The space used by the smallest solar charger I’ve seen on Amazon seems to be similar to 6 or more batteries in the format the N900 was taking - so if you look at space, slow charging from solar charger, and reliance on sun conditions taking individual batteries seems to be the better option for a few days hike. It’s also easier to stow individual batteries to wherever you still have space left.
One exception nowadays: Business notebooks - and that’s only because the rest of the notebook market went to shit. If you want a somewhat compact notebook with more than 64GB of RAM, decent CPU performance and good battery life Apple currently is the only one offering something.