Orbital defense generator

An Orbital Defense Generator, or ODG, was a large groundside power plant used to provide the energy needed by Orbital Defense Platforms to fire their primary weapon.

Prior to the Battle of Reach, Reach had seven ODGs to power twenty ODPs. As demonstrated during the battle, ODGs were vulnerable to ground assault, and the failure of UNSC ground forces to protect the power plants and the resulting nullification of the planet's ODPs was a major factor in the battle's outcome.

Speculation
Although never explicitly elaborated upon, it seems likely that the means of power transmission from the ODGs on the ground to the ODPs in orbit was through a means similar to rectennas.