correct, which is why:
- buy a meter
- learn the relationship between sunshine and output
- learn how your device tolerates erratic and lower current.
So I'll give you two examples:
- my Moto G4 Play is my "camping" or "off-grid" phone because it lasts a long time, it is a less powerful phone, I put a meter on it when attached to a 2A output wall charger and saw it is incapable of ingesting more than 1A
- my smaller tablet for "off-grid" is an old Nexus 7, it cannot ingest more than 1.6A. These two devices cost total of $187, given if I'm hiking an accident can only crack a low cost device.
- in summer noon sun my 21W gives about 2.6A output, so both the phone and the tablet can be recharged concurrently at their fastest possible speed.
- in less than ideal situations I see about 1A output, so then I choose, I plug in the most discharged device.
- if my devices are fully (or mostly, >70%) charge, then in weaker sun I would probably be best recharging the most discharged device, or the most important device (phone first, then tablet).
- in strong sunshine, if I my devices are mostly fully charge, then I'd plug in a 10000mAh Powercore to make use of the energy to story to carry me through any future wet / less-sunny period.
- I find an average of 5Wh to 10Wh/day is realistic from solar, then look at your devices and calculate. The Moto G4 Play in an off-grid situation its 10Wh battery and lasts me 2 days (phone needs 5Wh/day) so you're talking even in a sustained bad weather period, a few days worth is valid, and then weather turns better and "too much energy" to which the 10000mAh soaks that up then handle the next rainy days, repeat, so pretty much can be off-grid indefinitely.
Each device is different.
Water and solar - I'd not advocate it in general, Anker products are well made and probably tolerate rain but not by design, try to avoid it. One way is to place indoors, inside your tent, and even though the tent shades from sun, you might get something useful. Remember though batteries hate heat. Much better though is to take advantage of dry sunny periods and soak up the energy, either recharge device, or if they are mostly charged store that energy in a 10000mAh Powercore to see you through a damp dull period.
Buy a USB testing meter, they are far more reliable information of what a device tells you, because most devices, when you turn on the screen, to operate an app, they reduce their input due to thermal throttling, so the figure you see is lower than true. You don't necessarily need to take the meter with you (but they are light and small so why not) but if you spend a few days learning your solar / device interaction at home, it really helps when off-grid, to make best use of your solar budget.