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The rapid development of photovoltaic technology provides more possibilities for the efficient application of solar energy in buildings. This research proposed a phase change material (PCM) heat storage wall system with a “four-layer” structure. A performance test platform using low voltage DC was built to study the mechanism of electric thermal conversion of the graphene electrothermal film and the heat transfer characteristics of the “four-layer” structure. As shown in the experimental results, under the voltages of 24 V, 32 V and 42 V, (1) with the increase in voltage, the temperature of the electrothermal film increases, while its electrothermal conversion efficiency decreases from 85% to 75%; (2) during the heat storage process, because of its latent heat storage characteristics, the temperature of the PCM wallboard is 3~5 °C lower than that of the cement wallboard, but the effective heat storage increases by 59~65%; (3) during the heat release process, the effective heat release of the PCM wallboard increases by 41–78%, and the maximum heat storage and release efficiency is 98%; and (4) at 32 V, the PCM can completely change phase. The theoretical calorific value of the electrothermal film is equivalent to the hourly power generation of 1.45 m² of photovoltaic modules. The results provide basic data for the integration of photovoltaic and phase change technology and their efficient application in buildings.