# LED Watts per Sq. Ft.??? Unanswerable question?



## DankDave420 (Oct 18, 2017)

Hey guys. Just trying to get an answer on this. The sources online range from 25-60 watts of LED per sq ft for flowering optimally. I don't have a par meter and I am just looking for a general goal. I know that LED should be measured in Par umols. I know LED's aren't that far off though that nobody can give a decent range. Somebody out there knows a good wattage to aim for. I just want a good up to date answer from someone with experience. If it matters much I am not using any COB's. Is 25-60 watts per sq. ft.the best answer I can get? Is this question unanswerable? If it is, what is the question I should be asking to get the answer I am looking for? How much light? Seems like a very important question that shouldn't be too hard to answer for anyone with experience. Even a link to a reputable source that isn't a light dealer would be helpful. But I really would like some first hand experience, I know it is out there. How much LED light is optimum through flowering?


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## GroErr (Oct 18, 2017)

I've found ~30w sw. ft. in flowering and 20-25w in veg to work well, that's with COBs, likely applies to the newer strips as a guide. With older diode based tech you need more like 40+w sq.ft. in flower for any decent numbers. So the answer varies a bit depending on the tech which is likely why you've seen wide ranges. Some like to use more, some less. I did a run at 22w/sq./ft. and that produced well, but added more than a week to a few phenos.


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## Rolla J (Oct 18, 2017)

25 to 40 watts a sq ft
Id go with 50 if you could. 25w the lowest id go. Be about 100watts per plant.
Im running 43.75watts a sq foot right now


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## DankDave420 (Oct 18, 2017)

Awesome guys. Thanks. Those are good answers. I guess that range is always going to be wide with the range of quality these things are made. I'm at 28 watts per sq ft now with a Viparspectra 400w (200w actual), I believe that falls into the less efficient category. I am thinking of just getting a second one then and if it is too much I can mess with alternate timing maybe to act as like a light mover. Wonder if anyone has done that. Wonder if those light movers are legit. I am not trying to be the failed experimental grower. I wonder how much light would be too much. 56w per sq ft pushing too much? I have plenty of ventilation. Thanks again guys. Anyone else? Experience with this particular light would be fantastic. Also I can not give likes or anything yet, your input is very much appreciated. 
Also I am not growing 4 sq ft plants. I have about 2 foot of plant space vertically and will likely just do a small SOG type. I never calculate number of plants, only sq ft of canopy. Im stuffing as many small untopped plants(9 max) in there as I can at first to see how it works out. I can always take some out if there are problems. I want a fast harvest then I'll work on a high yield next time. This is all just for personal use and I don't smoke that much. I'll give away any extra to friends. Thanks again guys.


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## Seshwaan (Oct 20, 2017)

Hey man. I am actually in the process of building my own LED board for my 1sqft PC case so have been looking into this extensively. Obviously I am new to this but feel I have a pretty decent basic understanding.

You cannot really use Watts per sqft as a general measure for LED's due to the fact that depending on which LEDs/COB's you use they each have different lumens per sqft.

For example, off the shelf LED bulbs may have under 100 lumens per watt, Most CFL's you will see have only 60lm/W.... I am using some bridgelux BXEB-L0280Z-35E1000-C-A3

And they have 156 lm/W. There are some samsung strips for 3-4x the price that have 189lm/W. So you may have 3 different LED setups all giving 50W per s ft but the each of them can range greatly with how many lumens they each provide to the space.

I have found multiple places stating that the between 4000-6000 will suffice so I have gone for 6000 by using 5 of those bridgelux strips above.

Beauty of DIY is that you can tailor the light to exactly what you need. I haven't done the math but I am sure that you wouldn't get anywhere near 6000 lm/sq ft with a viparspectra or UFO


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## Budget Buds (Oct 20, 2017)

Lumens per watts dont mean shit, u got the right idea for umol's....I use 50 watts per sq foot with vero 29's , using a Dimmer for less intense light in veg, works great for my testing and main rooms


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## Seshwaan (Oct 20, 2017)

Budget Buds said:


> Lumens per watts dont mean shit, u got the right idea for umol's....I use 50 watts per sq foot with vero 29's , using a Dimmer for less intense light in veg, works great for my testing and main rooms


Could you expand on why lumens per watt don;t mean shit? Genuinely interested.

