Casio fx-3400P. A solar-powered programmable scientific calculator. Yes, you can take the battery out and it will run completely on the solar cell. Even without it, the battery is projected to last for up to 7 years. It has some dozen microwatt energy consumption. It was manufactured _before_ 1989 (probably 1988 or even 1987) and still works perfectly fine despite how thin and fragile the PCB is inside. A friend of mine gave it to me in exchange for some other interesting calcs. He bought it at a flea market for under a dollar or so. Still in a mint condition in the stock holster.

This is how pre-consumerism era electronics looked like. 35 years, 1 battery change, still rocks.