains constant, whereas offline UPSs start running when power is lost and there will be a slight delay when it takes over. Line-interactive is a combination of the two and provides more power protection thanks to its line conditioning.


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