Computer Power Supplies
A vital part of a computer, computer power supplies provide the very power your computer needs to turn on and run all of the parts inside. An inadequate power supply can cause crashes or prevent the computer from even turning on. Choosing a power supply is important and the choice boils down to two factors.
Your computer's 'overall wattage' is the amount of power required to run it. This can be calculated by hand yourself or via online calculators. It is always best to go over what you need in case you adjust other devices later that have a higher power requirement. It is always better to have too much power than too little, as your computer will limit the amount of power to what it needs. Extra power will not hurt your computer system or the CPU .
Rail specific power
This is how much power each individual component or device uses up from the power supply you have chosen. This isn't important in cheaper, less powerful computers, but in larger gaming computers that possess overclocking you should be careful about drawing too much. The instruction manual that comes with your graphics card and other accessories should contain this information. This is where aiming higher for the overall wattage comes in handy.
A power supply may have a 12V rail to draw power from or multiple rails, which are safer for your interior devices. 12V power supplies can easily protect them from there being too much power drawn from a single rail. Once again, this applies to only very high quality machines and for most computers does not make much of a difference.
Power supplies are rated for their efficiency, which basically looks at energy loss. The best ones are 80 Plus certified, whereas less high quality power supplies will waste more power.