A method for estimating a thermal quantity of a water tank, the water tank extending along a substantially vertical axis and having: - an intermediate portion in heat exchange with a device for heating the water of the tank, the device comprising a heating means powered by an electrical network; - a lower portion having a water inlet; and - an upper portion having a water outlet; the water inlet and/or the water outlet being equipped with a flow rate sensor (21, 22) measuring a drawn off water flow rate; the method comprising the implementation, by data processing means connected to said flow rate sensor (21, 22), of steps of: (a) determining an operating regime among a first regime of drawing off water from the tank, a second regime of heating water of the tank without draw-off, and a third regime without draw-off and without heating according to data representative of the energy consumption of said heating means of the device, and to measurements of drawn off water flow rate; (b) from an initial volume (V2 i) of the intermediate portion and initial temperatures (T1 i, T2 i, T3 i) of the lower, intermediate and upper portions (P1, P2, P3) of the tank, determining a final volume (V2 f) of the intermediate portion and final temperatures (T1 f, T2 f, T3 f) of the lower, intermediate and upper portions (P1, P2, P3) of the tank, according to at least the determined operating regime, the data representative of the energy consumption of said heating means of the device, and the measurements of drawn off water flow rate; (c) from initial volumes (V1 i, V3 i) of the lower and upper portions (P1, P3) and of said final volume (V2 f) of the intermediate portion, determining final volumes (V1 f, V3 f) of the lower and upper portions (P1, P3); (d) estimating said thermal quantity of the water tank according to the final temperatures and volumes (T1 f, T2 f, T3 f, V1 f, V2 f, V3 f) of the lower, intermediate and upper portions (P1, P2, P3) of the tank. - an intermediate portion in heat exchange with a device for heating the water of the tank, the device comprising a heating means powered by an electrical network; - a lower portion having a water inlet; and - an upper portion having a water outlet; (a) determining an operating regime among a first regime of drawing off water from the tank, a second regime of heating water of the tank without draw-off, and a third regime without draw-off and without heating according to data representative of the energy consumption of said heating means of the device, and to measurements of drawn off water flow rate; (b) from an initial volume (V2 i) of the intermediate portion and initial temperatures (T1 i, T2 i, T3 i) of the lower, intermediate and upper portions (P1, P2, P3) of the tank, determining a final volume (V2 f) of the intermediate portion and final temperatures (T1 f, T2 f, T3 f) of the lower, intermediate and upper portions (P1, P2, P3) of the tank, according to at least the determined operating regime, the data representative of the energy consumption of said heating means of the device, and the measurements of drawn off water flow rate; (c) from initial volumes (V1 i, V3 i) of the lower and upper portions (P1, P3) and of said final volume (V2 f) of the intermediate portion, determining final volumes (V1 f, V3 f) of the lower and upper portions (P1, P3); (d) estimating said thermal quantity of the water tank according to the final temperatures and volumes (T1 f, T2 f, T3 f, V1 f, V2 f, V3 f) of the lower, intermediate and upper portions (P1, P2, P3) of the tank.