This International Standard is applicable to heat pumps for space heating and cooling, heat pump water heaters (HPWH), and heat pumps with combined space heating and/or cooling and domestic hot water production, in alternate or simultaneous operation, where the same heat pump is used for space heating and domestic hot water heating. This part of ISO13612 provides a calculation method under steady conditions that corresponds to one calculation step. The results of this calculation are incorporated in larger building models and take into account the influence of the external conditions and building control that influence the energy requirements for heating and cooling supplied by the heat pump system. This part of ISO13612 specifies the required inputs, calculation methods, and required outputs for output thermal power generation for space heating and cooling and domestic hot water production of the following heat pump systems, including control: electrically driven vapour compression cycle (VCC) heat pumps; combustion engine-driven vapour compression cycle heat pumps; thermally driven vapour absorption cycle (VAC) heat pumps, using combinations of heat source and heat distribution listed in Table1. Table1 Heating/cooling sources and energy distribution Source Distribution Outdoor air Air Exhaust-air Water Indirect ground source with brine distribution Direct condensation/evaporation of the refrigerant in the appliance (VRF) Indirect ground source with water distribution Direct ground source [Direct expansion (DX)] Surface water Ground water