I always thought lumens per were an indicator of efficiency and the more efficient an LED the more photons you get out of the same wattage


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## DankDave420 (Oct 20, 2017)

Seshwaan said:


> Could you expand on why lumens per watt don;t mean shit? Genuinely interested.
> 
> I always thought lumens per were an indicator of efficiency and the more efficient an LED the more photons you get out of the same wattage


I am asking for a goal Watt per Sq. Ft. assuming that most grow LED's would be similar PAR and efficiency. I know there is a range, I was just hoping to get a smaller range since most of the info is outdated and the tech is changing quickly, and there is much more experience with them now. I realize it is not the best way, but I don't have a PAR meter and do not plan to aquire one. So I figured it would still get me close because alot of the sources still recommend a wattage per sq. ft. as a goal.

I am no expert yet, but from my understanding. The reason we use PAR in umol's for LED's is because it is the amount of useable light reaching the plant. Some of the spectrum of light that is unusable to the plant is often reduced, as they focus on only the usable spectrum. Also they focus on specific colors like reds and blues because they are the ideal color for growing. This would make using the full spectrum lumens insufficient as you can reduce the lumens without reducing the amount of usable light your plants receive. Some colors like green are not used as efficiently, considering our green plants tend to reflect most of the green light hitting them. So the green can be reduced, reducing lumens. Not having a complete understanding of all of this is another good reason I didn't build. I relied on someone else to do the engineering and went on user feedback to make the purchase. This is still a top rated light anywhere you look even though it is cheap. Definitely not the best money can buy. I will certainly document my experience with it.
Thanks for the input guys. It is truly appreciated.


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## Tim Fox (Oct 20, 2017)

i am going to run 62 watts per square foot on my next grow , thats 4 citi cobs and 2 cree cobs a mix of 4k and 3K


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## Budget Buds (Oct 20, 2017)

Seshwaan said:


> Could you expand on why lumens per watt don;t mean shit? Genuinely interested.


Dank dave pretty much explained it in the post below yours


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## dandyrandy (Oct 20, 2017)

~700 in 16 sq at 18" would work... Trees


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## Ryante55 (Oct 20, 2017)

DankDave420 said:


> Hey guys. Just trying to get an answer on this. The sources online range from 25-60 watts of LED per sq ft for flowering optimally. I don't have a par meter and I am just looking for a general goal. I know that LED should be measured in Par umols. I know LED's aren't that far off though that nobody can give a decent range. Somebody out there knows a good wattage to aim for. I just want a good up to date answer from someone with experience. If it matters much I am not using any COB's. Is 25-60 watts per sq. ft.the best answer I can get? Is this question unanswerable? If it is, what is the question I should be asking to get the answer I am looking for? How much light? Seems like a very important question that shouldn't be too hard to answer for anyone with experience. Even a link to a reputable source that isn't a light dealer would be helpful. But I really would like some first hand experience, I know it is out there. How much LED light is optimum through flowering?


Your question is too broad if you are using old blurples you need alot if your using 200lm/w white led you probably only need 30w/sf if your running 150lm/w white led you might need 35w/sf in my experience you don't need more than 45w/sf for optimal growth with led that have been released in the last year or 2


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## Rolla J (Oct 20, 2017)

155 watts max and 30watts the lowest i can dim down on just these 2 bars. All 4 bars 36 chips should run atleast 345 max in flower. Will hook it up tomorrow. And also check out my other diy light see what that pulls. Was alll so curious to see what im pulling from the wall, so i ran out and got a P3 kill a watt today


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## mauricem00 (Oct 20, 2017)

DankDave420 said:


