The wife and I have gone on 2 E-Bike tours (trips) together, about 850Km each. We took out camper as our sag vehicle so one of us drove and one of us rode the bike alternating, so each of us actually only road about 425Km each trip. We used the solar panel on the camper to re-charge the batteries every day never plugging into a wall socket the whole trip... so Yes it can be done using a folding solar panel the size of a computer bag that you can carry on the bicycle if you want to go solo, but they are big $$$.
Or you can carry a spare battery but then you need to plug in somewhere...
We can easily get 120Km the wife, to 160 Km me, per charge out of a battery
depending on how much you pedal, if I don't pedal and use motor only I would only be able to get about 30Km per charge,
so 30Km is about the total assistance I can expect from my 350 BionX set up per charge out of the total 160Km ridden, same for the wife 30Km of assistance per 120Km ridden. As for riding the bike without power, yes you can, it's just like riding a regular bike no real difference at all, with my set up, tho there can be depending on the system, some of them have very noticeable drag so one needs to test/find out if out first before purchase...
EDIT: So this is how it breaks down in our case...
Manufacturer's numbers; generally work out to 30Km per 100Kms traveled. =30% assistance would be used.
Wife's numbers; generally seem to work out to 30 Km per 120Kms travelled. = 25% assistance used.
My numbers; generally seem to work out to. 30Km per 160Kms travelled. = 19% assistance used.