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Thermochemical energy storage is promising for building applications as it offers high energy density and near-lossless storage. For example, inorganic salt hydrates that undergo reversible solid-gas thermochemical reactions can be used for thermal load shifting and/or shedding in buildings. However, this technology is still in early stages of development and drawbacks need to be addressed to make such a thermal battery viable. As salt hydrates differ in their morphology, crystal and/or particle size, and hygrothermal stability, it is critical to characterize thermochemical reactions accurately under specific operating conditions. Not only is the amount of heat delivered important, but so is the rate at which this heat is extracted for thermal end-use in buildings. However, the latter is not well reported in the literature, which is largely focused on energy storage rather than power density during the hydration reaction (battery discharge). To address this gap and the lack of standardized measurement methods, this work lays out a systematic simultaneous thermal analysis (STA) method for characterizing five different salt hydrate thermochemical materials (TCM). The effects of particle size, temperature and vapor pressure are analyzed to obtain the energy storage density and thermal power density across a full hydration-dehydration cycle under controlled conditions.