Highlights: ? Dedicated Mechanical Subcooling (DMS) increases significantly the COP at high outdoor temperature. ? Optimal size of the DMS, subcooling degree and activation temperature can be found, depending on climate. ? Time dependent cooling load profile is important for the identification of the optimal conditions. ? The investment cost of DMS is 13–20% of the total cost in a 10 years life span. Use of Dedicated Mechanical Subcoolers gives rise to the need of an optimisation process involvin g gas cooler pressure, activation and setpoint temperature values, subcooling degree and subcooler size, with the aim of further improving the energy efficiency while considering costs. In this paper a thermoeconomic analysis is performed on a commercial r efrigeration plant, at four climate conditions from warm to hot, with time-dependent refrigerating load depending on the location, resulting in some design and control rules for the DMS. In terms of energy efficiency, the size of the DMS appears to be more crucial at hot climate conditions (difference up to 3.5% in energy saving), with sizes ranging from 35% to 45% of the total cooling capacity. Optimal control rules should be preferentially adopted at mild-warm conditions, suggesting to exploit the highest subcooling rate available. The economic analysis shows that energy use is the most important cost item.