According to reprap.org under the title "Choosing a Power Supply for your Reprap project" its written :
"5 stepper motors, each run at up to 1.2A @ 4V peak per phase = 5 * 4 * 1.2 = 24W, times 2 to allow (conservatively) for driver losses and back emf during movement = 48W"

So my main question is why multiplying by 2? why not 1.5 or maybe 3? is there a certain standard used here or what?

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is there a certain standard used here or what?

I doubt it. That feels more like a general electronics rule of thumb for allowing some headroom in the power supply. It would be the first factor I would have thought of when sizing a psu - a starting point against which you can check cost and form factor.

Edited 1 time(s). Last edit at 03/29/2016 08:33AM by JamesK.