> Awesome guys. Thanks. Those are good answers. I guess that range is always going to be wide with the range of quality these things are made. I'm at 28 watts per sq ft now with a Viparspectra 400w (200w actual), I believe that falls into the less efficient category. I am thinking of just getting a second one then and if it is too much I can mess with alternate timing maybe to act as like a light mover. Wonder if anyone has done that. Wonder if those light movers are legit. I am not trying to be the failed experimental grower. I wonder how much light would be too much. 56w per sq ft pushing too much? I have plenty of ventilation. Thanks again guys. Anyone else? Experience with this particular light would be fantastic. Also I can not give likes or anything yet, your input is very much appreciated.
> Also I am not growing 4 sq ft plants. I have about 2 foot of plant space vertically and will likely just do a small SOG type. I never calculate number of plants, only sq ft of canopy. Im stuffing as many small untopped plants(9 max) in there as I can at first to see how it works out. I can always take some out if there are problems. I want a fast harvest then I'll work on a high yield next time. This is all just for personal use and I don't smoke that much. I'll give away any extra to friends. Thanks again guys.


 that viperspectra 450 uses 206 watts of power but only produces 27 watts of radiant power. a high end white C.O.B or 561 strip light will give you close to 45 watts or radiant power for 100 watts of input.you would need over 340 watts of those cheap LEDs to match the output of a high end cree or osram LED. how much power you need depends on the quality of LEDs you are using.



 cheap LEDs are not worth the money.


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## ChaosHunter (Oct 20, 2017)

GroErr said:


> I've found ~30w sw. ft. in flowering and 20-25w in veg to work well, that's with COBs, likely applies to the newer strips as a guide. With older diode based tech you need more like 40+w sq.ft. in flower for any decent numbers. So the answer varies a bit depending on the tech which is likely why you've seen wide ranges. Some like to use more, some less. I did a run at 22w/sq./ft. and that produced well, but added more than a week to a few phenos.


This


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## Rolla J (Oct 20, 2017)

@horribleherk has a CLS blurple grow as well as a cob setup. I believe he runs more watts with the CLS led light.

I run a cree and osram blurple led, the unit farm Ufo80 led in a 2x2. The draw is 175w +-5 182watts in my kill a watt
Here r some specs of that light.
I also run that diy flower light with full potential which runs 2x as much power in a 2x2. Like 90w sq ft lol compared to about 44w sq ft with the high end blurple


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## Tim Fox (Oct 20, 2017)

Rolla J said:


> 155 watts max and 30watts the lowest i can dim down on just these 2 bars. All 4 bars 36 chips should run atleast 345 max in flower. Will hook it up tomorrow. And also check out my other diy light see what that pulls. Was alll so curious to see what im pulling from the wall, so i ran out and got a P3 kill a watt today
> View attachment 4029792 View attachment 4029793


oh yea man looks great,,, here is my current citi panel, i am adding a couple of cree 3070's to get up to around 60 watts in flower next run


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## Rolla J (Oct 20, 2017)

Tim Fox said:


> oh yea man looks great,,, here is my current citi panel, i am adding a couple of cree 3070's to get up to around 60 watts in flower next runView attachment 4029902 View attachment 4029904


Got you a nice frame there! Thats a killer light right there. Have ubhad a harvest yet with the light?
The other diy one i have is for veg runs 6000k yeah yeah looks like a phoney qb haha
Guna hook it up tomorrow and see what im pulling from the wall. I dont think anymore but 110w at max. But we shall see


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## horribleherk (Oct 20, 2017)

I have an osram diode light (200w.) I run a 2x2 Scrog inside a 3x3 tent so I pretty much use 50w=1sq.ft.with this light my other light is [email protected] it is over a 30 x 30 Scrog which comes out to about 45.3w=1sq.ft. these terms & numbers for leds are mind boggling & im on my first run in this configuration the top pic is the cob light bottom pic is the osram blurple light the plants are sisters from same mom same batch of clones I flipped the cob tent 3 weeks ago today & the blurple tent 2 weeks ago this seems to be working out for me these are single plant grows


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## Rolla J (Oct 20, 2017)

Here's the link to the light i use
https://www.unitfarm.com/ufo-80-cree-osram-led-grow-light-for-sale


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## cheddars (Oct 21, 2017)

Would something like this work for one plant in place of 2x 5500k CFLs *125w)


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## blake9999 (Oct 21, 2017)

Stick with the CFL's. they better quality than that cheep light.


