Hi guys. I am starting new grow in couple of days and I am dealing with dilemma 
I have 75x75x160 tent to grow in. Until now I used 250W HPS for bloom and now I have to buy u new one and I have a chance to buy 400W HPS but I am not sure if its now to much. Wont there to much heat in the tent? Isn it to much light for 9 plants? Is it big difference between 250 and 400? Thanks for opinions. I dont know what to do now I dont want to buy 250 and after few week realise that I could easily run 400W. Thanks! 
I think the 250 will fit the space better unless it is dimable. Excuse my imperial measurements, but 75x75 is about 6 sqft. With the 250 you will get about 41 watts/sqft. With 400 you would get about 66 watts per square foot. The recommended is 50 watts/sqft. The 400 will work, but you will most likely need to also upgrade your ventillation.
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you could easily vent the heat of a 400w HPS out of there with a centrifugal fan. My only concern would be the 160cm height. The tops of the colas need to be about 12-14 inches, minimum, away from the bulb. If you tend to grow taller plants that could be trouble.
But the move from 250w to 400w HPS is huge, I can still fondly remember when I made the upgrade. The 400 watter brings the intensity necessary to form larger and denser buds similar to 1000W grows. The 250w will not penetrate down into the canopy as well. The electricity difference is small. I would try the 400w.
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maybe the 400 with a cool tube
Thanks a lot guys I got the new tent (75x75x160) and it seems to be ideal for the 400w. There is a lot more space so I am not worried. I Have cooltube and I can eventually buy better fan. I will put the HPS bulb in to the tent in a week. I will write if its ok or overheated Bye
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