I ran into this after a fight, where the ship was pretty badly damaged. Probably hit one of the generators.
The ship had 16/21 energy being used. All the generators were manned and active and on the same grid. The UI listed the energy as +5 on the top of the screen for this particular ship.
Despite this, the batteries emptied and the ship ran out of power. Life support failed and started killing employees.
Restarting the generators seems to have fixed the calculation, which I did before screenshotting so I have no screen dump. The ship was still on 16/21, the top UI said +5, but the batteries filled up and everything worked again.