I use a 430w HPS on a light mover in a 3' X 6' area. My electric bill went up about $50.00 a month. Two 400's maybe $80.00 a month, I'm guessing. The mover allows you to cover more plants, and gets down inside the bush. I found this to be a good set-up for me. I just finished my 7th week of 12 X 12, and am very happy.