Just bought one of those garden night lights the other day. Have not taken it apart yet. It has a 2cm x 2cm cell on top and says it contains a 2/3 AA NiMH battery IIRC. Cost about $1.29.
That low price doesn't compare to a larger panel in efficiency, but the installation story is hard to beat.
Tagent: I was obsessed with this pi zero socket server design where you fit it into a wal-wart plug as they say and you just stick it on an outlet with a little rubber ducky antenna.
I also have an outdoor solar light that I’m not using and a raspberry pi zero. I dunno how solar controllers work or what a small one might require. I also wonder if I could run this off of 4 or 8 AA rechargeable batteries and then just feed the solar panel into those. If it worked, they may not work long due to charge/discharge without a proper controller, but maybe that’s okay with replaceable batteries. I have no idea if it could start this simple.
Just rattling thoughts off the top of my head, my advice would be to go to a forum like allaboutcircuits.com and look around/ask. Actually could probably get help on a forum like RaspberryPi/askelectronics on reddit.
Probably not going to work honestly about the outdoor light (need more solar cells). The voltage charging the battery has to be higher than the voltage of the battery. Also have to look at the current being drawn by each piece, when the radio goes on that causes a surge in current draw too. For the Pi Zero (non-W) something like 5V * 70-200mA say current so up to 1W needed * duration. The batteries could do that but the solar cell can't keep it topped off. Also yeah you need the charge controller, some diode to make sure voltage doesn't go back to the solar cells from the batteries... lead acid battery is "safer" to work with but BMS chips for lipos are pretty inexpensive. Need a voltage regulator to take your batteries and drop it down/keep it at 5V to keep the Pi happy.
There's probably way easier approaches to do this with regard to pre-made parts but yeah. I guess depends how much you want to learn/do yourself. I remember the first time I soldered a 2x16 LCD display there were micro shorts and it was smoking.
What I like about the capacitor idea and something low power like an ESP01 is it just accepts what the solar cells put in and that's it of course I still use a voltage regulator to put 3.3V to the ESP. Doesn't last as long as a battery but it's not as complex.
Anyway this guy made a solar-powered rc plane/charging it in midair pretty cool.
I was just looking at small things like those garden night lights you can stake into the ground, has a solar cell on it.