I just stood on one side of a wall of light as 10 guards charged me. I just fired a bunch of bolts through the wall to activate it vaporizing them as they charged. One shot vaporized 3 at once . Also I think that either the guards are capable of turning off the wall or it runs out of energy after so many charges because it stopped before just after the last guy got there.

yeah, it seemed to be that if you drop an object into the wall of light it will deplete all its energy in a few seconds trying to vaporize it.

Its possible to change the depleted whale oil battery with a new on and get the wall of light running again though