Method to operate a hot water storage device, the hot water storage device comprising a tank configured to store water, a heating unit positioned at a first distance from a bottom wall of the tank, the heating unit being configured to heat the water stored within the tank such that heated water rises up within the tank and is stratified above unheated water within the tank, at least one temperature sensor (19, 20) positioned at a distance from the bottom wall of the tank being greater than said first distance, a controller configured to control the heating unit, the method comprising the following steps: determine from a water temperature measurement signal provided by the at least one temperature sensor (19, 20) a water usage profile, said water usage profile providing for defined time intervals of a day and/or for defined days of a week a nominal temperature and a nominal volume of the heated water to be stored within the tank, wherein the water usage profile is determined on basis of a frequency and of an amount of a hot water demand in the respective time intervals of the respective day, operate the heating unit in such a manner that for each defined time interval of a day and/or for each defined day of a week the actual temperature and the actual volume of the heated water stored withing the tank corresponds automatically to the respective nominal value of the water usage profile, characterized by the following step: determine for each respective time interval of each respective day a confidence factor associated with the nominal temperature and the nominal volume of the heated water to be stored within the tank, wherein the confidence factor depends on the frequency and the amount of a hot water demand in the respective time intervals of the respective day. a tank configured to store water, a heating unit positioned at a first distance from a bottom wall of the tank, the heating unit being configured to heat the water stored within the tank such that heated water rises up within the tank and is stratified above unheated water within the tank, at least one temperature sensor (19, 20) positioned at a distance from the bottom wall of the tank being greater than said first distance, a controller configured to control the heating unit, the method comprising the following steps: determine from a water temperature measurement signal provided by the at least one temperature sensor (19, 20) a water usage profile, said water usage profile providing for defined time intervals of a day and/or for defined days of a week a nominal temperature and a nominal volume of the heated water to be stored within the tank, wherein the water usage profile is determined on basis of a frequency and of an amount of a hot water demand in the respective time intervals of the respective day, operate the heating unit in such a manner that for each defined time interval of a day and/or for each defined day of a week the actual temperature and the actual volume of the heated water stored withing the tank corresponds automatically to the respective nominal value of the water usage profile, characterized by the following step: determine for each respective time interval of each respective day a confidence factor associated with the nominal temperature and the nominal volume of the heated water to be stored within the tank, wherein the confidence factor depends on the frequency and the amount of a hot water demand in the respective time intervals of the respective day. determine from a water temperature measurement signal provided by the at least one temperature sensor (19, 20) a water usage profile, said water usage profile providing for defined time intervals of a day and/or for defined days of a week a nominal temperature and a nominal volume of the heated water to be stored within the tank, wherein the water usage profile is determined on basis of a frequency and of an amount of a hot water demand in the respective time intervals of the respective day, operate the heating unit in such a manner that for each defined time interval of a day and/or for each defined day of a week the actual temperature and the actual volume of the heated water stored withing the tank corresponds automatically to the respective nominal value of the water usage profile, characterized by the following step: determine for each respective time interval of each respective day a confidence factor associated with the nominal temperature and the nominal volume of the heated water to be stored within the tank, wherein the confidence factor depends on the frequency and the amount of a hot water demand in the respective time intervals of the respective day.