You should buy solar charger with builtin lithium battery.
Price of this is about 12-15eur with 2000mAh battery and about 7eur with 500mAh:
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Such devices GPS navigations, mobile,.... my opinion is that is not good to charge over PC USB port direct, better use USB HUB with power supply adapter (power injection).
You can use only solar panels without battery, battery is good to use he give continuity of power, when shading or no light source. I make solar chargers for small project like sms alarm, I use small solar panels extracted from cheap solar garden lamps.