You'll need 6 HIGH-QUALITY 150w HPS ballasts. Get ones with a remote base so you can keep the ballast out of the grow area. (they produce a bit of heat, even 150w ballasts). I say high quality because they draw more than 150 watts between the bulb and ballast. A cheapo ballast will draw even more wattage. String 6 of those together and you'll most likely overload even a 20amp circuit. Especially if you don't replace your bulbs every grow. There have been studies that showed a crappy ballast and an old bulb can draw up to 450 watts. (Might as well get a good quality 400W hps and ballast and be done with it) Is there a reason you want to go with HPS? If height is your issue (150w have only about a 9" optimum foilage penetration.) and you're going to use Scrog or LST to keep your plants within the useable light of the bulbs... you might as well go with CFL's. For the same price you'll probably be able to get more bulbs and sockets with CFL's. Just make sure of a couple things. 1) Use lower wattage bulbs like 23w or 27w. I prefer the 27watt. You're going to use about 100Watts per plants. 2) Keep every part of the plant (except bare stem) no more than 6" away from a bulb. 3) Mix daylight spectrum bulbs with your 2700K bulbs during flower. One daylight per every 2 2700K bulbs seems to give most people the best results. I've grown with 150w HPS bulbs and you can do a great job. They put out more heat and require more ventilation though. (More cost) That and CFL's can be picked up nearly anywhere dirt cheap these days and work with just a standard light socket. Good luck!