The study evaluates the performance of two medium-sized supermarkets, R1234ze(E)/CO₂ cascade system and CO₂ transcritical system, started operating in 2023 located in northern Italy. Both supermarkets feature two stages, medium-temperature (MT) and low-temperature (LT) that provide cooling to MT and LT cabinets. The primary objective of the study is to assess the efficiency of the refrigeration system based on field measurement. Since both systems provide heat recovery when it is required, it is essential to analyses how the coefficient of performance (COP) of each stage is affected by the heat recovery operation, independently of the benefits derived from the recovered heat. To achieve this, field measurement data were gathered at one-minute intervals throughout a year. Models were developed in MATLAB for both supermarkets using REFPROP. The required calculations are explained in detail, average cooling demands, evaporation temperatures, internal superheating at each stage are discussed and COP was calculated. The field measurement of electrical consumption of the entire component systems permitted a comprehensive study into the electrical consumption of refrigeration systems, heating, and cooling. The results show that the R1234ze(E)/CO₂ cascade system in comparison to CO₂ transcritical has higher efficiency at MT stage when the ambient temperature exceeds 20 °C, and at LT stage when the ambient temperature is above 16 °C. COP in ambient temperature lower than 20 °C is influenced by different heat recovery operation in these two systems. R1234ze(E)/CO₂ cascade system frequently operates at higher fixed condensation pressures to meet heat recovery needs in ambient temperature interval, leading to more consistent COP of MT during this period. In contrast, the COP of the CO₂ transcritical system in MT varies as it operates in both transcritical and subcritical modes to meet the heat demand. The heat recovery analysis and its efficiency are presented in Part II of this article.