How to calculate motor parts and energy requirements?
The DC motor torque is proportional to current which results in acceleration unless opposed by a spring whose torque increases with deflection.  

The result is the calibrated deflection = a calibrated current.   When added with a scaled series resistor, then it measures voltage. 

The analog meter is often measured in kΩ/V.  Large meters were 50kΩ/V while small meters were 1kΩ/V.

BLDC motors use a Hall magnetic field detector to sense a position used to commutate the coil current with electronic toggled power switches called a "Half &amp; Full bridge driver". There are many different types using FETs.

The rules for ammeters are similar to motors with variables like magnetic field strength, number of turns, diameter and gap between the coil and magnet, as discussed in white paper.

http://www.ni.com/white-paper/14921/en/

As far as using solar power of tiny photo diode (PD) to drive a tiny motor, I recall a small 5mm PD response of 0.5mA per mW of optical power, thus the power in the motor cannot exceed this.