For indoor low-light applications, amorphous silicon solar panels are the best. For this reason, this is the only type of solar panel you see in solar-powered calculators. I was recently trying to find a source of such panels and all I was able to find turned out to be quite expensive. After much frustration, I decided to check out my neighborhood Wal_Mart; this place never ceases to amaze me.
A calculator with charming name “Le World” will cost you $1.00 (one US dollar ) plus tax. It contains A-si panel with indoor output of 40-70microamps and type LR1130 alkaline button battery( which is worth 30-40 cents itself). I compared a panel with older Sanyo solar calculator panel( both panels pictured on the right); the little one produces approximately half as much electricity being 3 times smaller. Because it’s so cheap, one can easily buy several and run them in parallel, if larger current is necessary.
The weather was pretty bad last couple of days and I haven’t had a chance to test this panel outdoors. As soon as we get some sun, I will measure max.output and post results. I am expecting 5-10ma output in the full sun; and 2-3 panels is going to be enough to run PIC-based sensor on north-facing window or under artificial light indefinitely. China FTW!