You would think that with the incredible advances in chip power efficiency, they could produce an edition that would be able to be run off a solar cell instead of 2 stupid and expensive CR2032 batteries.
I have an 11c. I bought it new in 1985. On its 4th set of batteries.
I imagine making your programs persist would be more challenging on a solar-powered device. Rechargeable batteries would be ideal, but the case is probably too flat for those.
The standby current of the original HP Voyager series was extremely low, a set of batteries (3 x LR44) would last a decade or longer. Even with batteries removed, the internal capacitor could retain the memory contents for hours.