The major concern to evaluate the transformer loading ability to feed a load is the capability to transfer the internal generated heat of the windings, core, and structural parts to its ambient. The climatic conditions play an important role for evaluating the loading ability of a transformer. Hence, in the planning stage, the long-term climatic characteristics are a crucial issue. The thermal limit of the loading capability is referred to the winding hottest spot temperature. This paper is aiming to present the long-term ambient temperature change effect on the loss of life and cost effectiveness of a transformer located in two different regional climatic conditions. The climatic conditions are modeled on basis of daily ambient temperature. The transformer loading was measured for a 2.5 MVA, mineral oil, and ONAN cooling type transformer at certain region. The same loading is considered for the same transformer size and type at another region to show the impact of the diversity of regional climatic characteristics on the thermal performance. The results demonstrate the loss of life and cost effectiveness of the transformer is a function of the ambient temperature. The considered transformer is a 2.5 MVA, mineral oil, and ONAN cooling type.