Our aim was to examine the energy efficiency at Nesjavellir geothermal power plant by calculating its EROI (energy return on investment). For calculations, real data was acquired from stakeholders on materials used in the construction, maintenance and operation of the plant. A previously proposed standard for EROI calculations is tested in this study. Also calculated is the EPT (energy payback time). The paper presents a new EROI metric which is given the name EROI ideal which looks at the maximum theoretical EROI, which shows the potential room for improvement in efficiency. Findings show that the infrastructure to deliver energy to Reykjavik is the largest contributor when looking at the embodied energy of used material. The power plant itself uses 12 MW of the produced 120 MW of electricity and 300 MW of hot water, which makes the plant itself the single largest energy consumer. EROI3,i was found to be 32.4 over a 40-year lifetime, EROIstnd 33 and EROIide 80.9. However, excluding hot water, the EROI dropped to 9.5. This indicates that the efficiency has not improved in electricity production using geothermal technology since the 1970's. Hot water production for domestic heating was therefore found to increase the EROI significantly.