How to calculate stepper motor power?
The power absorbed by stepper motors varies depending on the state of the motor and driver.

When holding position using full step or half step, the power will be governed by: 

\$P = I^2R * Coils\$  

where


\$I\$ is the holding current
\$R\$ is the DC resistance of an active coil, and
\$Coils\$ is the number of coils active.


If you are holding a micro-stepped position then it gets more complicated since you need to calculate the impedance of the coils at the PWM frequencies and do the appropriate math for whatever coils are on and off with the duty cycle involved.

Similarly, while being driven at some speed, the impedance of the coils at the applied step rate also needs to be calculated. 

At higher speeds where the step time is less than the time constant of the coils the calculation gets more complicated and the power absorbed actually drops off markedly as it becomes harder and harder to establish the drive current.