Tech Explained - what you need to know about PSUs

by HEXUS Staff on 6 March 2014, 09:00

Tags: Enermax (8093.TWO)

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What is a PSU?

Power Supply Units (PSUs) are an often overlooked part of a PC system build. While central processing units (CPUs), graphics processing units (GPUs) and solid-state drives (SSDs) regularly command more attention, it's hard to ignore the fact that the PSU is arguably your PC's most important bit of hardware. Why? Because a high-quality supply often outlasts other components and can be used in subsequent builds.


PSUs work by converting general-purpose alternating current (AC) available from the mains, which is 230V here in the UK, to low-voltage direct current (DC) power that's better-suited to your PC's hardware - generally 3.3V, 5V and 12V. That DC power is then routed to various PC components to give them the electrical juice required to run.

Sounds simple enough, but the importance of the PSU is such that its reliability and efficiency should not be overlooked. In this Tech Explained, we'll describe which features to look for when purchasing a new power supply, and we'll aim to bust a few myths along the way.

A brief history of the ATX PSU




ATX 2.1, announced in 2005, added dedicated 75W, 6-pin power connectors for power-hungry PCIe graphics cards, and was promptly followed by another minor upgrade - ATX 2.2 - that introduced 8-pin PCIe connectors capable of delivering a meatier serving of 150W for beefier graphics cards.

A recent revision, ATX 2.3, dates back to 2007 and requires all power supplies offer at least 70 per cent efficiency, with an optional recommendation of at least 80 per cent.

The need for efficiency

You've already heard the word efficiency mentioned in this Tech Explained, but exactly why is it so important for a PSU?

Let's look at a real-world example. If a power supply is rated as 70 per cent efficient, it would require 200W of AC power in order to create a 140W DC output that's fed to the components. The remaining 60W (30 per cent) is wasted in heat, and the heightened temperature can result in more noise as the PSU fan speeds up to keep the unit cool. And, as a general rule, higher temperatures are detrimental to long-term component longevity.

Using a similar example, a 90 per cent efficient power supply could produce the same 140W DC output by drawing only 155W of AC input - resulting in just 15W of waste. Less waste equals less heat produced, and of course lowers electricity bills. As a real-world example, requiring 200W DC, eight hours a day, five days a week, 52 weeks a year, would mean an 80 per cent efficient PSU would need an extra 58 kilowatts over a 90 per cent efficient supply. Priced at 20p per kilowatt-hour, that's a saving of over £10, which increases as PSU load is heightened. Run a coin-mining computer chewing through 1,000W and the saving jumps to a not inconsiderable £58 on a single PC. Enermax, for example, has an online calculator that works out potential cost based on load.

The higher the power-supply efficiency, the better, but it's important to note that efficiency can vary at different workloads. Using the efficiency graph of an Enermax Platimax 80 PLUS Platinum-rated power supply, below, we can see that the unit is most-efficient when a load of between 40 and 60 per cent is applied.

As a result of the reduced efficiency at low loads, it's important to match the power supply rating to the needs of your computer. For example, using an Enermax Platimax 1,000W PSU unit to power a system that typically consumes around 100W (representing an approximate 10 per cent load) would not be optimal, though such is the engineering excellence that, even at such low loads, efficiency is over 90 per cent.


How much power do I need?




Modern applications have put renewed focus on PSU capacity. An example of one is coin mining, where a bunch of graphics cards are housed in a single system and their general-purpose compute capability is used to digitally mine for coins that, at the moment, have a high resale value. The last few months have seen multiple AMD graphics cards shoehorned into a system that requires 1,000W-plus of DC power. Companies such as Enermax have numerous supplies that deal with this kind of continuous load with utmost stability.

What about multiple 12V rails?




Fortunately, that inappropriate layout is becoming a rarity, and most PSUs, such as the Enermax line, feature one 12V rail solely for PCIe connectors, and another for the PC's other components.




Here's a brief summary of the cables to look out for:


Modular or non-modular?



Unlike non-modular PSUs that have a grouping of cables connected to the unit at all times, a modular power supply (such as the Enermax Platimax 1,200W, pictured above) features an array of ports to which cables can be easily attached. A wide range of cables are typically bundled with the unit, and the cables are often available in different lengths - making it easier to choose the right cable for your chassis. Modular PSUs are undoubtedly neater, but be warned, they're normally more expensive than their non-modular counterparts.

Other considerations


EuP / ErP (Energy-using Products) is a European Union directive that "establishes a framework under which manufacturers of energy-using products will, at the design stage, be obliged to reduce the energy consumption and other negative environmental impacts that occur during the product’s life cycle". For personal computers, it aims to ensure that the total system power consumption does not exceed 1W when turned off or placed into standby. In order to meet the directive, you'll need to ensure your motherboard and PSU are EuP / ErP compliant.

With regards to efficiency, from July 1, 2014, all PSUs sold within the EU need to be at least 80 PLU Bronze efficient. This means having at least 85 per cent efficiency at 50 per cent load and at least 80 per cent efficiency at 20 per cent and 100 per cent load, respectively.

ATX PSUs typically measure 150mm x 86mm x 140mm in size, but the depth can vary depending on brand, and if you're working with a small-form-factor chassis, you'll be happy to hear that mini-ATX and micro-ATX, SFX and TFX power supplies are also available.

Firms such as Enermax have considerable experience in producing the smallest, most efficient supplies at a particular wattage. The powerful Platimax 1,500W PSU is just 180mm deep, compared to over 200m for rivals, and low-wattage models such as the Triathlor ECO (350W-600W) are just 140mm deep.

Technical considerations

Not all PSUs are created equal. The best supplies have low ripple (caused by the AC current being transformed into PC-compatible DC), low internal noise, high amperage on the key lines, and high-quality capacitors, preferably Japanese, that should last longer than cheaper ones. Newer supplies also feature what is known as a hybrid-fan mode, where, due to high efficiencies, the unit's fan doesn't even need to switch on until, say, 40 per cent of its capacity is reached.


Opting for a well-respected power-supply brand such as Enermax will stand you in good stead, and remember, always take the time to check reviews at reputable online publications.

A wide range of Enermax PSUs are available from and

For full disclosure, this Tech Explained is sponsored by Enermax.

HEXUS Forums :: 4 Comments

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What's the difference between bronze to platinum ratings on PSUs? A clear explanation of that is missing from the article IMHO.
The ratings refer to the “efficiency” of the PSU (i.e. how much of the electricity is used to provide your components with power, rather than being “wasted” as heat). You'd have to Google it, but basically each unit offers x % power efficiency at x % usage. So the better the efficiency, the higher the rating (starting from bronze to silver, gold and platinum). No idea what the ‘numbers are’ tho.
They may have figured it out in the intervening four years.
They may have figured it out in the intervening four years.

Couldn't have put it better myself! :)