The production of energy causes different kinds of damage to the environment depending on the specific type of technology used in producing a given energy supply. The common term that expresses the costs of these environmental damages is externalities. These costs are not included in the cost and price structure faced by the producer and the consumer. During the last few years, external costs related to power production technologies have been calculated making use of different methodologies. The external costs may turn out to be very different:for the same fuel cycle depending on the methodology that has been used to assess the externalities. The article will focus on some of the most important reasons for differences in the numbers. To illustrate the importance of knowing the exact data and assumptions used, two studies using the same approach and with integrated computer models are compared. The models are based on the same concept with air dispersion modules and dose-response functions for the calculation of impacts. Although the models are comparable, the resulting external costs turn out to differ with a factor of five in the two studies for the same power plant due to different assumptions, different dose-response functions used and different impacts included in the studies. In the paper the most important differences to be aware of will be illustrated.