Review on MakerHawk Raspberry Pi UPS Power Supply Uninterruptible HAT with Battery Charger Power Bank & Management Expansion Board 5V for Raspberry Pi 4 Model B / 3B + / 3B (Battery Excluded) by Domonic Roberts

I used this in Central Africa to make my own cheap solar charger there. worked great. First I put them all in a 20 watt panel, then scratched my head when that didn't work, the arithmetic is watts divided by volts equals amps. So yes, I should have done it first. They work well when connecting a dual USB charger to a 20W panel. You could do with less, but don't expect to overcharge, I can only assume the math works like this: 20 watts / 5 volts = 4 amps. but in reality the panel outputs 14 or 15 volts at 20 watts. So it could be that a lot of energy is wasted by stressing the crystals in the solar panel. I'm sure someone smarter than me could answer this question quickly. Anyway it works.