That's the part I'm confused about.
So after steps, the motor is released (not holding the torque). There are 450 steps in total which is stretched across about 4 hours or so. So essentially the motor is active for less than a second, 450 times across 4 hours. So at 12v, I should be good with 3Ah or so power supply, assuming the board itself will drain a lot over the hours.
@Billysugger: this motor shield doesn't seem to have any current limiter. Is there something I'm missing? And yea, in a hindsight, it should have been a smart servo motor. But I got this stepper for free!