I bought this meter from Amazon to measure. Amazon sells quite a few, but many don't measure over 100W. This measures to 150W. https://www.amazon.com/gp/product/B07JYVPLLJ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Thanks for this very informative write up! Thanks also for keeping the rabbit hole open that I find myself going down often. I also want to tag /u/ilreppans who has passed on lots of information that I have used.

Here are few personal observations and further tips:

I found it useful / essential to have an inexpensive USB multimeter to see both the output of the solar panel and the charge transfer of my batteries in real-time -- whether I bring the solar panel or not. The meter confirms for me that I have my cords, cables, and adapters all connected properly, so that I am getting the most efficient and fastest charge transfer possible.

From other reports of yanking off the electronic components from the backs of panels, I first added a bead of E6000 glue to those components and have never had a hint of failure. Some folks have used epoxy. :https://i.imgur.com/S32ytSk.jpg Close-up also shows 2 holes drilled for shock cord strain relief and keeping USB-cable inserted in solar panel jack while still allowing "break away" action: https://i.imgur.com/qzXommU.jpg That was my solution to keep the plug inserted in the jack instead of your yellow electrical tape

I posted this video clip of using an umbrella to help keep solar panel tilted both horizontal and vertical (longitude and latitude) at the Sun: https://imgur.com/a/Dtira1M At one time I did have the 2170 battery mounted on the back of the panel with a short cable, but switched to the longer USB cable so the battery could go in my hip belt pouch without any stress on the jack. Another possibility would be to use the 12 g Nitecore MPB1 (magnetic power bank 1) and a 21700i battery. This would allow magnetic "break away" under adverse stress, but I am not sure if that product will remain available in the future. Preliminary testing suggests though that "break away" happens too easily, so maybe shock cord plus magnetic force would be better. https://i.imgur.com/0y2kWE9.jpg

I've added an insertable straight piece of plastic to act as a "sun dial" to let me know when my panel is pointed directly at the Sun. If the piece has no shadow, then it is pointed directly at the Sun. https://i.imgur.com/IDCJXkp.jpg

I do not live in the American West nor at high altitude so I am glad you highlighted environmental conditions. I have measured my typical power consumption and it is 9 to 10 Wh per day. My experience is that a single panel can produce that in nearly ideal conditions (i.e. sunny all day, low humidity, above 5000 ft), but I'd typically run out of power in 3 days if I had the setup you showed. And certainly, 3 or 4 cloudy days in row - especially if cloudy from hours of 11 am to 2 pm -- would make me stop using my phone for photos and videos as you mentioned.

I received one of those new batteries yesterday and preliminary testing shows it works fine, but is probably between 80% and 90% efficient, so one does not get a full 18Wh of usable charge out of it when it is fully charged. Also these batteries are not rated to output the 18W of the Nitecore NB10000 and F21i though they certainly are able to do so despite warning text all over the battery. This is not a problem since one would probably charge their electronic devices when Sun is not available. They do not seem to have PD and QC protocols (chipsets?) that phones use.

Finally, when I was using my solar panel to keep all my backpacking electronics charged for more than 2 months I had to ABC: Always Be Charging and constantly thinking about it especially when stopped. While I sat in the shade, I had to make sure my panel was propped up pointing at the sun.

Thanks again!

Added: Be sure to pay attention to any of the LEDs that these batteries use to tell you what they are doing. Mine have red, green, white, and blue LEDs. Since the new 2170 RX battery can both charge and be charged through the same port, I found that my phone battery was being used to charge it when I wanted it to charge my phone! While I haven't completely sorted this out yet, I think that happened because I was too quick changing from charge-the-RX-from-wall to charge-the-phone-from-RX. I think one should wait until all the LEDs are out (indicating the RX has reset) before switching to a new task or configuration.

Don't bother with calculations when you can DIRECTLY measure what is going on. Get one or two of these by tomorrow: https://www.amazon.com/Eversame-Multimeter-Voltmeter-Indicator-DC3-6-30V/dp/B07JYVPLLJ/ Weight is about 16 g, so take it with you and don't ever be in the dark again about transferring electrons from/to any of your devices and batteries.

I am not at all sure about your calculations since mAh often assumes 5V while Wh doesn't assume that especially since devices charge with different voltages nowadays and Li-Ion chemistry provides about 3.7V that gets stepped up to whatever.

Also if you have shitty cables, then a cable could use up to 20% of the charge and convert it to heat which is another way of wasting the charge.

Im using this meter when charging OPs. It can check voltage and current. very useful.
https://www.amazon.com/Eversame-Multimeter-Voltmeter-Indicator-DC3-6-30V/dp/B07JYVPLLJ

Not the exact same one I bought (mine has poorer reviews)

Eversame 2 in 1 Type C USB Tester Color Screen LCD Digital Multimeter, USB C Voltage Current Voltmeter Amp Volt Ammeter Detector USB Cable Charger Indicator DC3.6-30V/0-5.1A https://www.amazon.com/dp/B07JYVPLLJ