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Specs

Model - 17PM-K402V

6.00V 0.80A/phse 7.50 Ohms 3,400 g cm torque 7.0 mH 75.0 rotor inertia (g-cm^2) 200 detent torque 350 g

At 6V, the stepper will have V=IR=6=I*7.5 - so I=0.8A per phase. Period. Power is IV = 0.8*6V=4.8W. Per phase. Period.
Note power is double: there are 2 phases. So 9.6Watts. but only 1 phase is on at a time, unless there is microstepping, in which case there will be much overlap of phases and near 9.6 watts albeit with lower power.
Note power is not determined by provided voltage - such can be 24V or significantly higher than rated voltage. The Power formula refers to the voltage that is actually used, not that which is provided. Provided and used voltage are different with stepper motors, while these 2 powers are the same in non-electronically controlled motors.
Stepper motors are electronically controlled by turning on phases at the required timing.
By limiting current, we can alsoreduce power use.