Solar

A solar watch has a photovoltaic layer sitting under a translucent dial. Light passing through the dial generates current, which charges a rechargeable secondary cell, and that cell powers an ordinary quartz movement. Accuracy is quartz accuracy, because light only changes where the power comes from. Expect around 15 seconds a month on standard models and far tighter on high accuracy versions.

A full charge usually runs about six months in complete darkness. Most models add a power save mode that stops the hands while the movement keeps counting internally. That mode stretches the run to one or two years. Charging is undemanding: a few hours of daylight a week is enough. The exception is a watch worn under long sleeves in an office all winter, which can drift toward empty without you noticing.

The claim to check is never needs a battery. The cell is rechargeable and lasts roughly ten to twenty years. But it is still a component that eventually holds less charge and gets replaced at a service. Gaskets age on the same schedule as any watch, so a solar watch you swim in still wants pressure testing every few years. Dial design is constrained too, since light has to get through, which rules out some very dark or heavily textured dials.