Something I learnt from the Atomic mag.

Power Supply Unit (PSU) is one of the very important components in most PCs. It supplies all the juices that all the other parts need. The equation is simple: The more parts and the more powerful the parts, equals more power needed to keep the machine alive. So buying a good and efficient power supply is a must in building a stable machine.

Efficiency

Nowadays, PSU come in power ratings ranging from 400 Watts to 680 Watts (maybe higher). There are a few things to note when buying a PSU. First is to check the efficiency of the PSU. If a 500 Watts PSU is rated at 80% efficient, then it will need to draw 625 Watts from the mains to produce 500 Watts of power, the rest is dissipated as heat. Remember, more heat = less efficient. Therefore a higher efficiency PSU will save you on your bills and produce less heat.

Power Rails

The next more important factor is the power supplied through the Power Rails, namely 12V, 5V and 3.3V rails. 5V power things like Hard Disk Drives and DVD-ROM Drives while 3.3V feed parts like the PCI cards. The processor (CPU) and Graphics Processor Unit (GPU) draw power from the 12V rail. Current GPUs require a lot of power from the 12V. Similarily, overclocking will also up the comsumption. Most PSU have at least two 12V rails, however some do have just one high amperage 12V rail.

Amperage

Ampere is the rate at which charges pass a fixed point. Thus higher amperage = better. For instance, the latest nVidia 7800GTX needs at least 26A and 34A in SLI mode. So if you are going to get two cards running in SLI mode, do check if the PSU is “SLI-Ready”.

Conclusion

The bottomline when buying a right PSU is to know the type of machine you are going to build and what are you going to do with it. A Dual-Core plus SLI combo will suck more power, and same goes if you plan to overclock your machine.