In 1x05 - The Tell, Kate Argent uses an electrified baton as a taser, to knock Derek down. She claims the baton produces 900,000 volts, as though this were a rating of pain or ability incapacitate. In fact, amps are what damage people, volts are how quickly the electricity is delivered.

Watts = Amps x Volts

Picture electricity as water flowing through a pipe. The rate of flow of electricity through the pipe is amps. Electricity is moved through the pipe by electrical pressure, called volts. Both together produce watts, a measure of how much energy flows per second. What this means is that volts alone doesn't tell you how "powerful" something is electrically. 1 million volts at a very low amperage will still be weak. 

Effects

The shock from Kate's baton makes Derek shift back to human and largely incapacitates him. He writhes in pain, cannot stand, and can barely speak. It only takes a few minutes for him to recover enough from the shock to be able to run, however, so the effects are shortlived. Tasers, as opposed to stun guns, are specifically designed to disrupt the body's ability to send signals to your muscles, globally. A stun gun delivers pain to the particular area it touches, but it does not disrupt the delivery of signals from the brain.