It does sound still doable. Low-power mcus can be really low power these days, I checked a random one and they advertise 25uA/MHz active mode, and for a pocket

It does sound still doable. Low-power mcus can be really low power these days, I checked a random one and they advertise 25uA/MHz active mode, and for a pocket calc you dont need many MHz. https://www.microchip.com/wwwproducts/en/ATSAML11E14A https://www.microchip.com/wwwproducts/en/ATSAML11E14A On the other hand, these small solar cells are rated 5V/30mA, but I think they'll be more in the uA range when used indoors. https://www.amazon.com/AOSHIKE-Electric-Materials-photovoltaic-53x30MM/dp/B07BMMHMSJ https://www.amazon.com/AOSHIKE-Electric-Materials-photovolta... So might be bit of a stretch, but not too far off