How to put a switch on a power supply?

[BrianG]

The rating of the switch to use depends on how you are doing it.

In JAILBIRD's case, he wants to use the "logic" method, which is the low voltage/current "enable" wire on many supplies. Simply using any SPST switch works here since you don't have to worry about current or voltage ratings. Larger/heavier switches can be used, but is just overkill.

The other method is what brijar describes, or what I call "power interrupter". As the name implies, the switch is placed inline of the input AC mains voltage, and so the ratings must be suitable. If the PS is the kind that uses 220/240v, make sure the switch can handle it. As he said, current ratings much beyond 15A is rather pointless, but keep in mind heavier/duty switches meant for heavier currents have better contacts and are less apt to pit from turn on spark (similar to how an ESC sparks when you hook up your batteries). Whatever you get, make sure it is rated for at least 50% more current than the PS uses. In the OP's 540w example, the input current is ~6A: ((540w/80% efficiency) / 120VAC. If using 220/240v mains, the current is even less at around 3A. For the former, use at least a 10A switch, for the latter, a 5A switch is fine.

FYI: Depending on availability, SPST (single pole single throw), DPST (double pole double throw), SPDT (single pole single throw), or DPDT (double pole double throw) can all work, but only SPST is needed. If using a DPxx type, parallel the connections for better current handling (double pole means there are two isolated/seperate SPxx switches in one package).