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## Rolla J (Oct 21, 2017)

Rolla J said:


> Got you a nice frame there! Thats a killer light right there. Have u had a harvest yet with the light?
> The other diy one i have is for veg runs 6000k yeah yeah looks like a phoney qb haha
> Guna hook it up tomorrow and see what im pulling from the wall. I dont think anymore but 110w at max. But we shall seeView attachment 4029906


***
Just checked and it maxes out at 163 watts


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## ChaosHunter (Oct 21, 2017)

It all comes down to Photons and how many per watt if I'm not mistaken. 200W of diode LED vs 200W of COB LED at the wall is different. Maybe @robincnn @CobKits @Stephenj37826 can better explain.


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## mauricem00 (Oct 21, 2017)

cheddars said:


> Would something like this work for one plant in place of 2x 5500k CFLs *125w)
> 
> 
> View attachment 4030279
> View attachment 4030280


if you can not afford a high end led than you would be better of with this https://www.ebay.com/itm/T5-HO-Indoor-Grow-Light-2-ft-8-Bulbs-DL828S-Fluorescent-Hydroponic-Fixture-Veg/222107421531?ssPageName=STRK:MEBIDX:IT&_trksid=p2055119.m1438.l2649 or this https://www.ebay.com/itm/400-watt-400w-Dimmable-HPS-MH-Grow-Light-System-Set-Kit/150656047425?_trkparms=aid=555018&algo=PL.SIM&ao=2&asc=41375&meid=640c44e43b6b46569b63aab2a36825f7&pid=100005&rk=3&rkt=6&mehot=pp&sd=282506500516&_trksid=p2047675.c100005.m1851 those cheap LED grow lights are worse than CFLs. you would need a vipraspectra 600 watt to match the radiant power of those 2 CFLs you have. vipraspectra is the only company that I have found who provide independent lab test results for their lights so with other lights you are just guessing. you would need the best LED grow light on the market to match the performance of HPS and CMH or T5s will give you better taste and potency. LEDs may look cool but many of us growers do not want to look cool. I want to look invisible. I don't want anyone to know what I'm growing. and I'm a legal MMJ patient and PU grower.


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## maximum autism (Oct 21, 2017)

mauricem00 said:


> LEDs may look cool but many of us growers do not want to look cool. I want to look invisible. I don't want anyone to know what I'm growing. and I'm a legal MMJ patient and PU grower.


I don't think that the coolness factor is a selling point outside of the low-end, like the cheap panel that OP linked, and these models are mostly 2010 surplus parts thrown together as a sort of low-info scam. 
As far as things being proven for effective growing, I'd say that CFL's and HPS have been proven to be less energy efficient and less cost effective in terms of PPW, which is the reason to make the switch to begin with. You can try to dissuade people from looking into LED's at all, it will fail as the post-2015 stock (including COB's) becomes more widespread. Enjoy your 1970s-tier setup.


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## mauricem00 (Oct 21, 2017)

maximum autism said:


> I don't think that the coolness factor is a selling point outside of the low-end, like the cheap panel that OP linked, and these models are mostly 2010 surplus parts thrown together as a sort of low-info scam.
> As far as things being proven for effective growing, I'd say that CFL's and HPS have been proven to be less energy efficient and less cost effective in terms of PPW, which is the reason to make the switch to begin with. You can try to dissuade people from looking into LED's at all, it will fail as the post-2015 stock (including COB's) becomes more widespread. Enjoy your 1970s-tier setup.


in the 70's I was growing bag seed with T12 grolux bulbs but HID and fluorescent technology has gotten a lot better since then I am not trying to dissuade anyone from anything. just giving fact based options to low budget growers. the aquarium crowd was the first to use LEDs to grow plants but they have returned to T5s and MH lights for growing coral and plants because they work better. even as structural lighting LEDs are failing. the air force and naval observatory tried them and went back to more conventional lighting because LEDs did not live up to the claims.but I guess you know more than the government and commercial green house and garden center operators. they have tested LEDs and went back to conventional lights. I've done test grows with LEDs and prefer other forms of lighting for taste and potency.


