My wall charger draws 90 watts, to charge a 36v Prodeco ebike battery. The charging voltage is in the mid 40's, so the amps run about 2. If my battery (16ah) is fully discharged, it takes 8 hours or so. A 100w solar panel would be fine, but the voltage output would be wrong. Given that most ebikes put some of the charging circuitry in the battery box, and some in the charger, the only 'really safe' way to do this is with another battery that you use to charge the bike, with the bike charger (and an inverter).
You can't take a 5v solar panel (which is USB voltage) and charge much of anything else. I can link you to a DC-DC upconverter that would let you charge a 12v deep cycle battery, but at 14w it would take too long to be useful. Relatively speaking, 100w panels are cheap.
Solar systems tend to be 5v for folding stuff, 17v for lead acid batteries people use in RV's. An ebike is a 36v to 48v system, so mixing and matching is not simple.
This only weighs 20 pounds, so you could put it in a little bike trailer. I guess this isn't where people want to go, or how they want to get there. Something like that.