Nematic elastomers undergo a phase change from an orientationally ordered nematic phase to a disordered isotropic phase when subjected to a temperature change. It is shown that such a phase change is both entropically and energetically driven. Isostrain data on a liquid crystal elastomer material is analyzed in terms of classical rubber elasticity theory. By separating the contributions to the total elastic restoring force into energetic and entropic components, the role of internal energy is assessed and shown to be significant.