What is Casio Tough Solar?
Casio uses a proprietary solar charging system in their watches called Tough Solar.
It is designed to charge a rechargeable battery and power all internal systems (sensors, time-keeping etc.) for approximately 2 years on lower power.
It operates not only from the sun, in full daylight, but also from low-light or indoor lighting such as fluorescent lighting. It is quite unique in this way as it grabs power from just about anywhere and keeps your Casio watch almost all the time.