Everything you need to know about power supplies

Each DC output is known as a ‘rail’, and PC PSUs are required to deliver a number of rails: +3.3v, +5v, +5VSB (standby – active as long as there is current from the wall, lets you turn your PC on from the front button), -5v, -12v, and the ever important +12v.

When it comes to buying a PSU the main stats you’ll see advertised (on the box and the PSU itself) are the total wattage and how this is divided among the various rails. The output of a particular rail is determined by multiplying the voltage of the rail by the amps – for example, +5v at 30A would be 150W.

The eagle eyed among you may notice that, depending on your PSU, the values don’t always add up – you might see the +5v rail at 30A and +3.3v rail at 24A given as a total of 170W. But if you do the math, it should be around 230W.

What’s going on? Even though the two rails have a maximum of 30A and 24A respectively, the total output power is 170W between them – meaning one or the other could be maxed out at a time, but not both at once. Usually this isn’t a problem; you’ll often find the +3.3v and +5v rails aggregated this way (because the +3.3v is actually drawn down from the +5v source), and they provide enough power for their given duties.

Note that once upon a time CPUs used to run from the +5v rail, but these days they use +12v. Any motors also draw from +12v – so fans, hard drives, optical drives, pumps and so on – as well as your GPUs. In fact, most of your PC’s load, as much as 90 per cent, comes from the +12v source. This is one reason it’s good to look at the distribution of power on a PSU’s rating – it doesn’t matter that it’s a 700W beast if only 400W is on the +12v rail and you need more than this to satiate your gear.