How do I properly calculate the requirements for an on/off switch?
You need to figure out seven things:


What's the maximum total current (amps) I want to switch (including inrush if applicable)?
What is the maximum voltage I need to switch? 
How many lines (poles) do I need to switch? 1, 2, 3...
Do I need more than just on/off (more than two "throws")?
How do I want to mount the switch? PCB Mount, Panel Mount, etc.
How do I want to connect the switch? PCB Mount, Solder Tabs, Crimps.
What "style" of switch do I need? Lever, button, weather-proof, etc.


Then find the switch that exceeds BOTH 1 and 2 and meets conditions 3&ndash;7.