AbstractThe voltage loss, determined by the difference between the optical gap (Eg) and the open‐circuit voltage (VOC), is one of the most important parameters determining the performance of organic solar cells (OSCs). However, the variety of different methods used to determine Eg makes it hard to fairly compare voltages losses among different material systems. In this paper, the authors discuss and compare various Eg determination methods and show how they affect the detailed calculation of voltage losses, as well as predictions of the maximum achievable power conversion efficiency. The aim of this paper is to make it possible for the OSC community to compare voltage losses in a consistent and reasonable way. It is found that the voltage losses for strongly absorbed photons in state‐of‐the‐art OSCs are not much less than 0.6 V, which still must be decreased to further enhance efficiency.