Power supplies need to be able to power your entire system which can be as simple or as complicated as you like. High end systems can draw 500W+ of power, which requires an good power supply manufacturer.
Of recent, multi output power supplies have been released which split their total ampage over several rails. This stops any noise from connected components (for example, the motherboard and graphics card(s) are on a separate circuit) this generally means that the rails are a little more stable.
Amps (A) are a measurement of the amount of electrical charge per second. Watts (W) are joules per second, while Volts (V) are electrical potential difference.
The watts of a power supply are worked out by all the volt rails x the amps per rail.
For example, the 12V rail may have a rated ampage of 10A, and so pumps 120W.
Components are often rated in watts. For example, a graphics card could pull 180W of power, which equates to 15A on the 12V rail. It’s possible to work out the exact peak amount of power that your PC uses and buy a respective power supply to deal with the load. A switching power supply rated as 500W would be able to deal with a PC which uses at its maximum (peak) 500W. While this is technically true, most people do not work out the peak wattage and it’s advisable to buy a power supply that provides more than enough watts, to allow for future upgrades and simplicity.
If a power supply is rated as 240W and only had a 12v rail (in actual fact they have several different voltage rails) then you could either have a single 20A rail, or two 10A rails. While this means that the current is split and you have two separate rails, you also have more transformers to step down the voltage, and you can’t have a single device pulling more than 10A.
Power supply - single rail
In a single 12V rail power supply, a single device can take up the entire rated ampage. You could run, say, an 11A graphics card on a single 20A power supply rail, but not on a twin 10A rail one. This gives weight to the argument of only have one rail. If the graphics card idle’s using 5A yet can peak to using 11A, you will get system instability if you used this card on the twin rail system in this example; the single rail unit wouldn’t suffer from this.
Usually, running a multi rail system will leave power ‘wasted’ on each rail. For example, say there are 4 separate 12V rails, each sending 10A (120W each rail). If there is a graphics card on each rail which pulls 8A then 2A (8A overall) isn’t being used. If one of these cards peaks at 11A, while the power supply can provide enough power, there aren’t enough amps on that particular rail.
However, a single rail with a lot of amps running through it requires careful monitoring and thicker cables. The more power running through a single transformer in the power supply means more heat in one area, rather than two smaller transformers. A sufficient cooling system must be installed to cope with the heat.
Power supplies split their rated wattage between their respective rails. Most of the amps go to the 12V rail, then either the 5V or 3.3V rail. The 12V rail is the most widely used and is key for a hard disc, or a graphics card to run for example.