I think that the variable you want is the convective heat loss between the exterior wall and the interior. From my understanding, heat loss through exterior walls in EnergyPlus is calculated as convective losses between the interior air space and the wall. The wall interior surface temperature is calculated based on the wall construction and interior and exterior conditions.
I have 5 walls exposed to outside. I want to know the amount of heat loss through them in total. But, in Output:Variable there isn't any variable for calculating the mentioned amount.