Solar is a complicated topic, more complicated than is reasonable for a power focus.
I'd say Anker is representative of the current leading technology in portable solar, and the issues are not specific to Anker but to solar in general so getting aware of solar in general is a healthy use of time.
A good place to start is with this guy, go back through solar videos.
Ahead of you spending that time, as a very crude guide, and none of this is a complaint about Anker but of solar in general, the Watts claims are somewhat high in they relate to the internal peak theoretical and not what you see from a USB port and in real world situations there are further drops in performance. As a general rough guide, take a Watts claim and assume 1/3rd for most parts of the sunny part of day. So say 21Watts, well cut that to 1/3rd to be 7Watts, then at 5V (Watts = VoltsxAmps) you'd be talking about 1A so in a 7 hours of sunshine you've got 7Ah or so. That is enough to charge one of the larger Anker batteries.
That is very good in some situations, if you were in strong sunshine you could charge a phone at about the same speed as being on a mains charger (Powerport) or battery (Powercore).
Then you must think about losses. If you use the sun to charge a battery, which then itself charges a battery, all the conversions lose energy.
So you'd be better thinking of using a solar panel to charge your phone directly for fewer losses, or be thinking of leaving the solar panel at a base camp and it charge a battery which charged a device later.