The solar version of Suunto Vertical has a solar charging feature that will charge your watch when it is exposed to light. Some watch faces have a complication icon that shows how much solar energy is currently being harvested.
On a sunny day, the sun can provide your watch with 30% more battery life during training and even more in time mode (calculations made with an impact of 50 000 lux).
Do not leave your watch in direct sunlight too long during days with high temperatures as it can damage the watch.
In addition to the complication icons in the watch faces there is a solar widget where additional data is shown. From the watch face, swipe up or press the lower button and scroll to the solar widget.
Do not use a screen cover, or similar, over your watch display because it can affect the solar charging.