We bought a 100w solar panel from eBay for about £30 which came with a 30A controller.

We also bought a 12-volt 7AH battery and connected it up as shown in the picture.

During the lovely sunny summer this year it worked well we were able to charge up our phones and tablets off the USB ports on the controller most days. One to two devices a day during the sunny days.

Now the weather has turned cloudy and rainy, the solar panel doesn’t work quite so well but on the sunny days it works ok.
once the battery is charged you can still charge a device for a bit when its dark.