Andy, sorry for the late reply. Somehow, working on a small phone screen without my glasses I had inadvertently marked you as ignore. Only saw your post when back home on a larger screen and not logged in!
The answer is that it is a balance between battery capacity, panel power and the trip details of weather, distance per day, hilliness and so on.
My setup is a version 1, just how things fell together rather than a considered in detail conscious design. It has a particularly small effective battery capacity of only about 300Wh, which means I need to live fairly closely to within what I am generating moment by moment. That's 30 miles at my typical 10Wh per mile, and I have just completed 470 miles without any external charging. The panel size of 330W is roughly what I have read of other solo solar bikes using, and has proved itself sufficient to work with the small battery in all conditions so far encountered.
Riding with the panel area fully deployed is an essential part of long distance infinite range touring, because the middle six hours of the solar day are the most powerful sunshine. There is useful charging at the ends of the day especially with the trailer tilted to the sun, but it can't replace those middle hours.
A bigger battery would make the greyer days less stressful, and might allow two panels instead of three do the job.
Folding, or more likely stacking panels, would be useful for going by train or plane, but that is not on my agenda. Also for a compact setup for riding into wilder places. Having said that, unless the trailer was made a one wheeler, a small top would not make much difference to the way this Burley based one rides. It is remarkable given the roughly 1 inch of suspension travel!
In full sun, at rest with the trailer tilted I have seen 8amps or slightly more occasionally, at 38 to 41 V, so these nominally 330W panels get pretty close to full rated output. Thus if my 300 usable Wh are empty, an hour of good sun is enough to fill them up again! But the key is to deplete slowly, in parallel with panel output, to stretch those 300Wh over enough miles or hours to get to the next sunny rest stop or evening. The numbers I posted yesterday show that approach in detail.
A different type of expedition could take a very different approach. For example, going into the wilds to camp and climb for a few days, having a battery just about big enough to get there but not back, a single panel, which would mount fairly well on a standard size of kiddies or cargo trailer, maybe with a slightly extended towbar, and a charge controller is all that is needed to charge at camp to come home. 3kg of weight, 1.2m by 0.6m area. Assuming a generic type ebike system.
