Method for determining the output required from a heat generator, by means of which, in a property, service water within at least one service water circuit and also hot water for room heating within at least one hot water circuit is heated, characterized in that for a measurement period of at least one day, preferably for several days, in each case repeatedly, a. a daily value for the quantity of energy consumed for the service water heating within one day is determined from at least one measured value from a heat meter in the at least one service water heating circuit, b. a daily value for the quantity of energy consumed for the hot water heating within one day is determined from at least one measured value from a heat meter in the at least one hot water circuit, and c. a daily output value for the service water heating is determined from the daily value for the quantity of energy expended for the service water heating on the day, and d. a daily output value for the hot water heating is determined from the daily value for the quantity of energy expended for the hot water heating on the day, and e. depending on at least one daily output value for the service water heating and at least one daily output value for the hot water heating, the output required from the heating boiler is determined f. and wherein, depending on whether the heating of the service water takes place with priority before the heating of the hot water or whether heating of the service water takes place chronologically in parallel with the hot water circuit, the output required is determined differently from case to case, g. wherein it is determined automatically whether priority operation or parallel operation is present, in that both for the hot water circuit and for the service water circuit a state value is respectively determined at time intervals, which indicates whether active heating is present within a time interval and, for each time interval, the state value of the service water circuit is compared with the state value of the hot water circuit and, by means of the comparison, it is concluded whether priority operation or parallel operation is present. a. a daily value for the quantity of energy consumed for the service water heating within one day is determined from at least one measured value from a heat meter in the at least one service water heating circuit, b. a daily value for the quantity of energy consumed for the hot water heating within one day is determined from at least one measured value from a heat meter in the at least one hot water circuit, and c. a daily output value for the service water heating is determined from the daily value for the quantity of energy expended for the service water heating on the day, and d. a daily output value for the hot water heating is determined from the daily value for the quantity of energy expended for the hot water heating on the day, and e. depending on at least one daily output value for the service water heating and at least one daily output value for the hot water heating, the output required from the heating boiler is determined f. and wherein, depending on whether the heating of the service water takes place with priority before the heating of the hot water or whether heating of the service water takes place chronologically in parallel with the hot water circuit, the output required is determined differently from case to case, g. wherein it is determined automatically whether priority operation or parallel operation is present, in that both for the hot water circuit and for the service water circuit a state value is respectively determined at time intervals, which indicates whether active heating is present within a time interval and, for each time interval, the state value of the service water circuit is compared with the state value of the hot water circuit and, by means of the comparison, it is concluded whether priority operation or parallel operation is present.