Calculus...
Well POWER consumed in any period of time has no meaning. Energy consumed over a period of time does, it IS the power. Power at an instant is defined as the derivative of Energy with respect to time which means it possible to state the instanteous power consumption in the same way it is possible to state the instantaneous speed of a car, even though over zero time the car moves zero distance. So certainly the original poster is correct, the power being dissipated during the shot IS the sum of the laser and the normal consumption of the US, however it's not actually being drained from the grid, since it got stored up earlier over a longer period of time.