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## CobKits (Oct 21, 2017)

DankDave420 said:


> I know that LED should be measured in Par umols.


ALL lighting should be measured in PAR (gross light output - umols/s) or PPFD over an area (umols/s/m2)



DankDave420 said:


> Hey guys. Just trying to get an answer on this. The sources online range from 25-60 watts of LED per sq ft for flowering optimally.


you are kinda correct that it is "unanswerable" if the efficiency of the fiture is not known. fixtures like the one you posted NEVER list efficiency because in almost all cases it is no better than the HPS it is designed to replace

so there are two parts to your question:
1. how much light do i need?
2. how can i tell how much my light provides?

A:
1. light requirements vary by growth stage, plant health and to an extent strain. you can be as low as 100 umol/s/m2 ppfd for seedlings, 250 in veg, 600-900 for flowering (or up to 1500 ppfd in a super dialed super vigorous garden with CO2 supplementation

2. so its pretty easy to convert ppfd to target watts/SF if you know the efficiency of your fixture. you can divide target PPFD by 45 to get PAR W/SF. so if you are targeting 900 ppfd (this would be considered strong lighting by most growers in flowering), divide by 45 and you need 20 PAR W/SF

so what is a PAR W? that is actual usable light on your plants. if you buy one of the elcheapo lights like lsited at the top of page 2, thats probably going to be around 30% efficient, so to get 900 ppfd aka 20 PAR W/SF you divide 20 by 0.3 and find you need 66 wall watts of LED

if you spent good money on a real fixture (any of the multitude of systems you could build around current offerings from citizen/cree/luminus/bridgelux, or any of the board or strip designs featuring samsung of bridgelux chips, and *ran them at a low current, say 30-50% of their max nameplate current*), you could have a fixture thats approaching 60% efficiency. 55% is attainable

so in that case 20 PAR W/ 0.55 = 36 wall watts/SF

quite a difference from 66W/SF, eh? both are producing the same amount of light to your plants. the good light is throwing 16W/SF of waste heat into the room, the crappy fixture is throwing 46W/SF of waste heat into the room!

you get what you pay for....


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## CobKits (Oct 21, 2017)

DankDave420 said:


> I am asking for a goal Watt per Sq. Ft. assuming that most grow LED's would be similar PAR and efficiency. I know there is a range, I was just hoping to get a smaller range


that is an erroneous assumption. there is still lots of bullshit lighting out there and the best LEDs are literally twice as efficient as the worst


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## CobKits (Oct 21, 2017)

DankDave420 said:


> I am no expert yet, but from my understanding. The reason we use PAR in umol's for LED's is because it is the amount of useable light reaching the plant. Some of the spectrum of light that is unusable to the plant is often reduced, as they focus on only the usable spectrum. Also they focus on specific colors like reds and blues because they are the ideal color for growing. This would make using the full spectrum lumens insufficient as you can reduce the lumens without reducing the amount of usable light your plants receive. Some colors like green are not used as efficiently, considering our green plants tend to reflect most of the green light hitting them. So the green can be reduced, reducing lumens. Not having a complete understanding of all of this is another good reason I didn't build. I relied on someone else to do the engineering and went on user feedback to make the purchase. This is still a top rated light anywhere you look even though it is cheap. Definitely not the best money can buy. I will certainly document my experience with it.


search "mccree absorption" on this site. you will find that while green is less absorbed it still is up to 80% as absorbed as reds and blues, and has important plant response effects. which is partially why everybody who moved from blurples to white LEDs saw great results (apart from the fact that the white light is more efficient


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## cheddars (Oct 22, 2017)

mauricem00 said:


> if you can not afford a high end led than you would be better of with this...


Thanks, I did buy a 600w HPS which was overkill and produced as much heat as the core of the sun. I'll look into a 200w version. I only need it for one plant, continuously throughout the year.


