power through scattering boundary condition

Hello all;

Im using scattering boundary condition as power source to illuminate my structure. Everything is Ok with that but I just don't know how to calculate the amount of power that is going through it. I mean by using ports you can define a specific amount of power trough them, but by scattering boundary condition you can define just the electric field and when I use regular formulas for calculating EM power and put the result as the amount of electric field the result is different, and it seems that its not that calculated amount of power. I would appreciate if somebody that knows about this problem could give some directions.

Thanks
Alireza