![apple wireless solar powered keyboard apple wireless solar powered keyboard](https://www.hitechreview.com/gallery/logitech-wireless-solar-keyboard-k750-for-mac/logitech-wireless-solar-keyboard-k750_05.jpg)
Each of them can provide surprisingly high current levels (set mostly by the specific chemicals used, and by how they are layered and things like that), while the total amount of energy roughly matches the amount of those chemicals that are in the battery - not too surprisingly the big fat "D" cells would last the longest. Thus the big fat "D" cells, the little "AAA" cells and the tiny little button cells all provide roughly 1.5V of "push".
![apple wireless solar powered keyboard apple wireless solar powered keyboard](https://www.macfixit.com.au/media/product/a22/macally-solar-powered-rechargeable-slim-bluetooth-keyboard-for-mac-space-gray-dcb.jpg)
The pipe size is what determines the water flow rate, and the volume of the reservoir limits how long it will last. A 20m high water tower supplies the same water pressure as a 20 meter high dam across a huge lake. Voltage (more accurately "electrical potential") is roughly analogous to pressure or height in a mechanical/gravity system. Roughly speaking, the voltage is set by the chemistry, not the amount of sunlight. Keep in mind the following is a pretty "toy" description of some of these items - don't be too rough on me.