Everything you need to know about power supplies

How much power do you need?
There are a number of power calculators you can use online, but keep in mind they’re only a guide. To give you a rough idea, however, an overclocked dual-core SLI system with a couple of drives might use around 450-500W at peak, and 250-300W at idle. If this sounds like you, and allowing for a minimum 30-40 per cent headroom, a 700-750W would be sufficient.

Is bigger better?
In our culture the tendency is to view bigger as better and PSUs are no different – we now have 1200W+ PSUs on the market. In truth, however, most enthusiasts – let alone normal people – would have a pretty hard time trying to load these babies. In other words, these PSUs are more for e-peen value than actual functionality, with one exception: efficiency on load. A 1000W PSU should, for example, be able to deliver 600W without skipping a beat, ensuring that its efficiency remains high under load and doesn’t heat up too much. Overall, however, you don’t need the beefiest PSU you can find. Even a quad-core three-way-SLI system can get away with an 800W PSU nicely.

Single or multiple +12v?
The ATX specification states that no more than 240W can pass through a single wire, as this much power could cause enough heat to melt the insulation and possibly start a fire.

Contrary to what you might think, bar very high end (1000W+) PSUs that actually do have two or more seperate +12v sources, most PSUs with multiple rails draw from the same +12v source. That is, when you see ‘+12v1, +12v2, +12v3’ on your 850W PSU it’s actually a single +12v rail split into ‘virtual’ rails, all in the name of safety. These ‘rails’ are simply limited connections that cap the maximum draw through any one rail.

So how does a single rail +12v PSU with an output more than 240W on the +12v rail get away with it? Herein lies much of the debate: if you think about it, both single and virtual rail PSUs all still draw from the same single +12v source, and all use multiple leads with various connectors. In other words, the distribution of power is inherently limited by the cabling – a single PCI-E 6-pin cable for your GPU, for example, isn’t going to be supplying more than the GPU demands (which is far less than 240W), and loading up a SATA cable will use even less. Across your whole machine you may use 500W or more, but no set of cable individually carries this much. It’s not surprising, then, that many high end PSUs, even with multiple +12v rails, sometimes list output power above 240W for a given rail.

Additionally, there’s one argument in favour of a single +12v rail: the maximum output power is available to use, whereas with a multiple rail configuration you can ‘lose’ output power from the headroom of each rail that doesn’t get used (you’re limited by what connectors are attached to each ‘virtual’ rail).

So the argument basically comes down to flexibility; a single +12v rail is more flexible for whatever your system demands, though technically a multiple ‘virtual’ +12v rail PSU is supposed to be safer.

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