I've always felt that this was a meaningless PR stunt with no practical application. We already know what PV-battery planes are good for, HALE missions where speed is irrelevant. There is no development path to make this practical at all - "Yes, now you can fly around the world in more time than you could cycle and sail around it, in extreme discomfort at a high price." Kind of reminds me of Lindbergh's non-stop New York- Paris trans-Atlantic flight which also bore no relation to any practical development, and was essentially a dangerous stunt. The necessary distance and endurance with a suitably reliable engine (Wright J-5C Whirlwind, used by all the successful 1927 and 1928 trans-oceanic a/c bar one*) had already been demonstrated a few weeks before by Chamberlain and Acosta in the Wright-Bellanca WB-2 "Columbia" ( which Lindbergh had previously tried to buy and failing at that, had the "Spirit of St. Louis" built to his specs), and that was with two pilots.
There was no practical value to be gained by flying the Atlantic solo in a single-seat, single-engine a/c with no direct forward visibility, no radio, and no way to take star sights or use a drift meter, where the pilot had to fly non-stop for 33.5 hours. Lindbergh was actually 'awake' (although there were numerous periods during the flight in which he was briefly asleep or hallucinating) for about 60 hours straight before he finally got to bed in Paris. Lindbergh used dead reckoning only, and was very lucky he didn't hit worse weather or winds significantly different than forecast - he also had extra fuel (calculated after landing in Paris to have enough to go about another 1,040 miles), and as he himself said before the flight when asked by some Navy pilots in California how he planned to navigate solo, said that even if he was way off he could hardly miss all of Europe. Commercial aviation requires a lot better accuracy.
Commercial aviation wasn't going to fly with a single pilot, would require multi-engines with single-engine out capability, wouldn't do a trans-Atlantic trip non-stop (at that time) but would make shorter hops with stops in Newfoundland and Ireland (the critical North Atlantic leg which Alcock and Brown flew in 1919, and which remained needed stops for fuel westbound up until about 1960), and would also require a radio operator and dedicated navigator with full facilities for celestial and dead reckoning nav. including measuring drift, given the tech at the time (and would also provide a toilet, moreover one accessible to all crew and pax). After Lindbergh made his flight, Chamberlain and Levine followed a few weeks later and flew on (in the same WB-2 that had set the distance/endurance record) past Paris trying to reach Berlin, coming up a bit short and landing in Eisleben, Germany.
Finally a few weeks after that, Balchen, Acosta, Byrd and Noville in the Fokker Tri-Motor "America" equipped with full radio and nav. provisions made the crossing, the only one of the three which had any development potential for a commercial capability (after reaching Paris they were unable to land owing to fog, flew back to Normandy looking for clearing, and ultimately had to ditch just off the coast). I don't blame Lindbergh or the others who attempted the flight in a/c that were unsuitable for anything other than showing that the distance could be flown - that was the fault of the organizers of the Orteig Prize, who didn't require more practical a/c and more safety equipment. Enough people died trying, in that year and the next on both oceans (cf. the Dole Race to Hawaii) to demonstrate just how irresponsible many of these attempts were.
Even with appropriate a/c, equipment and crews, there was at least five more years of technical advances needed before trans-Atlantic flight could be commercialized, as all the a/c making trans-oceanic flights in 1927-28 had to be severely overloaded to carry the necessary fuel, and couldn't carry any passengers. They needed the development of variable geometry (flaps and variable pitch props), improved engines with lower power loadings and SFC, metal stressed-skin fuselages, retractable landing gear, NACA cowlings, longer and smoother runways (preferably hard surfaced) or else use seaplanes (as Pan-Am did because they couldn't afford to build long, hard surface runways) and blind landing aids (radio D/F, homing and nav. beam aids were already available, and one or more was used on the trans-Pacific flights that year and the next), autopilot, better blind flying instruments plus pilots trained in instrument flying (only a few of the 1927-28 pilots were, including Lindbergh, Chamberlain, and Balchen), plus much better weather reporting/forecasting. Realistically, given 1927 tech you needed a three or four-engine a/c for any commercial trans-oceanic passenger flight, and that remained the requirement for passenger flights up until the 1980s.
In 1928, Kingsford-Smith and Ulm (pilots), Lyon (Nav) and Warner (radio) flew the Fokker Tri-motor "Southern Cross" from Oakland to Brisbane, Australia via Hawaii and Fiji (3,128 miles and 34.5 hours non-stop from Kauai to Suva, a much greater over-water distance than that across the Atlantic), and were in radio contact pretty much the whole way, using radio beams plus D/F from ships and celestial nav. Hell, Kingsford-Smith with various crew flew the "Southern Cross" around the world over a period of about two years, and that was interspersed with multiple other flights for different purposes (including the first crossing of the Tasman Sea from Australia to New Zealand, and back) rather than an a specific attempt to do so, so I'm completely underwhelmed by this stunt and don't see any value to it beyond PR, other than the purely personal accomplishment for the pilots.
*The Junkers W 33 "Bremen" flown by Kohl, Fitzmaurice and von Hunefeld with a Junkers L5 engine that made the first successful westbound heavier than air crossing in 1928 from Ireland to Greenly Island, Quebec, arguably more significant than the longer distance eastbound (downwind) flights of Lindbergh and Chamberlain/Levine in 1927.
