Best wattage for 3x3 veg metal hallide?

I've been using a Mars II 900 for vegging until now and it did the job just fine but I am going to re-purpose that light with a custom COB setup and combine it with my existing flowering light (400w Vero 29 COBs). I am considering a metal hallide is a smarter option as its cheaper and easier to replace a bulb. I was thinking 250w would be fine for my 3x3 tent. Usually 4-5 plants in there. I have an exhaust fan and I am hoping that should be enough for controlling heat. I'm not about to get a full venting setup. Temps are high 70s with the Mars II 900. What temps should I expect with 250w or 400w metal hallide? Thanks!

250w mh for veg is fine. 400 would be faster growth but would be hotter than your mars.
250w will run cooler than the mars been 100w less power.
Could also get a 315cmh, but might be more than you want to be spending.
Exact temp will be related to the rate of air exchange and ambient temp but at a guess low 70s.

three x three is an awkward size. i like vegging under t5s a lot better than mh, but the two foot fixtures aren't strong enough imo, and you can't use the 4 footers in your tent.
i'd look at getting a 250 for the larger plants, and hanging it slightly off center, with a 2 bulb, 2 foot t5ho for the little guys on the other side of the tent.

250 watt would be fine, 400 watt maybe cheaper to buy. if you don't veg real large plants and are handy you can make a diy CFL fixture, run all sorts of bulb colors, add/remove as needed, cheap and easy to replace and low temp.
I am playing with a screw in led (flood bulb type) and it is working pretty well. I know others are using them so my have more info there.

315 cmh would fit that build perfect imo. about $200 on amazon.

if u mean HID wattage, the industry standard for 1 square metre is a 600w HPS.
that being said, and given what i've seen from running 315's.. u can pull more by using 2x 315w.. i mean two lights, not a single fixture 630. Like all double fixtures ever made, the sockets are too close. u need the freedom to move both light bulbs independatnly. a single 315 is not enough either.. a square meter needs at least one and half 315's (2 per every 2 squre metres) and u cannot cut a bulb in half, so run two 315w's..
two 315s will run you under $400 (growers supply).. IDK how much DIY cob kits run.. a single 600w HPS will be in the low $100's.. and a DE 600w will be a bit more..

i cant edit posts over darknet (dang scripts, smh).. i meant THREE x 315 per every 2 square metres.. which equals to 1.5 of the 315's per m2.

i thought it was 400 for a 3x3, 600 for up to a 5x5..maybe you mean in a commercial application, where there's a lot of overlap from the adjacent lights. in a grow tent with no other lights to overlap with, i think my wattage's are closer

Roger A. Shrubber said:

three x three is an awkward size. i like vegging under t5s a lot better than mh, but the two foot fixtures aren't strong enough imo, and you can't use the 4 footers in your tent.
i'd look at getting a 250 for the larger plants, and hanging it slightly off center, with a 2 bulb, 2 foot t5ho for the little guys on the other side of the tent.

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I use a 2’x8 bulb t-5 and a 2’x2 in my 3x3 to cover all the way across. Does leave 6” on the sides uncovered but does very well.

ive got an 8 bulb 4 footer in my veg tent, which is 32 x 60, so it fits pretty well. it has two switches so you can run the outside 4, the inside 4, or all 8.