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.