I'm not sure. Although he didn't have a whole lot of time to react. Hence him not doing anything, like Force flight, to escape the reactor shaft. Perhaps he believed that his Lightning would stop Vader. I don't think it is confirmed if he turned up the intensity or not. It doesn't really matter though. Vader survived the Lightning blast anyway.
EDIT: I did some quick research on the Star Wars wiki and it didn't say whether or not his lightning increased in power or not. But it doesn't really matter. The fact remains that the Lightning didn't knock him back or kill him outright like many before him.
Hm, don't the more powerful blasts of lighting shown the victims skeleton? Did so with Mace and Anakin, so that could imply that he turned up the volume. But anyway, ya regardless Vader survived from the throne room all the way down to the hanger bay on a moon sized battle station.