Solar cell comparison is generally based on an arbitrary maximum terrestrial intensity and spectra (of 1 sun, 1000 W/m2) at 25 °C perpendicular to the cell plane [1] referred to by specialists as AM1.5. In practice, no solar cell experiences such conditions, yet few alternative bases for comparison exist [2]. Our interest in this paper is to explore the correct design of indoor Photovoltaic (IPV) products. Given that the indoors, when compared with the outdoors, are characterised by much lower radiant energy intensities, various spectra (including artificial light sources), complete comparison data for indoor conditions are not freely available. More general level reports have been published [3–6]. Twenty-one different solar cells representing eight different Photovoltaic material technologies are reproducibly electrically characterised under laboratory based simulated AM1.5 (1 sun; solar spectrum) from 1000 W/m2 intensity down to the 0.1–1 W/m2 decade. Some were measured under an artificial light source (fluorescent tube) in the 1–10 W/m2 decade. The results are used to validate a phenomenologically based model.