First, USB is 5 volts. You will likely need minimum 3 of the solar cells in series to generate near 5V, maybe 4. Then there is rate of charge, minimum worthwh

First, USB is 5 volts. You will likely need minimum 3 of the solar cells in series to generate near 5V, maybe 4. Then there is rate of charge, minimum worthwhile is probably around 250mA, maybe 500mA depending on what you charge. Based on area and efficiency, eBay lookup, a 54x54mm solar cell is quoted at 2V, 100mA. So allowing for efficiencies of conversion etc, then you need 3 series, 3 parallel of these cells to get close, running direct in a buck/boost convertor to make steady 5V 250mA under as wide a range of conditions as possible. Maybe something like this https://m.alibaba.com/product/60841019812/DC-6-24V-12V/24v-to-5V-USB.html?__sceneInfo=%7B%22cacheTime%22%3A%221800000%22%2C%22type%22%3A%22appDetailShare%22%7D&from=Android https://m.alibaba.com/product/60841019812/DC-6-24V-12V/24v-t... Though you might want to run more cells/voltage into it to maximise economy. So a standard laptop USB port does 500mA, so you are looking at maybe 18 solar cells to replicate one laptop USB port capability. Plus a one dollar part, and wiring and mounting. So at $1 a solar cell capital cost that is around $8-10 per watt. (5 volts X 0.5A is 2.5W, $18 worth of cells, roughly). If you go to AliExpress and buy any decent sized solar panel you can get them for under a dollar a watt. If you buy the raw cells themselves and assemble them, because you have more time than money, under 25c a watt, maybe down towards 6-7c a watt - https://m.alibaba.com/product/1600163540077/Cheap-5BB-Polycrystalline-Solar-Cell-.html?__sceneInfo=%7B%22cacheTime%22%3A%221800000%22%2C%22type%22%3A%22appDetailShare%22%7D&from=Android https://m.alibaba.com/product/1600163540077/Cheap-5BB-Polycr.... Interesting project, but totally uneconomical. Edit, fixed up my costs in the starter config of 18 cells for 5V 0.5A