Air Handling Unit (AHU), as a system for space heating and cooling is one of the most relevant causes of energy consumption in both residential and tertiary sector buildings. As the energy efficiency of AHU is closely linked to the climate conditions, a special attention should be given about varying yearly climate conditions in different geographical locations. This paper presents an approach for calculating the energy efficiency by using the Psychrometric diagram which has been divided into five zones based on different functions of AHU; for each zone outside climatic has a particular status with yearly weight that is corresponds for specific AHU’s operation. To achieve this, different combination of outdoor climatic parameters (Dry and wet bulb temperature and humidity ratio) has been considered as varieties for defining zones. Sum of AHU’s energy consumption in different zones based on related weighting factor will result in the total annual consumption of energy.