To get more energy in Magic, you must build more 'Energy Sources'. Each one supplies you with 14 energy.
I think that he meant the energy of units in a fight (Battlemages etc.), which runs out and units are not able to do any damage until they get it back. Is that right Vanquish?
I suggested adding a new feature to the Tower of Souls in
Megaglest, that would increase the speed of gaining new energy. If they implement it, you may consider playing MG instead Glest (which is already in many ways improved in comparsion with Glest).