Energy torpedoes are not truly torpedoes, regardless of the name. They are small, rapid firing, powerful balls of quickly decaying plasma energy that can be highly destructive. They have two major drawbacks however. First, the rapidly decaying nature of the containment field means that the maximum effective range of this weapon is under 4 LM (6 hexes). The second is that energy torpedoes are completely ineffective versus a shielded target. The containment field is unable to penetrate the electromagnetic shields, and "fizzles" when it hits. Essentially, the shields disperse the energy of the containment field and plasma without ill-effect.
Energy torpedoes use the beam weapons chart to hit, and the following chart for damage.