Casio fx-991 CW ClassWiz – Get Yours Now on Sale! Dual Power Source : Solar power with battery backup and auto power-off for longer-lasting performance.
Casio FX-991CW the best calculator I have had - so far
Online manual is ok but the pdf version is better.
The physical constants are not explained as to their meaning. Maybe Casio assume if you need the value you'll know the symbol and the units. I noticed at least one value is fractionally different from the latest values.
I hope the next generation of calculators will have units and unit conversions built into the arithmetic e.g. X feet squared times Y psi equals Z kN per metre squared.