Ballasts are necessary for grow lights to operate. Grow Light Ballasts power the high intensity grow bulbs and lamps via 120 volt or 240 volt power. You connect a ballast to a reflector or socket by the cord that comes off the reflector or socket you screw your grow light into. Some ballasts power specific bulbs such as HPS or MH, and others known as "Switchable" can power both HPS and MH bulbs. Digital ballasts are also often dimmable meaning the wattage can be dimmed down e.g., 1000w ballast can be dimmed to 600 and 400w.