This paper presents three approximate solutions for the energy density distribution in plates subject to harmonic excitations. These solutions are obtained considering a plane wave approximation in the energy flow equation in plates. Galerkin, least-squares, and Ritz methodsare used to solve this equation. The energy density distribution is analyzed in simply supported square plates of aluminum with 1 m in length and 1 mm in thickness, respectively.Two excitation frequencies (239 Hz and 487 Hz) and four values of loss factors (0.01, 0.05, 0.10, and 0.20) in plates are considered. The results obtained using the approximate solutions agree well with exact solutions reported in the literature with a relative error lower than 10%. In addition, the proposed solutions are simple and easy to use in the prediction of the approximate-energy density distribution in plates.