AFAIK the *time* for which the electric current is flowing in the body is important. So, you can get away with a pretty low voltage as long as you can maintain it, and you don't want to send it through the air. If you do want to send it through the air, then you would need a very high voltage which would change with the range of the lightning.
An additional problem is that if you try to send lightning through the air, the lightning will simply hit the ground unless the person is closer to your hands then the ground is. You could simply let the have more control over the lightning but that would imply that there are much easier ways of killing someone with their powers then shooting lightning at them.
Oh and, lightning can't do anything to metal armors or the person inside it, unless you make it so strong that the metal heats up enough to kill the person inside.