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plankton
Alright, so back EMF is only affecting voltage, not current.
It doesn't affect the motor current (unless he back emf is too higher for the supply voltage to overcome), but it does affect the power drawn by the motor, and therefore the current drawn from the 24V supply.
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plankton
Agreed that for this estimate I probably would use the max motor rated current of 2A, rather than my expected current limit of 1.5A, so that gives 5.6W when stationary, what would be a reasonable multiplication factor to account for movement and back EMF? I'm not looking for an exact figure any more, when I first asked the question I expected to find a simple equation to say that a stepper will use x watts (peak) at a set voltage and current, clearly it's not as simple as that, so I'll be happy with just a ballpark figure with a reasonable safety margin built in.
I'm guessing here, but if you allow 10W per motor, I think that will be more than enough in practice.