Even though we run our mains at 230V here in the UK, some might wonder why electricity pylons operate at such incredibly high voltages (50kV+), while others might have been paying attention in physics class. The reason is that of efficiency - the higher the voltage, the less power is lost when travelling over distance.
Every power supply converts your mains supply into a selection of different voltages. Each of these voltages is called a rail. In older machines, the 5V rail was the rail of concern as this is what powered the CPU. However, as CPU consumption increased, efficiency became a concern. Borrowing from the same principle as the electricity pylons, it was decided to change the voltage to 12V. After this change, PSUs went from conforming to the ATX standard, to conforming to the ATX12V standard – a subtle but major difference. In a modern machine the processor, graphics, hard drive, optical drives and fans all run off 12V. The other rails are used for small components on the motherboard, system memory and essential functions such as turning on a machine in the first place.
The ATX12V 1.3 specification involves a single 12V rail. If a single rail is drawing over 32A, this can become potentially dangerous if the end user comes in contact with it. When the Prescott processor was introduced drawing huge amounts of current, Intel decided to change the specification to make running such a processor a little safer. They decided upon multiple 12V rails (ATX12V v2.2 and v2.3) as a method of supplying more power to the system in a safe manner. They also suggested that this should be no more than 20A per rail to make things a little safer. To enforce this, Intel also suggested an over-current protection that cuts the power should more than 20A be drawn. Other safety features include a thermal cut out should the power supply overheat and it also specifies that any failure should not cause a startling sound, excessive smoke, molten material or flames. These sound about as obvious as putting “may contain nuts” on a packet of dry roasted, but considering the number of power supplies that blew up in previous group tests, they obviously need emphasis.
You may be wondering how each 12V rail is distributed. Unfortunately, there is no real rule on this, but you can usually work it out depending on the colour of the wires used. Generally speaking 12V1 is the ATX connector and 12V2 is the four pin auxiliary CPU connector. 12V3/4/5/… could well be anything, from a PCI-E connector to just the SATA connectors. That is why the power available is just as important as the distribution of these rails.
So the question is, how much power do these units actually supply? The quoted wattage on packaging is a total across all the rails on the power supply. Wattage is worked out by multiplying voltage and amperage together. For instance 10A on a 12V rail is 120W. Totalling up all of these for each rail will be considerably higher than the total power output of the power supply.
For backwards compatibility purposes, the 5V rail is generally pretty beefy (150W+) so that older machines can run with them. However, in a modern machine the 5V rail is used to a much lesser extent, yet this power is included in the overall wattage calculation, somewhat skewing the quote. As well as the overall power limitation, specific areas also have limitations. For instance, a power supply may allow 23A on the first 12V rail, (known as 12V1), equating to 276W (23 x 12), but the combined limit for 3.3V, 5V and the 12V rails may be 280W. Considering you will use at least 40W across the other rails, it starts to eat in to the amount available for the 12V rails to use.
Another scenario, could be three 12V rails with 20A available on each, which is in theory 720W of power (12 x 20 x 3), but when combined you are not allowed any more than 40A or 480W. This seems a little like cheating, and it would be if they quoted 60A on the 12V rails. However, what this does mean is power can be distributed as necessary. One rail might only be using 5A meaning one of the other rails can take advantage of the full 20A allowed, without going over the limit.
Finally, another common trick is to quote peak figures, often achieved by using under-specced equipment with added airflow to compensate. These are about as useful as a flannel in a tsunami. What you want to study is the continuous output ratings.