Geothermal resource assessment requires several input parameters at a time when field information is normally very limited. The power plant efficiency is an important parameter as it affects investment cost and profit. However, there is significant debate in the geothermal industry on the appropriate performance measure to implement. In this paper we attempt to determine the best performance measures for financial decision making during the exploration stage of geothermal power developments. We compare and contrast current performance measures commonly used by the geothermal industry. This includes thermal efficiency, geothermal brine effectiveness, utilisation efficiency and enthalpy efficiency. We show that enthalpy efficiency, categorised by the geothermal reservoir enthalpy, is the best financial performance measure because it is similar to other measures on most criteria, but superior in terms of comparability across different geothermal sites, while satisfying homoscedasticity. Utilisation efficiency, on the other hand, was demonstrated to have the least reliability while requiring additional input parameters. We recommend using the modified enthalpy efficiency developed in this work for resource assessment of new geothermal fields, benchmarking with existing development and for comparison with conventional fossil-fuel thermal plants. This should also affect the choice of resource assessment methodology. (C) 2016 Elsevier Ltd. All rights reserved.