While the average user doesn't need a second power supply, power users and overclockers alike (that's us) tend to use gadgets that require a lot of power, much more than the ordinary power supply can handle. Fans, TEC units, water pumps (some of them), and more recently lights or neon are all things witch you can plug on your power supply. If you system require more power then your power supply can give, your system will be highly unstable.

But why not simply get a new, more powerful power supply? Well first of all you have to understand something. You have some parts of the system like the memory witch require 3.3v while others like hard drives require 12v and 5v. What really interests us here is the 12v line. Since most, if not all overclocking gadgets work on 12v, this is the voltage that you will need most.

A typical 250w power supply is able to put out 10amps at 12v. This means that out of 250w total, only about 120w is available through 12v (amps x volts = watts). This is exactly the reason why you can't plug a 120w peltier on a 250w power supply. When buying a bigger power supply, make sure it gives you enough power at 12v. The 300w power supply I have only gives 10amps at 12v, just like my 250w. The additional 50w is available for the 5v and 3.3v lines so it does not help me at all. Other 300w power supplies will give 12amps at 12v. The biggest power supply I have seen was rated at 450w and could supply 16amps at 12v, this gives you 192w of power at 12v but will cost you over 100$. What you could do instead is buy two cheap 250w power supplies and link them; this would give you about 240w of power at 12v for under $50. As you can see, multiple smaller power supplies will give you more power for less money.

As you probably know, an ATX power supply will not work if it is not plugged to a motherboard. Since you cannot plug two power supplies into a single motherboard, the first logical step would be to find a way to make a power supply confer power without being plugged to a motherboard.

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