This paper verifies the representation of power input and subsystem energy in terms of modal density of subsystems in Statistical Energy Analysis (SEA) by means of FEM calculations. First, vibration response in the modal form is used to derive the representations. It is shown that the utilization of values averaged over space and frequency for the power input and the subsystem energy of the subsystem can be represented in terms of the modal density. Furthermore, the coupling loss factor between plate subsystems is also analytically estimated by using the wave theory and is determined on the basis of the modal density. In order to verify these representations, FEM calculations are performed to compare subsystem energies, power inputs and coupling loss factors of three types of structures with different subsystem shapes and the same modal densities. The estimations obtained with analytical SEA are also compared with the FEM calculation results.