Most plug-in USB chargers have an output of 1A for smartphones or 2.1A for tablets/smartphones, so the output of the batteries should be good enough. I guess it depends on the output of the solar panel as to how long it takes to charge the batteries. Typical 4V solar panel spec here. I'll let someone with a better knowledge of electronics work out what that means!
Most plug-in USB chargers have an output of 1A for smartphones or 2.1A for tablets/smartphones, so the output of the batteries should be good enough. I guess it depends on the output of the solar panel as to how long it takes to charge the batteries. Typical 4V solar panel spec here. I'll let someone with a better knowledge of electronics work out what that means!