Ebike Range Extender

I want to do a long trip that involves 100 miles in a day and then another the next day. I could take a spare battery and charger. The 20AH 48v battery weighs 4kg. The cost of a spare one is £330, which works out at £1.65 per mile plus electricity cost. I can do about 50 miles on one charge with my battery if I pedal with medium effort, 25 miles if I rely mainly on the motor and 100 miles if I pedal really hard, which I don't think I'd be able to sustain for 20 hrs in two days. Charging is always going to be a problem. It looks like I'd need about 5 charges, each taking about 8 hours, so that's not really going to work unless I could charge two at a time, which adds even more weight for the extra charger.

That's the problem: Are there any creative minds out there in the ebikeasphere that have creative solutions?

I thought of solar charging to help, but it's never going to be able to keep me going, and it might be a dull day.

I also thought of putting a shaft on the bike from front to back, with a propeller on each end. As i go forward, the wind on the front one would turn the shaft, which would turn the back one and push me forward. Does that still count as a legal 250w ebike if i use it the same time as my motor?