Sometimes the lighting spell does 20 points of damage, more often it does 1. I've been running around chased by the moth monster while it does 1 point of damage repeatedly. What determines damage? the amount of mana I have, the distance to the target, how long I stand still?
You've probably figured this out by now, but the Lightning and Energy Bolt spells do rapid damage over time and break if you move. As you hold the beam, it does "ticks" of damage, with increases in the level of the spell increasing the damage per tick. As long as you have mana, you can pretty much hold it until the enemy dies or gets close enough to kill you. Lightning has the added benefit of chaining to other enemies, while Energy Bolt does strong damage to a single enemy.