At present there is a lack of a concept which can be used to evaluate and compare efficiency for installed refrigeration systems using short term measurements. Traditionally COP (Coefficient of Performance) has been used but it has a weakness as a comparator as it is strongly dependent on operating conditions. System Efficiency Index (SEI) is the ratio between the measured COP and the COP for an ideal process, known as the Carnot efficiency operating between the same temperature levels. In collaboration between SP Technical Research Institute of Sweden, IOR and VDMA the method has been further developed. This paper will focus on the method and results from the implementation of SEI from field measurements for, in the first step, condensing units and chillers. The principle is the same for all kind of refrigeration systems. In the next step more measurements for refrigeration systems will be evaluated.