I would assume that the activation of the defense system would require at least some decent power draw from the vehicles' power plant.... Possibly enough to require activation of the system for certain periods of time.
From my understanding, the electric field generated when an incoming round is fired, would create a current in the ammo (as all ammo is conductive metal), causing it to lose kinetic energy, and heat up prior to hitting the armor. You *could* nullify this with non-conductive ammo, but unfortunately, the ballistics of such rounds are not usable in military scenarios. I guess the only way to overcome such a force field would be sheer brute strength, if it really works as advertised. ....
Could you imagine a scenario with dozens of terrorists shooting at a Bradley, only for it to come out of a fight unscathed? 
Back from the dead, I'm afraid.
