Yes. It depends how you shadow it.
Solar products are a parallel connection of serial cells. So if you part block 1 panel then the whole panel turns off. So in say the 21W Anker 3 panel product if you part-blocked 1 panel then the output would drop by 1/3rd.
So say take this (non Anker but the principals apply to Anker) solar panel
It is 3 panels.
If you block 2 panels in a manner where 1 panel is not blocked but 2 are blocked, the output drops but not to zero.
But if you partially block all 3 panels it drops to zero.
So when I blocked more total solar panels, blocking 2/3rds of then in one direction, I had a lesser effect than when I blocked 1/3rds of then in a different direction.
So the point here is:
- you would never know this without a meter. Hence meters are excellent!
- if you placed the meter in a location where to see it, you blocked the sun, you can alter the result to invalidate the conclusion. Depending on the shape of the shadow can have wildly different outcomes.
Meters are good things but bundling them into some products is not viable, bundling with solar in the panel is an example of a bad idea. Putting them on the end of the wire next to the product, so in-band in the cable, and if the cable is long enough, is more viable, but if you recall, the most common cause of failure is cables, so the solar investment lasts longer if you just have a USB port and you source the cable separately, so you end up what is best is what we have now, the most experienced person would buy a meter and buy a cable and then use the meter at times.