The role of the ballast is to help provide enough power to get the arc started when you turn your light on. After this, the ballasts job is to regulate the current, providing a steady beam of light. Without a ballast, the grow light would get too hot and pop. As HID grow light age, they require more power for operation, and a ballast also helps with this. When shopping for a ballast, it is important to consider the watts of your grow light. If the wattage of your ballast does not match up with your grow light, you will see poor operation. If the wattage of your ballast is too high, it could blow your lamp. If it is too low, it may not be able to get the lamp to fire.