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For flower we need 50 watts of HPS per square foot to get best buds for most strains. You can have as many plants as you can fit in 3 square feet, though you could probably get a 4 square foot grow with no problem. How many plants depends on your style of grow - from 1 large plant to 16 budcycles in 6" pots.
What you're going to be more interested in is your yield. A beginner can expect in the 1/4 grams of bud per watt HID; an intermediate grower 1/2 gram per watt; an expert 1+ gram per watt HID.
So with a 150 watt first time out you can expect 40 - 75 grams of bud if your garden is full and you have no great disasters.
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With such a small wattage bulb we get very little penetration into the canopy, the bulb will be wasted with a tall plant. Look into SOG and SCROG style growing. I have a simple low stress training style of growing I call Crop Circle Of Bud, I'll post a pictorial from veg, training, through to flower to give you an idea how simple LST is.
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If you have any electrical experience you can open your ballast and put an extension wire to the socket and bring only the socket and bulb into your grow area, keeping the heavy and hot ballast out of your grow area. I did the same with a 150 watt ballast I picked up at a building store for $70, I've had it since the 90s with no problem.
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