These subseries come with Solar Assistance
Watches with Solar Assistance use a solar panel to extend the battery lifetime of the watch. Solar Assistance is found on watches with sensors with high energy demand (e.g. gps).
Other than ToughSolar, the solar panel does not provide enough energy to keep the watch at full charge at all time. Especially if the functions of the watch are used extensively. Watches with Solar Assistance are equipped with a charging port to recharge via cable.