The escalating demand for heating and the widespread use of CO2-emitting fossil fuels during cold seasons have imposed significant pressure on our natural resources. As a promising alternative to coal-fired boilers, electrical thermal storage devices (ETSDs) for space heating are gaining popularity. However, designing ETSDs for space heating involves significant challenges, which involve their storage rate and operational stability. In contrast to the research of directly developing mid-temperature ETSDs to manage heat release during long heating hours, this study proposed a new ETSD that uses K2CO3−Na2CO3 for high-temperature storage to match the off-peak hours and thereby gain potential economic benefits. This study used experimental and simulation methods to investigate the ETSD’s temperature distribution. An operational strategy was also proposed to achieve more efficient temperature distribution and higher economic benefits. The ETSD with two steel plates and two insulation layers with a power rating of 1.6 kW was found to be the optimum structure, due to its improved heat storage rate (2.1 °C/min), uniform temperature, and material heat resistance (<750 °C). An energy analysis, economic analysis, and a 7-day cycling operation performance of the device were then conducted by comparing the proposed ETSD with a traditional electric heater. The results revealed that the proposed ETSD released 53.4% of the stored energy in the room, and stored 48.6% of it during valley electric time. The total cost of the proposed ETSD was consistently lower than the traditional electric heater in the second heating season (by the 213th day). The efficiency of its valley heat storage for users was 37.2%. Overall, this study provides valuable insights into the development and practical applications of ETSD systems for space heating.