If you have 0 energy you are disabled. If you take damage when you are disabled, you die.
i thought this was funny and Padawan could still move -- Gorded
If the energy is lower than 0.1 but more than 0.0, it shows 0.0 but you can still move. It is wise not to shoot in such a case, as the minimum power of a bullet is 0.1, meaning you get disabled immediately. -- GrubbmGait