Power compacts are a newer version of flourescent lighting...they claim to be more intense then the VHO, but I have both and like the VHO better...VHO is another version of a flourescent light...dont quiet know the particulars, but the VHO is a brighter more intense, and higher wattage lighting then the normal flourescent...neither are similar in any fashion to a metal halide, which is a complete different ball game...