Electric heat pumps are a well-established and widely adopted solution for space heating and cooling, offering an efficient solution for decarbonization. In particular, large-size heat pumps are increasingly important in new and retrofitted district heating machines. These systems typically employ large fields of fin-and-tube evaporators, in which frost formation is a critical issue that significantly impacts the seasonal performance, leading to reduced airflow, lower heat transfer efficiency and the need for defrosting cycles that temporarily switch off parts of the heat exchanger field. Consequently, energy consumption increases to meet the heating demands, highlighting the need for accurate design processes focused on evaporator field geometry and defrost strategies. This study presents a seasonal performance model capable to identify the best configuration of evaporators to minimize the seasonal energy consumption. A set of simulations involving variable geometrical parameters such as fin spacing and evaporator field size in terms of number of modules with different airflow velocities for the climatic conditions of Helsinki has been run assuming a fixed defrosting strategy. Through a detailed tube-element modelling approach for the evaporator field, focused on frosting and defrosting cycles, a maximum gap in total costs between the best and the worst solutions up to almost 100% has been obtained starting from a 7.5% without taking into account frosting/defrosting cycles.