To get a better feeling for the limits of the PV-powered device under indoor LED illumination I modified one of our ikea floor lamps with a phillips hue LED bulb as can be seen below. The color of the light can be changed and also the intensity can be configured between 0...~3000Lux. The solar cells are oriented towards the LED bulb; the rest of it is covered with a beautiful hand towel... my wife was really happy with this setup too:-) To measure the light intensity I used a light meter MT-912 for less than 20 bucks, which works for me much better than some of the available mobile phone apps, especially with LED and dim light.

With this setup I was now looking for the minimal light intensity which is just recharging the supercap back to its last state for a given update rate (e.g. 5mins.). So the clock demo was running and the light intensity was changed until I found a stable condition (for around 12hours) at which the supercap voltage remained unchanged during each update (because it just reloaded before the update). Luckily the voltage is measured via a voltage divider and printed on the screen so no manual measurement needed here. This measurement was done for 5 mins update rate, three, one and 10mins update frequency, the related minimum LED light intensity can be seen below.

Typical indoor intensities for living rooms may vary around 200Lux..400Lux, so what we see is not super performand for indoor usage  yet... the used mono crystaline IXYs PV cells work really well when exposed to full sun light but with this dim artifical lights we can just harvest a couple of Micro-amps.

Nevertheless; i repeated the same measurement by using the earlier mentioned epaper update mode with low power settings(shorter update time, 2GLs). The arduino clock sketch uses this mode when the supercap voltage is below a certain voltage threshold (e.g. 4.2V) automatically. It can now be seen that about half of the light intensity is only needed for a given update rate.  I thinks thats some good data to share and a big step forward.