From Eric Kvaalen, Les Essarts-le-Roi, France
Ben Haller calculates that a 25-square-centimetre solar panel in a room lit by a 100-watt incandescent bulb will gather about 0.0001 watts (Letters, 23/30 December 2017). He assumes that the bulb converts 2.2 per cent of the electrical power into light, but the figure is more like 5 per cent. He also implicitly assumes that the walls, ceiling and floor of the room absorb all the light falling on them.
