A few months ago I tested a Blavor 10,000mAh solar charging power bank and discovered that it took over 130 days to full recharge using sunlight alone. Some of the feedback I got on these series of testing video was that the reason it took so long was because the solar panel was indoors behind a window.
While I am aware that clear window glass does filter out some sunlight (especially UV). But glass does not reduce the power output of a solar panel by as much as some claim. Many outdoor solar panel arrays are covered in glass to protect them from inclement weather.
But to prove my point, I conducted a revised test using the same Blavor solar charger I tested previously. I placed outdoors on my windowsill facing almost directly upward. Exposed to direct sunlight (and the weather) for 7 days to see if I could detect any improvement in charging speed.
I started the test with the battery at 1/2 full as this seemed take the longest to make progress in my earlier tests. I checked the device 7 days later and found no change in the charge indicator. So I left the device to charge an additional 3 weeks (a total of 35 days) and checked it and found, no change. I have no intention of waiting another 100 days to see if charges any faster. To my satisfaction at least I believe this proves my point that the window didn’t make much or any difference to the outcome of my earlier tests.