Which is also according to brochure. Well, brochure states 20V as output voltage when on battery … but the it wouldn’t surprise me if output voltage was not exactly DC due to DC-DC voltage conversion which is probably realized by high-frequency switching and if output would be observed with a decent instrument (e.g. oscilloscope) some AC component might become visible and most of consumer-grade instruments get slightly confused when they’re presented with (relatively small) AC component when they are requested to measure DC.
So to the “used source” dilemma: the only way of prioritizing one power source (e.g. solar panel) over another is to choose the “non-preferred” to have lower voltage. In your particular case, using 18V wall power adapter would do the trick, mUPS would use solar power until voltage dropped below 18V (or whatever wall PA output) and would switch over to the wall PA. But if solar power voltage is not stable (and I guess it’d rise sharply after disconnection of load) it would start the ping-pong between both power sources (as mentioned in post #2 above). Not nice to both PA electronics at all.