A computerised electromagnetic modelling methodology is described and applied, which side-steps difficulties in calculating equilibrium fields in cavities filled with complex distributions of diverse materials by focussing instead on the rate of power ingress to the cavities via slots and apertures. The energy entering a cavity within the reaction time of sensitive components may be used as a worst-case limit on the amount of energy available to perturb or damage such a component for the purposes of safety analyses and similar investigations.