I want to build a hydro system and I need to know how many plants a 400w MH/HPS can grow from start to finish.

Welcome to the forum..uhhh kid. The key number you would want to use is the finishing number. A400 watt hps will cover a 3' by 4' area if the plants are placed about 1' apart. So (12) plants. There will be some that will point out you can fim, top, prune and etc. to put more plants under that light and they may be correct. I just don't recommend depending on doing that for a new grower.
That is why the 600 watt light are so popular. 200 watts, or 50 % more power used, 92,000 lumens, 100% more light. I believe you could increase your production by 60%, by using the 600watt, because you could finish 20 plants in that same space.
As far as the number you can start, I wouldn't recommend 'starting' any under that light. Use cfl's floro's ecolight etc. until your plants have grown a little. The hid lights are to hot to be close enough to give seedling good light. Trust me on this one I can show pictures to back up what I'm saying. VV

i get even less coverage out of a 400 watter i feel with most reflectors a 3x3 is max as for plants about 6 generous sixed will fit under this light without overcrowding

Does it depend in how long you veg? If you flower at 10 inches you can get more than if you flower when they are at say 20 inches.

I have 12 (1 week into flowering) under a 600W and the light just covers them until I can get rid of males

(Just a thought!)

I drew out my plans for my hydro system (Im building my own) and I think Im gonna go w/ 5 plants. My gf is weird about me growing. She wants me to have only one but after she sees the buds she'll quit bitchin. Ive already built a small grow room that uses 3 CFLs so i already have the light for sprouts.
Im moving in 2 weeks and once I get into my new appt. I will scope out the best place for my hydro system. Im going w/ the 400w conversion setup because I dont wanna spend a shit ton of money and I seen them on eBay for $200.

Ohhh and I have a new question. Will a 3.75" net pot be big enough to support full grown plants in a hydro system?

when you get leaf tips crowding other plants they react by getting taller. if you flower @ 15-24 inches like i do your looking @ 9 plants 3.5 footers finished and i use 1000 watters. i have done SOG i prefer large bud sets i find well spaced plants adverage 4-7 oz per plant

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I have an aero setup doing 28 plants and use 3 400 watt lights which I am seeing isnt enough coverage.My grow area top which I refer to as the table is 4 feet wide by 7 feet long and the outside rows dont get enough light.I use 3 inch net pots all the way through from clone til harvest.I vegged mine 10 days and they hit about 16-18 inches flipped the 12/12 light for about 10 days now and am at 38-40 inches.If you do a simple rubermain bubble ponics which sounds best for you as the right size or even easier a rubbermaid aerponic setup you can do 6 plants easily with the 400 watt light and have a good grow.Id veg til they hit 10 inches then go 12/12 and you should end around 24-30 inches assuming your not doing a sativa grow then they could go higher.Get a switcahble ballast its cheaper then a conversion bulb setup.

she is right some states will ruin you life. some states have a tax of 1100.00 per plant if caught. this is on top of the criminal charges. five plants is still around medical limits. you do take a huge risk in growing. dont forget about this.


Usually one become two becomes six then years later your just like abudsmoker

what do you mean a tax of 1100.00 per plant?

Yeah. Im not too worried about about being busted. Its all for personal. Shes just paranoid about everything. She refuses to go anywhere near the the drains along the road for fear that she might step in it and the first 30 min. of interstate driving are full of "ohhh my gods" and gasps. Shes very irrational. But she loves pot as much as I so I think she'll come around. Ohh and I meant switchable instead of conversion, my bad.