Watt per square foot

Correct me if I'm wrong but the general rule of thumb for growing is 50w/5000 lumen per square foot of growing space as a medium expectation.

So my grow space is approximately 28 in (length) x 17 in (width) x 21 in (height). I'd assume you'd just multiple length and width; 28 x 17 = 476 right?

So with one square foot being 144 in, 476/144 = 3.3. So 3.3 x 50/5000 = give or take 165 watts/16,500 lumen

Am I using the right formula?

My second question is if my plant of course takes up less space, can I get away with less watts? Or if the whole point to fill up every square foot regardless of the plants size?

Also, if you had to choose, is it better to have less heat or more lights?

Your math seems good.

In a small spot like that, enough light for the square footage of the grow space, your plants will stretch as well and fill more area.

It's a balance, you want enough light but need to exhaust enough heat. Too little light is bad and too much heat is bad, I don't know which one is worse, always strive for optimal conditions.

150 watt hps.

SnapsProvolone said:

150 watt hps.

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I think I will listen to you. I thought it would be crazy expensive but it turns out I can find a bulb for 10 bucks. You can screw the bulb into anything right? And do they get really hot? More than 150 watts of CFL?

You can get a bulb / ballast / reflector for 60 bucks at 1000bulbs.com

CFL is for kindergarten play time. If you can't cool 150 watts of HPS I doubt you could cool 150w cfl.

JuicyyFruit said:

I think I will listen to you. I thought it would be crazy expensive but it turns out I can find a bulb for 10 bucks. You can screw the bulb into anything right? And do they get really hot? More than 150 watts of CFL?

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Well, in theory they (the hps) don't put off as much heat because they are more efficient, but the heat they do put off is concentrated in a much smaller area, so while the total heat put off may be more with a 150w cfl, it will be distributed over the cfl bulbs much larger surface area so the bulb itself would be much cooler to the touch than the 150w hps, at least that is my understanding.

As for what 150W hps screw into, I believe they are mogul based (e39) and I believe you still need a ballast, you can't just screw them into an e26 socket (standard us light socket size) the way you can with a cfl (cfl have internal ballasts in the large plastic base part)

21" tall is the problem. My clones plus medium plus light reflector and space between light and canopy would blow that away.

I payed attention to H & W, damn......21". I tie my plants down and they are still 21" tall.

You going to go 12/12 from clone? An 8" tall bucket and even 4" of light leaves you 9" for plant and space between light and canopy.... That's why I can't wrap my head around pc grows, seems so difficult to grow anything worth 3 months in such a tight spot.

Yep, another micro.... such a waste to be risking cultivation for no rewards.

I'm running a 150w right now, and you can check it in my signature.

From experience, I'd say you will be better off looking into COB LED lighting, LEDs like the Vero 29 and the CXA3070 are tha bomb. There are a lot of DIY light builds here on RIU.

Search for growers like SupraSPL, Stardustsailor, SomeGuy and Guod. They are having AWESOME results.

All in all, these new LEDs will outperform the 150w HPS, using less energy and outputting better lumen numbers and quality.

Welcome to the future.

Um, can I suggest listening to provolone here. 150 hps give it juice and watch it grow. As big a douche as he may be he knows his shit.

note - For the love of god don't build your own led. Your growing a viable plant that rewards you like no other. Why dick around.... Go big... Get Zips

Im using at least 150 W per Sq foot, I wouldn't use less.

JuicyyFruit said:

I think I will listen to you. I thought it would be crazy expensive but it turns out I can find a bulb for 10 bucks. You can screw the bulb into anything right? And do they get really hot? More than 150 watts of CFL?

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No, a HPS bulb will require a Ballast, a mogul base, and wiring. You will never get it to work using a regular socket.

I am amazed by the number of people that think they are going to grow a great crop with 2x 42 watt CFL. The truth is that 84 watts of CFL will give you maybe a few grams of the worst shit you ever smoked.

21" of height? Really? Are you trying to grow an awesome crop in a small cardboard box or something?
Not trying to be a dick, I can be abrasive at times, apologies are out to you.
Read more FAQ, keep it basic, Quality and wattage of light are directly proportional to yield. Anything less than 400 Watts of HPS will be lacking. Can you grow with less? Sure you can, but don't expect spectacular results.

JuicyyFruit said:

So my grow space is approximately 28 in (length) x 17 in (width) x 21 in (height).

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With that height I would go with 9.5w Cree LED lightbulbs from Home Depot. (<<link showing how I use them, mounting options, alternative "corncob" lights from AliExpress.). I'd shoot for about 20w/sq ft. in veg, 30 in flower. Your space is 3.75 sq. ft, so you need 112w in flower, or 12 bulbs. Sounds tedious mounting/positioning/powering that many light sources, but it will conserve a lot of your limited vertical space. Two in each corner, four in the center (assuming two plants in that space). You can position them so the highest light in each corner gives top-lighting at a 45-degree angle, the bottom light gives side lighting.This would let the plant grow as tall as possible (no fixture on top).

JuicyyFruit said:

My second question is if my plant of course takes up less space, can I get away with less watts? Or if the whole point to fill up every square foot regardless of the plants size?

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You can disable some of the lights in veg, or disable the lights on one side if you're only growing one plant.

You might do a perpetual grow in that cabinet, where veg and flower share the same light cycle. Either autoflower at 18/6, or photosensitive started at 12/12 from seedling. If you do it this way, one side of your space could accomodate a larger plant in flower, while the other side would only need modest space for seedling. Harvest a plant every 6 weeks, adding a new seedling.

JuicyyFruit said:

Also, if you had to choose, is it better to have less heat or more lights?

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Depends on your climate. If you're in a cold area, a less efficient (hotter) light might be useful to keep your space warm. In the summer you might need cooler lights. To some extent your extraction fan can control this. Put it on a speed control. You want to replace the air in your space every 5 minutes. In the summer you can turn up the speed to extract heat.