Here's some deep physics analysis into the requirements for using solar panels to charge an electric motorcycle or other EV.
Some vehicle makers might add solar panels as a way to woo people into thinking the car is extra cool. Such as Fisk'ers karma which has solar panels on the roof ... but the roof is so small it only provides a modest amount of power, enough to run the in-cabin stuff like air conditioning.
Anyway.. the guy goes to the trouble to calculate wind resistance to estimate power requirements. I think it's a bit simpler, that you can simply measure the power used to recharge the battery averaging over several rides. I've done that with my motorcycle and know it took 0.2-0.3 kwh per mile (over a 10 mile journey I took several times a week).
So if you know you have n kilowatt-hours to put into the bike, how many solar panels for how many hours are required to recharge the bike? And can you manage to fit that many solar panels onto the bike?