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Power Supply Buying Guide

System failure and instability?  We all used to love blaming Windows, but poor power supplies are often the cause. Whether you’re solving issues, upgrading or buying a complete new rig – KitGuru investigates the dark art of PSUs for you.

Why do we need a PSU?
Your home is probably wired to deliver more than 7,000 watts (7kw) along 2.5mm twin core cables in your walls. Even the most demanding PCs rarely use more than 1kW.  For computers to work, we need to use a good quality power supply unit (PSU) to convert the high AC voltage of your house's power grid to a usable, low voltage, DC suitable for your system’s components.

Technology Factors Affecting Choice
The PSU you choose depends on the number/nature of components used in your PC. For example, different graphics solutions can have different connectors. Some graphics draw all the juice they need from the PCI-Express slot, while power-hungry cards like Fermi also need a 6-in AND 8-pin connector. If you dislike noise, you ought to check the acoustics and thermal performance of the unit you plan to purchase.

Some products offer unique features (such as Nesteq's EECS cable management system) which you might find interesting for the application you have in mind.

Also, some situations might require you to look for specific features. For example, if you are using a cheaper Uninterruptable Power Supply (UPS), you need to ensure that the PSUs hold-up time (measured in milliseconds) is longer than the switchover time for the UPS. You can see the ‘hold up time’ in action on your notebook PSU, when the green indicator light stays bright for a while after the wall socket has been turned off.

I’ve heard of rails, how many do I need?
Now this is a proper marketing war, over a simple difference in design.
Overall, units with multiple 12V rails are considered to be ‘safer’, while units with a single large 12V line are preferred by enthusiasts because they negate the possibility of a weak 12V line getting overloaded by a single power hungry device.
Let’s say this nice and clearly, to avoid misunderstanding…  High-end PSUs, from quality vendors, that use multiple 12v lines – are created in such a way that all of these lines exceed the ATX safety specifications.

Units with single and very powerful 12V lines are [said to be – Ed] dangerous, because people try to imagine a catastrophic event which might force most of the power in the circuit being channelled through a single connector, in which case the cable would surely melt – possibly even starting a fire. The example normally quoted is when a faulty component doesn’t cause a short circuit. Quality manufacturers integrate several safety methods to ensure this does not happen, which is why such catastrophic events are extremely rare (although there is an interesting story about a French journalist who ended up in hospital while testing a ‘famous brand’ PSU – or so the story goes).

Efficiency: How much of my expensive juice will get wasted?
Typically, if a supply loses less than 20% of the total power consumed, then it is considered ‘efficient’.  Look for the ‘80Plus’ sticker on modern PSUs. And remember; the more efficient a power supply is, the less energy will be lost as heat; this means that not only do efficient PSUs run cheaper, they also tend to operate quieter. There are five 80Plus levels:-

80 PLUS Test Type 115V Internal Non-Redundant 230V Internal Redundant
Fraction of Rated Load 20% 50% 100% 20% 50% 100%
80 PLUS 80% 80% 80% Not defined
80 PLUS Bronze 82% 85% 82% 81% 85% 81%
80 PLUS Silver 85% 88% 85% 85% 89% 85%
80 PLUS Gold 87% 90% 87% 88% 92% 88%
80 PLUS Platinum Not defined 90% 94% 91%

So how much power do you need?

Switching PSUs are designed to continuously work at 50-60% of their rated capacity; not at their maximum load. Having a power supply work very heavily loaded for prolonged periods of time not only will have it operating at lower-than-optimal efficiency, but it can also be disastrous for its lifespan. Be aware all you hardcore folders out there!

Taking these issues into account, a lot of ‘online wattage calculators’ tend to double the actual power consumption of your system when recommending a power supply. For example, they tend to add up the maximum thermal power design (TPD) for your components, just to be ‘on the safe side’, even if you almost never hit that level in everyday use.

Inefficiency happens at both ends of the scale
If you massively over-specify a PSU, then you will also incur a hit on efficiency. It’s important to choose the right wattage.

While power users might have 2 or more high-end graphic cards in their rig, most systems are much less power hungry. Here are 3 typical usage scenarios:-

Graphics CPU Other Total PSU
Power Fiend 250w 100w 80w 430w ≈850w
Normal Gamer 120w 80w 60w 260w ≈500w
Jo Average 50w 50w 60w 160w ≈350w

The price of a PSU can vary from £10 to well over £200 and there are a huge number to choose from.
You also need to decide if you need a ‘modular’ PSU, one where you only plug in the cables you need.
Having this option costs a little more, but can mean better airflow, quicker build times and easier upgrading.

Quality manufacturers
In reality, most of the PSUs you can buy will have been assembled in one of a small number of factories. However, each will have different components, features, noise/efficiency levels and warranty – so it is worth looking for a reputable brand, including (alphabetically)… Antec, BeQuiet, Coolermaster, Corsair, Enermax, FSP, OCZ, Seasonic and Thermaltake.

What does KitGuru like?
For PSUs up to 600w, KitGuru likes OCZ’s XStream and Antec’s EarthWatts series.
From 600w to 775w, there are great choices from Coolermaster (Silent Pro), Thermaltake (Toughpower XT) and Corsair (TX).
After 850w it is between BeQuiet, Corsair and Enermax. At the ultra high-end, Thermaltake has been creating 1500w PSUs for many years and Antec now has some strong designs in this area.

Buy links…


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