Magnetic ballasts are the tried, true, and less expensive option for grow light ballasts. They come wattage specific i.e., 1000w, 600w, 400w, etc... (not dimmable), and can either run high-pressure sodium (HPS), metal halide (MH), or both depending on their specifications--switchable ballasts can run both HPS and MH bulbs. Switchable ballasts allow you to have one ballast and switch your bulbs out from vegetative to flowering spectrums.