There are currently no international norms which define a method for characterizing photovoltaic solar cells for indoor applications. The current standard test conditions are not relevant indoors. By performing efficiency simulations based on the quantum efficiency of typical solar cells and the light spectra of typical artificial light sources, we are able to propose the first step for developing a standard by determining which light sources are relevant for indoor PV characterization and which are not or are redundant. Our simulations lead us to conclude that indoor light sources can be divided into three different categories. For the characterization of photovoltaic solar cells in indoor environments, we propose that solar cells be measured under one light source from each group.