A concise power law reaction kinetic expression of reversible crystallization phase change was presented for use in global modelling of crystallization and nucleation kinetics of solid-liquid phase change materials (PCMs). This was developed as a closed form, wholly mechanistic expression of phase change kinetics essential to predict coupled heat-mass latent heat evolution in PCMs. This constituted a significant departure from the semi-empirical and phenomenological formulations that have so far dominated PCM sciences. Notably, the presented formulation expresses mass supersaturation as the fundamental driving force for phase change as opposed to relying on supercooling degree or temperature rate. In contrast to practice in the industrial crystallization process industry, it was postulated that crystal size population balances could be neglected in PCMs used for thermal storage due to averaging effects of crystal size population balances since the rates of latent heat evolution and heat transport are exclusively prioritized rather than yield of a desired crystal size.