The Solar Station is a device which harnesses and contains solar energy, known as Soll, and stores it so that it may be extracted for later use. It is used mainly for recharging the player's energy/battery. The station charges when you manually input soll, or while walking around in sunlight. In Boktai 2: Solar Boy Django and onwards, you can withdraw soll from the station to the Solar Bank, where it is used as a currency.
So far the Solar Station has appeared in all Boktai games to date. In Boktai: The Sun is in Your Hand, the limit for the Solar Station is 999. From Boktai 2: Solar Boy Django and onwards, the limit is 9999. If the Solar Station runs out of soll, the player cannot recharge their energy/battery from the Solar Station until more soll is collected. However, you should not worry; you can gain soll from the sun even if you're walking inside a dungeon.
The Solar Station's value can be manipulated through the use of the Solar Bank in each game. In Boktai: The Sun is in Your Hand, the Solar Bank's one and only use was to put soll in the Solar Station. From Boktai 2: Solar Boy Django and onwards the player could choose to put soll from the Solar Station into the bank so that the soll could be used as a currency.