For commercial purposes, compression HVAC machines are characterized by means of performances rated in standard conditions. The actual operating conditions are really variable and may differ from standard conditions. As a consequence, heating and cooling performances vary within a wide range in real conditions, therefore constant values cannot be applied in detailed energy simulations. This matter is even more important in the nowadays trends in heating and cooling design, since new components and system solutions are oriented to minimize temperature differences between the room and the thermal delivery units. This paper describes a numerical model for the calculation of coefficients of performance for heat pumps and chillers out of standard conditions. The numerical model can calculate COP in all running contexts, using simple and “easy-to-find” inputs. Inputs required are really few and rough, practically based on rated performances and essential construction characteristics. The model uses these inputs for defining the basic parameters of compressor and heat exchangers. Then, the behaviour of the machine for different source temperatures can be calculated, based on internal subroutines defining the entire thermal cycle. The COP at partial loads is computed as well. The model was tested against real measurements and showed relevant accuracy, in spite of the low amount of inputs required.