Modular assembly for storing heat by phase-change material including a plurality of heat-storage modules attached to one another, the heat-storage assembly comprising a vessel, each module for storing heat by phase-change material being delimited by a plurality of external walls (5a, 5b), and comprising: - at least one heat exchanger including first (4.1) and second (4.2) connection ends intended to be connected to a heat-transfer fluid system, said first (4.1) and second (4.2) connection ends entering and emerging from the vessel, - a structure intended to contain a phase-change material, said structure comprising a porous matrix (MA, MA') with communicating cells through which said heat exchanger passes and in contact with the external surface of said heat exchanger, said matrix (MA, MA') being obtained by moulding around said heat exchanger, at least one of the external walls of the module, referred to as a solid external wall, being formed by moulding directly on the porous material so as to form a part of said vessel of the heat-storage assembly, at least one of the other external walls of the module, referred to as a porous external wall, being formed by the porous matrix (MA, MA'), and, among the modules of the heat-storage assembly, at least two adjacent modules are disposed so that a porous external wall of one of the modules is arranged facing a porous external wall of the other one of the modules, and so that a solid external wall of one of the modules is attached to a solid external wall of the other one of the modules. each module for storing heat by phase-change material being delimited by a plurality of external walls (5a, 5b), and comprising: - at least one heat exchanger including first (4.1) and second (4.2) connection ends intended to be connected to a heat-transfer fluid system, said first (4.1) and second (4.2) connection ends entering and emerging from the vessel, - a structure intended to contain a phase-change material, said structure comprising a porous matrix (MA, MA') with communicating cells through which said heat exchanger passes and in contact with the external surface of said heat exchanger, said matrix (MA, MA') being obtained by moulding around said heat exchanger, - at least one heat exchanger including first (4.1) and second (4.2) connection ends intended to be connected to a heat-transfer fluid system, said first (4.1) and second (4.2) connection ends entering and emerging from the vessel, - a structure intended to contain a phase-change material, said structure comprising a porous matrix (MA, MA') with communicating cells through which said heat exchanger passes and in contact with the external surface of said heat exchanger, said matrix (MA, MA') being obtained by moulding around said heat exchanger, at least one of the external walls of the module, referred to as a solid external wall, being formed by moulding directly on the porous material so as to form a part of said vessel of the heat-storage assembly, at least one of the other external walls of the module, referred to as a porous external wall, being formed by the porous matrix (MA, MA'), and, among the modules of the heat-storage assembly, at least two adjacent modules are disposed so that a porous external wall of one of the modules is arranged facing a porous external wall of the other one of the modules, and so that a solid external wall of one of the modules is attached to a solid external wall of the other one of the modules.