The design of latent heat thermal energy storage systems requires an accurate and comprehensive way to predict the phase change behaviour, considering the transient heat transfer during the melting process. This paper proposes the melting time liquid fraction method to characterize constant wall temperature melting experiments in rectangular enclosures. This method distinguishes itself from classical methods as the fitting coefficients are a discrete function of the liquid fraction, which implies a relation between liquid fraction and time. The method was validated experimentally for constant wall temperature melting experiments in a rectangular enclosure. Moreover, the method was compared to the classic characterization method. The melting time method systematically showed a lower root mean square deviation (RMSD) during the whole course of the melting process, with a maximum RMSD below 6%. This improved prediction follows mainly from the varying fitting constants, which capture the different heat transfer mechanisms during different melting stages more effectively than constant fitting parameters.