"This portentous structure reaches toward the sky, capturing the energy of lightning for the colony. In addition, it extends the power supply, acting as a huge Tesla Tower."
The Lightning Spire provides 800 energy, which is roughly equivalent to 27 Mills or 16 Advanced Mills. It also create an energy transfer radius within 30 cells. It costs 60 workers and 100 gold to maintain.
The Lightning Spire is the most space-efficient energy source in the game.
However, it's also the second biggest security liability in the colony after Command Center itself. If it gets infected, it will immediately create a 800 energy deficit which will be difficult to compensate. The colony may survive loss of The Crystal Palace or The Academy of Immortals, but losing Spire will usually disable all of its defences, therefore, Lightning Spire should be always placed as far away from colony borders as possible.
Having a large power range, The Lightning Spire can sometimes spread energy range through impassable terrain, like forests or water.
Note that a glitch may occur if Tesla Towers that provides energy radius to The Lightning Spire are destroyed, which can make its energy radius "blink".