So I do not own the Anker 21W but I own equivalent technology (which includes the zipped pouch you mention in your video) and a meter and I'd say in clear strong sunshine with good alignment to the sun you'd get a minimum of 2A 5V 10W output, the Anker has been metered at 11W. Each port of the two ports can be upto 2A so you begin to discard solar energy if you use only one port in perfect situations. The Powercore II 20000 you refer to has 2x2A so it can ingest all the available solar energy if you get into perfect solar conditions. The Powercore 26800 would also (but not any other Powercore, those are the only two which can).
Not all sunshine is the same, and the eyes cannot detect the difference because more energy per photon comes from UV which our eyes cannot see. This is stronger at altitude and when the sun is nearer noon.
Each of the 3 panels are a set of serial cells, and the 3 panels then connect in parallel. So if you partially block 1 panel it renders that 1 panel as no output. So (once you get a meter) if you have the shadow from tree branches over the 3 panels, each is partially shaded and each goes to nil. This is because within each panel the series cells are either resistor (absorbing energy) or conducting (and releasing energy) so the partial shading of part of a panel causes the whole panel to become blocked by the parts of the panel not conducting. So really cloudy skies are better than partial shade. The electronics in the Anker 21W (and equivalents from others) will see a dropping voltage from the cells in shade, and compensate via stepping down the current and stepping up the volts to hold to a minimum of 4.2V to push electrons into common devices (like the Powercore) so it produces in effect a varying current constant voltage output through the varying sunshine.
If you simply leave the Anker 21W pointed in roughly the right direction then most of the time it is not perfectly aligned, so say it produces an average of 7W (max of 11W) then the Powercore II 20000 which is a (Guessing!) 20Ah x 4.2V = 82Wh ? So you'd expect an average (assuming 10% power loss in the conversions) 82/7/0.9= 13 hours to fully recharge. If you simply left the Anker 21W at the southerly direction and left it all day you'd get say 6 hours @ 7W so you'd roughly half recharge the Powercore II 20000 in a day. If though you revisited the Anker 21W and moved it little between morning, noon, afternoon, that would improve and then possibly more 9 hours @ 7W so 70% recharge. Your words above align with this calculation. I'm wondering if you at altitude with more UV?
IF you metered the fully energy lifecycle you could then measure all the mAh into the Powercore from the Anker 21W and meter all the mAh out of the Powercore into your other devices. If you did empty to full to empty then you could "count all the electrons" and then calculate the efficiency of the Powercore.
So you see this can take days, or weeks of effort to do a full test of the full energy lifecycle. Practically none of that that commitment to measurement.
What I do:
- I don't touch Apple stuff whatsoever.
- I use a basic phone which is very tolerant of the variable energy output of solar.
- a smaller powercore (my fave is the Powercore 10000).
- The morning sun I use whilst cooking and packing etc is not as strong sunlight (less UV) the solar panel is not at peak output, say 7W but that perfectly balances the phone, so i can recharge the phone in the morning.
- If the Powercore is not fully charged, then I can just strap the solar panel in the general southerly direction and by end of the day be fully charged.
- Use the phone and/or Powercore in the evening.
- in a group the Powercore tends to end up empty by the morning. On one own tends to be nearly full by morning.
When on one own you "don't have enough demand" for the 21W so a little bit of weight shaving is perfectly fine, so the 14W is a better option in summer. In winter as the sun is lower in sky, passing through more atmosphere which reduces the UV then you really need the 21W to keep the voltage up enough to make useful energy.