And finally, there’s the power supply itself.  If your printer is a 12V printer, using a 12V electronics package like RAMPS, you need a 12V power supply.  The amp rating should be greater than the sum of your connected items.  For example, a heated bed can take 11 amps at 12V.  Stepper motors can take an amp or more each.  The hot-end can take 2-3 amps.  Any LCD panel, LED lighting, fans, etc… can all take additional current.  So I generally recommend a 12V 30A power supply for 12V 3D printers.  24V is becoming a more common option on 3D printers, so as time goes on, you should at least consider thinking about looking into a 24V system.  Here is a typical power supply: