Solar-powered watches are designed to convert natural and artificial light into energy. (Artificial means that a solar watch can be charged by a lightbulb for example). Solar panel of a solar watch absorbs light and converts it into electrical energy then store it into a rechargeable cell that powers the watch.
All you have to do is expose the watch to any source of light, and it will start charging. You can say that a solar watch is always charging (unless you keep it in total darkness).