This 400 watt switchable ballast is best suited for use with High Pressure Sodium (HPS) or Metal Halide (MH) hydroponic system. It provides you with the flexibility to select the ultimate lighting condition for your indoor garden by switching to the desired bulb, instead of having to maintain two separate grow light systems. Use MH grow lights for faster vegetative growth, and then change to HPS grow lights to promote flowering, fruiting or budding.