Formula #1 says that the optimum year-round angle is 25.6 degrees, but that subtracting 15 degrees in summer and adding 15 degrees in winter will further maximize sunlight (and therefore panel production efficiency).
Formula #2 is a formula that yields following angles: 24.1 degrees in spring and fall, 3.1 degrees (so basically flat) in summer, and 55.69 degrees in winter.
Which one? I haven't decided. At the moment, I've got it at about 45 degrees.
When I set it up early this morning, it was only producing a watt or two. Unsurprising with the sun being so low and also having some inconvenient trees that block some light at that time of day.
But circa 8:45 am, with the sun not that high in the sky, or anywhere near in the direction the panel faces (due south), I'm getting 14-17 watts out of it.
Which brings me to my mild complaint:
Most inexpensive electronics are of Chinese manufacture, and apparently most of the manuals for that gear are written by people who aren't especially fluent in English.
This whole time, I'd been under the impression (from diagrams, labels, and manual language) that the wattage measurement on a particular part of my power station screen was output. Which confused me, since theoretically my Raspberry Pi 4B and Echo Dot combined draw something like a maximum of 10 watts combined (closer to 5 watts at idle), and I was seeing 10-15 minimum, with spikes to more than 20. I wrote that off as including the consumption of the power station itself.
Well, that output level is actually the input level. I'm glad of that, but I wish I had known it before.
So, if I revert to an assumption of 10 watts maximum draw for the computer and Dot, that's a maximum of 240 watt-hours per day, and in theory the panel should produce an average of 672 watt-hours per day. I turn the computer off (and could unplug the Dot, since it's in my office and I'm not) at night when I go to bed, so that's less consumption (more like 160 watt-hours). So I could probably run another thing or two off this setup, although I'm not sure what I have that fits into that differential.
Another mild documentation complaint: The listed dimensions of the panel are 85" wide by 22" tall. The latter measurement seems to be correct, but it's about a foot narrower than the former. So the frame I built while waiting for it (as you can see from the photo) is not really correct in size. I could have saved myself both a little lumber and a little extra work piecing together scraps to get to that width. I'll be recombobulating things so that the panel is attached to, rather than just leaning on, the frame, and maybe buying some plexiglass to put over the panel and give it some weather protection.
Update, 1:37pm: In nice full noonish sun, I'm seeing panel production in excess of 30 watts, with spikes up to 40. I do see a problem, though: It looks like the power station stops drawing from the panels once its battery is full, and only starts drawing again once it comes down to some threshold. Which, as little wattage as I'm pulling, means I'm having "dead spots" where the solar production isn't being used. Hopefully the fact that I get up and start working at oh-dark-thirty will let me draw the battery down enough that I get maximum "yes, the battery wants that solar-generated power" time during the day, with it hitting 100% about the time I shut down for the day.
Update, 11am 12/09/22: So far so good. I got up this morning at oh-dark-thirty, went to work, and by the time the sun came up the "power station" was down to 64% of its storage capacity, having received no solar input overnight (obviously). Now it's at 99%, with the solar panel cranking as high as about 60 watts at times. If this is anything like typical, I should be able to run these two devices in perpetuity (well, until something wears out, anyway) without ever plugging into the wall outlet.