The required output of the heat pump is defined by the heat losses of the building and the energy required for heating the domestic water. The consideration of the energy for domestic hot water heating is particularly important in low energy and passive houses, where the energy consumption for domestic hot water can be up to 40 % of the total heating energy consumed. For new buildings, the energy needs are usually already included in the project documentation, while for renovations this can be obtained by looking at the average fuel consumption over the years. If you have heating with fuel oil, then divide the annual consumption by a factor of 250 to get the required output of the heat pump. In case we have an energy performance certificate, the annual heat required per unit of useful floor area of the building is given according to the class in which the building is classified.