Just some back-of-the envelope calculations to check feasability: up-to date light intensity data can be found here (for Germany, but the Netherlands should be close to northern Germany in terms of "sunniness"). It's given in kWh/month/m2. To obtain the total energy harvesting of your module, multiply this figure by it's power rating (as they are rated for 1kW/m2 incoming radiation power)*. So for a 1Wp (Watt peak) panel that you leave outside in the sun during the whole daylight time, you can expect ~50Wh per month to be produced in October, ~180Wh in June, and maybe ~15Wh in December. A full charge is 8Wh, plus about 20% charging losses (rough estimate) you need 10Wh to charge your battery. This means in December you can charge your mobile ~1.5 times during the month, leaving it in the sun the whole day every day. And this does not cover self discharge losses and the reduced efficiency at low light intensities.

So, a solar back might be nice for outdoor trips in summer, but it definitely
does not make charging a thing of the past. And most of us do not have transparent trouser pockets, in south direction... As cool as this project is in itself, IMHO an external panel with a buffer battery and usb charger is the much more sensible alternative.


* actually, most cell types' light conversion efficiency is significantly worse at low light intensities.