long answer: while solar is actually the best indoor energy harvesting solution its still in nA land. This uses 56µA for the DCDC alone. that means you would need at least 1mA for maybe an hour to just keep that going without any screen updates or communications.

A windowsill setup might work but thats hardly "indoor" to me.

This lasts 1 year on 2 AA rechargeables - just swap the batteries.