halogens are absolute crap for growing Mj as they dont have the right light spectrum..you need a high pressure sodium light which are also used in floodlights..you'll need a 250w + depending on how many your'e planning on growing...if its jus a few a 250w will do fine! any more an id go for the 400w an more..be careful though 400w's+ get incredibly hot so u'll need some way of getin' rid of that heat!..there is lodsa online stores that sell hps lights jus' do a online search for hydroponics, hps light, whatever an you'll find what you need! happy growing!!