Lightning will likely damage your Energizer if it strikes your electric fence. Gallagher Energizers have internal lightning diverters that give partial protection against small strikes.
Since lightning always finds the easiest way to ground, installing a Lightning Diverter will give added protection by providing a path for any lightning that strikes the fence to be diverted to the ground.
All permanent Energizers should be fitted with a lightning diverter to help protect from lightning damage.
Otherwise disconnect the Energizer from the fence and power supply during lightning storms.
Using the diverter does not guarantee complete protection. In bad lightning areas, Grounding the top fence wire helps significantly by encouraging the lightning get to ground without passing to the Energizer.