More solar than you could possibly carry.
To charge a 15kWh Zero in a single day with 8 hours of good sun, you'd need over 16 square meters of solar panels (4 meters by 4 meters). That array would provide about 2kW during the middle of the day but the electronics and batteries both lose some energy as heat. It would provide less power during morning and evening hours.

While solar is "simple" and works great with no moving parts, the energy is less dense than other sources. My off-grid house works great with all solar, but I don't want to move it every other day.

New lithium battery technologies are getting better and other chemestries like carbon-carbon are fare better. These are quickly increasing the range of electric motorcycles. But the energy per mile is still limited by friction and air resistance.

Have you read about Terry Hershner's Bun Burner? 1,000 miles in 24 hours. He spent just under 1/3 of the time charging, but he could plug into multiple high-amp car charging stations at each stop to dump huge amounts of energy into his motorcycle.

A RTW trip would require trying to stay along the the power grid. Even as close (to me) and civilized as northern Canada, there are whole towns operating on electricity generated from diesel fuel. Their electricity is expensive and the towns are not very close to each other. Petrol is portable -- you can run out of gas by the side of the road and have some brought to you. I suppose you could convince someone to bring a portable generator to you, but it'll be a bit harder.

EDIT: So how far COULD you go on solar? Electric "cars" cross Austraila with solar, but they're more like aerodynamic solar arrays with 4 bicycle wheels underneath. Back to Terry Hershner's Zero motorcycle, his trip used 121 Wh per mile. If you got 2 of the 300-watt folding solar arrays, you could move forward about 32 miles for every 8-hour day of charging. His trip was at high speeds but his Zero was very aerodynamic -- going slower you may extend your range. Each 300W array is 19Lb or under 10kg and folds to about 22" by 16" by 2" thick.