Thermal energy storage via latent heat provides an energy dense solution and, during discharge, a hot stream at a well-defined temperature. Utilizing metal phase change materials (PCMs) and high-temperature operation might enable fast (dis)charging and application in previously unexplored areas (e.g. high-temperature processing industry). But detailed characterization and design guidelines of high-temperature metal latent heat storage systems with tailored characteristics are missing. Here, we introduce a combined experimental-numerical characterization and design tool for high-temperature heat storage units. We apply it to a megajoule-scale test-bed operated with steel encapsulated Al68.5Cu26.5Si5 (weight%) (melting temperature 791 K) as the PCM and air as heat transfer fluid (HTF) for charging and discharging. Thermocouples positioned at multiple locations throughout the storage unit are used to characterize the performance and heterogeneity. For the inaugural experimental campaign, three encapsulated tubes with a total of 1.83 kg PCM were vertically arranged in one of the modular stacks and several multi-hour charging/discharging cycles were performed with the HTF heated up to 30 - 70 K above and below the PCM melting point at a flow rate of 1500 lmin(-1). Specifically designed open-cell cellular Si-SiC lattices were wrapped around the PCM tubes to enhance the heat transfer. The experimental results showed a near-isothermal (+/- 10 K) exit temperature discharge maintained for 0.39 2.67 h. Temperature variations throughout the test-bed indicated that test-bed design is important to ensure homogeneous usage of the PCM and controlled phase change. A quasi-1D lumped parameter multi-mode heat transfer model was developed to quantify the thermal stratification within the PCM, heat losses, and phase change characteristics in the thermal storage, and provided explanation and quantification for the experimentally observed heterogeneities. The model was used to extrapolate the heat storage characteristics to larger scales: a 16.84 MJ and 50.51 MJ latent heat storage with the modular stacks filled with 25.7 kg (42 encapsulated tubes) and 77 kg (126 encapsulated tubes) of the PCM, respectively. The coupled experimental- numerical approach allowed for model validation, explanation of experimental variation, and to provide general design guidelines for practically relevant latent high-temperature (metal) thermal energy storage.