Different lighting systems have different ballast needs. The HID (High Intensity Discharge) lighting systems used for indoor gardening includes MH (Metal Halide) lamps and HPS (High Pressure Sodium) lamps. Both of these types of lamps use a specific type of ballast. It is very important to choose the correct ballast for the lamp type and voltage because a incorrect ballast may burn out your lamp or even become a fire hazard.