Here's another idea for mobile units (and other units that take energy).
Have an "Idle" command. Once a unit is idled, it takes no more energy. However, it takes a certain amount of time to de-idle it. Maybe 25% of the build time, with a minimum of some reasonable amount of time (no more than minutes)? During the de-idling, you could have it use 2x (or Nx) its usual energy as a "powering up" draw.