I actually mentioned where the figure came from - warp core power generation figures, but apparently I didn't stress it heavily enough for all spectators.
The deflector beam weapon is stated to use the full maximum warp core output of the Enterprise, which - by the fact that the Enterprise can go to warp right next to a sun-like Star - is then around 100 GT/s. The total energy of the beam, then - recall that the beam is sustained for quite some time - is actually on the order of teratons, which fairly closely matches the ICS figures.
It could, by some evidence, be up to two orders of magnitude higher on the rate, but I find that rate of antimatter burn mildly implausible.
I finally found that posting on ST.Com. If you want a somewhat more conservative estimate; use the 12.75 million terawatt (three gigatons per second) power output of the E-D warpcore stated in "True Q". Given that the E-D fired it's deflector weapon for at least 30 seconds, that gives us a total equivalent delivered of 90 gigatons!
-Mike