An energy based circuit parameter extraction method for composite right/left-handed (CRLH) leaky-wave antennas with a symmetric unit cell is introduced. This method computes the CRLH LCRG equivalent circuit parameters in terms of the stored energy and power loss obtained by a full-wave eigenmode analysis via even and odd current and voltage considerations. In this fashion, the unit cell is accurately characterized for the infinite periodic case, with full account of electromagnetic effects. Compared to previously reported driven mode extraction techniques, the proposed eigenmode method does not require the separation of the series and shunt parts of the cell along with de-embedding, it automatically accounts for mutual coupling between the periodic cells, and accurately determines the radiation parameters (resistance and conductance)to assess the antenna efficiency and bandwidth. Furthermore, the method provides physical insight into the radiation mechanisms of CRLH antennas.