As anyone who has ever suffered at the hands of a cheap power supply knows, all PSUs aren’t created equal. Whereas once upon a time all you needed to know was what wattage your system demanded, nowadays the explosion in brands and types of power supplies makes the decision a little less clearer than it once was.
Part of this is the myriad possible uses for a power supply. Someone wanting to pack components into a cool and quiet Mini-ITX or Micro-ATX case will have very different needs to an overclocker wanting stability when running under liquid nitrogen. In between there are a surprising number of usage models, often with PSUs tailor made for them.
Before we get into the complexities, it is good to understand the basic way in which a PSU works. Modern switching power supplies are designed to convert the high voltage alternating current of the power grid to lower voltage direct current needed by PC components. While the grid in Australia supplies 240V of power, PCs use much lower voltages (12V and 5V is the most common), which necessitate the need for separate power supplies.
This power is then delivered to the components via the assorted wiring coming out of the unit. When talking about power supply in this manner, the commonly used terminology can be a bit confusing. Each voltage is supplied via a ‘rail’, which takes power from a transformer and distributes it to the wires. In a basic setup each voltage comes from its own dedicated rail, and is then spread out across the wires that need it.
When PCs were first taking off, the majority of power draw was over the 5V rail, but the addition of 12V motherboard connectors with the ATX12V specification used for the Pentium 4 marked a move towards 12V as the dominant voltage within a PC. This was reinforced by the rise of PCI-Express graphics cards, which use 12V connectors when they need more power than the motherboard can supply.
Because of this the 12V rail has become key to PSU performance, especially when using multi-GPU or overclocked systems. But it isn’t just the amount of power that can be delivered that is the true test of a PSU, it is the way in which it supplies the power.
At its simplest a PSU will have a transformer for each required voltage. However, a growing number of models features multiple 12V rails in order to support high end systems and multiple GPUs. This stemmed from an update to the ATX specification a few years ago, which sought to limit the amount of power deliverable on a single 12V wire to 20 Amps.
These can involve multiple transformers, but generally it means that a single transformer is connected to multiple bits of regulation circuitry. This power is then spread out across wires, but it acts to ensure higher-capacity, reliable power delivery without the dangers inherent in pushing everything across one wire.
80 plus watt
As we mentioned before, power supplies have historically been marketed on wattage and little else. This has led to very little standardisation between brands – cheap power supplies will often quote irrelevant numbers based on 5V delivery, while some high-end brands have multiple different models with the same wattage rating.
One of the best ways to differentiate between these models is through the 80 plus certification scheme. This is a third party initiative that classifies power supplies based upon their efficiencies. In this scheme, PSUs are given a rating based on their power conversion efficiency at different loads. In order to get the base level 80 Plus certification, a Power Supply must convert 80% of incoming power into usable supply (with the remaining 20% lost as heat).
There are then four more levels of 80 Plus certification for consumer power supplies, each having a higher efficiency requirement up to 80 Plus Platinum, where a PSU must be able to maintain 92% efficiency at 50% load and 89% at 100%. The good thing about these ratings is that they are wattage independent – instead they look at how well the power supply works. This provides an easy metric to let you work out what you need.
While 80 Plus level is a good way to judge a power supply, some manufacturers actually go above and beyond that. Standardised 80 Plus testing is done at 23 degrees, which is a rough estimation of room temperature, but considered a little on the low side. Some manufacturers will actually do their efficiency testing at more realistic temperatures (usually 40 or 50 degrees), which will usually be pointed out on the packaging or website.
Working out just how much power your system needs can be a bit of a guessing game. While the temptation is there to go for the highest wattage possible, the reality is that lower end hardware will be unable to draw enough power to keep a high wattage supply running smoothly. To this end the rule of thumb is that you want to look at the total 12V draw of your components and work off this. Build in 30-40% of headroom and you should have a good idea of just where your needs will lie.
If you are using a low power processor like AMD’s A series APUs, you’ll likely only want something in the 400 Watt range, while those running high end CPUs and multiple GPUs will need to look at much higher Wattages. For most uses an 80 Plus Bronze power supply will be fine, although overclockers and power users will likely want to go for a higher efficiency model in order to ensure smooth delivery.
This also means you stay away from cheap power supplies. While it may be very tempting to only spend $40 on one, the odds are that your experience will mean that you’ll end up having to replace it prematurely. This not only means added cost, but also the hassle of disentangling your power supply from your system.
Making a decision
Because of the relatively few standards used by power supply manufacturers, making the right decision involves a bit of research. For most PCs a mid-range supply will be fine, but if you have particularly heavy-drawing items then you’ll need to pay more attention. This generally means high-end or multiple graphics cards, but it also means large numbers of hard drives. Overclocking and enthusiast CPUs will also require more, and you should be able to get a rough idea of power draw from specifications for various products.
By not only getting enough wattage, but also getting a good quality brand, you’ll won’t need to worry about your power supply for some time. Having a little overhead in your choice is good if you plan on upgrades over the coming years, but don’t get too worried about futureproofing your choice.