Although the PTU will allow basic operation at input voltages down to 12 VDC, maximum speeds and torques will not be achievable. The motors are the major power draw in the system, so as the input voltage drops from 30 VDC, so does the current applied to the motors. The motors are connected to the power input (there is current limiting and low-voltage shutoff, but no voltage regulation). So if 30 volts is supplied to the PTU, then the motors see 30 volts; if 12 volts is supplied, then the motors see 12 volts. While not quite a V=IR relationship, the lower the voltage, the less current that the motor can draw. The power mode setting for the motors sets the max current. But if the voltage supplied can`t drive that current, then you don`t ever achieve max current.
So while the PTU can draw up to 60 watts in high power mode with 30 VDC input, it will not draw nearly as much power with a 12 volt input and torque/speed will be compromised.