FYI as its bright sun today I did a few experiments. I encourage watching about solar from this chap, he explains in detail slowly so you can really learn.
In particular focus on solar, and then on USB meters.
Right now I don't have anything sufficiently under-charged to pull much Wattage, I had a 80% charged tablet, it was only able to pull about 1A 5W but for a test of a point, related to some gawd awful reviews that exist out there...
I think these points done on a non-Anker Choetech 19W, will apply across to an Anker 21W, and it is do with how photovoltaics work, parallel and sequential, voltages.
Each panel, of which the Choetech 19W and the Anker 21W have 3 panels, is made up of small cells which are connected in series, then each panel is connected in parallel. So I'm guessing each cell is making a tiny amount of voltage and current, lots of in each panel in series then aggregate their small voltages to make around 5V at the current of each cell. Then the panels in parallel aggregate their 5V current to make more current.
A cell is a semiconductor means electrons flow when sun shines on it, when no sun then it is an insulator. Hence when cells are connected in series, to aggregate their voltage, if any of the cells in a panel is seeing insufficient sunlight to allow electrons to flow, then the series is broken and no current flows through that panel. This is what I tested and it very important for anyone not familiar with a particular construction of solar panel to understand.
(so given my device was only able to pull about 0.9A or so, ignore the 0.9A part)
- 3 panels fully seeing the sun = 0.9A @ 5V
- 2 panels fully seeing the sun = 0.9A @ 5V. So what you're seeing here is the 3 panels in parallel, one of the panels is shutdown, so its now 2 panels so given this as more of a 2A capable panel its about 1.3A capability on 2 panels which is >0.9 hence you don't see any drop
- 1 panel fully seeking the sun = 0.6A @ 5V. So this tallies with each panel being about 0.7A.
If however I opened all 3 panels and put a shade across the top of each panel = 0A, the meter shutdown.
Hence if you partially shade a panel, it shuts down the whole panel.
If you left the panel under a tree with branches partially shading each panel, you'd get no energy. You're better off having weaker sun to all of a panel than strong sun to part of a panel.
So in photos:
3 full panels in the sun:
and the meter reading (note ignore the fact it is only 0.92A the device attached isn't needing any more I have shown this panel can do 2A easily in strong sunshine):
Then to simulate a 2-panel or one of the 3 panels shaded, I tuck one panel under:
and the associated meter (ignore the fact it didn't drop, this is roughly a 0.7A per panel solar charger and the device attached is only wanting 0.9A)
Then to simular a 1-panel, here via 2 panels tucked under, now you see a drop, as the device attached is capable of ingesting 0.9A but the 1 panel is only giving about 0.7A maximum output.
and the associated meter
And finally, to prove the point that parallel vs series and semiconductors is very important to understand, back to all 3 panels, now someone less knowledgeable would think if 3 panels is about 2A and about 1/3rd covered I'd get say 1.4A and so the device can pull 0.9A, we'd see 0.9A like we saw off 2 panels full seeing sun... right?...
- buy a meter when you buy solar, learn your solar panels foillables
- partial shading of all panels can kill the whole solar panel
- a lot of devices can't ingest as much as a big panel can output, so pair up your panel with your device. In my experience a phone can pull about 1.4A-1.6A, a tablet about 1.7A-2A. A "21W" or even "19W" is more like 10A so 5V 2A so it is well capable of charging most devices, including useful in cloudier days where all of a panel receives some sun so it acts to make some voltage.
If Anker wants to me send me their solar products I'd happily review. I might even buy another meter so I can measure both ports concurrently, in perfect conditions you're beginning to exceed what a device can feed from 1 port.