The use of nanocomposites phase change materials (PCMs) as transient passive thermal solution for high power electronics have been growing in the last few years. Typical application loads are in timescales of seconds, milliseconds, or nanoseconds; where the use of traditional cooling solutions, designed for steady state conduction/convection, are no longer effective. Common combinations for nanocomposites, with a PCM matrix and metal nanoparticle fillers, are Wax-Ag/Au or Sugars-Ag/Au. However, there is a lack of a performance parameter to guide the decision making during the co-design process. In this investigation we propose the formulation of an effectiveness parameter (index), in terms of response time and effective PCM volume selection, to assist the tradeoff analysis required for complex systems. The index is defined by the ratio of the PCM volume’s phase change time to the melting time. When heating a PCM, with a finite thickness, a melt front is formed as soon as the interface with the heat source begins melting while the heat source continues adding energy into the material. This melting front will achieve a constant velocity through the thickness owing to the high volumetric latent heat; therefore, the volume changes phase with an accompanying linear temperature increase. This non-isothermal phenomenon drives the definition of our proposed phase change energy term, or modified latent heat, for non-isothermal transient phase changing systems such as those encountered in pulsed power electronics. The calculated modified latent heat was validated, with a 1.68% difference, when compared to Field’s metal experimental data using a 12W heater and the Temperature-Energy diagram. Furthermore, a 0.39% difference was obtained between the calculated modified latent heat of organic PCM PT-58 and the experimental data with a 2W heater.