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## GreenLogician (Oct 24, 2017)

The place I bought my cxb rig gives quite a lot of details, but all for the thing running on full power.
I'm wondering if I can figure out the efficiency when I've got it dimmed down, and then what's the ideal power level for my 2.6x2.6 flower tent (80cm)
https://pdgrow.en.alibaba.com/product/60484689669-803344651/Top_quality_netherlands_led_grow_light_cxb3590_x6_led_grow_light.html?spm=a2700.8304367.prewdfa4cf.5.5af4e5ffE4iApT


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## ChaosHunter (Oct 24, 2017)

GreenLogician said:


> The place I bought my cxb rig gives quite a lot of details, but all for the thing running on full power.
> I'm wondering if I can figure out the efficiency when I've got it dimmed down, and then what's the ideal power level for my 2.6x2.6 flower tent (80cm)
> https://pdgrow.en.alibaba.com/product/60484689669-803344651/Top_quality_netherlands_led_grow_light_cxb3590_x6_led_grow_light.html?spm=a2700.8304367.prewdfa4cf.5.5af4e5ffE4iApT


I'm not a numbers or charts guy. I run at 100w in early veg to the start of flower. 150w for early flower and 175w from mid flower until Finnish. This is in a 2x2.5 tent 5sq ft. My Light has dimming to achieve this. At max 175w divided by 5 puts me at 35w per sq ft. 

The ball park COB is you need 35w per sq ft for flower


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## GreenLogician (Oct 24, 2017)

Cool, thanks ChaosHunter 
Do you have 6 cobs on your unit?
Since I have 6, if you had 4 would that mean mine would run at a significantly lower wattage per cob for dimming to the same total unit wattage, therefore more efficiency and I should undercut your numbers a bit?


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## ChaosHunter (Oct 24, 2017)

GreenLogician said:


> Cool, thanks ChaosHunter
> Do you have 6 cobs on your unit?
> Since I have 6, if you had 4 would that mean mine would run at a significantly lower wattage per cob for dimming to the same total unit wattage, therefore more efficiency and I should undercut your numbers a bit?


I was one of the first to start growing with Quantum boards when they were released last year. I'm running two 304s


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## Chip Green (Oct 24, 2017)

My original 2x2 panels of Bridgelux EB series builds were based on 50w/sqft, Ive spread those collections out on a few panels to the 30w/sqft range, and still achieve satisfactory LUX readings.


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## Enigma (Oct 24, 2017)

It is all about efficacy.

From there you can determine how many Watts per square foot you will need based on which light source you choose, not all COBs and strips are the same.

I'm using Bridgelux Vero 29s and will be using the EB Series shortly.


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## Chip Green (Oct 24, 2017)

Before the leap to LED, we had to buy a lamp and build the space around it. Now we can choose a space, and build a "lamp" to fit the space....
What times we live in!!!!


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## ChaosHunter (Oct 25, 2017)

Chip Green said:


> Before the leap to LED, we had to buy a lamp and build the space around it. Now we can choose a space, and build a "lamp" to fit the space....
> What times we live in!!!!


We used to take street lights and modify them. Light pole security lights were used and modified for smaller grows. This was 25 years ago.


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## mauricem00 (Oct 25, 2017)

CobKits said:


> ALL lighting should be measured in PAR (gross light output - umols/s) or PPFD over an area (umols/s/m2)
> 
> 
> 
> ...


if par is so important than why do major manufacturers and lighting engineers use radiant power and spectrum to compare lights? PPF, and PPFD is just as useless as lumens for determining how well a light will grow plants.atleast lumens is useful for designing structural and area light but nothing in nature follows the PPF curve except silicon and CDS light sensors.vipraspectra is one of the few companies that post independent test data showing radiant power and those test results show a radiant efficiency of about 14%. the best white LEDs can achieve 47% radiant efficiency but only a very small amount of the light they produce fall in 3 of the 4 chlorophyll absorption peaks ( 434,465, 630,670nm). LED makers have always focused on hitting these peaks which is why top LEDs use mono's with a few white leds to provide green light. since the makers of LED grow lights target cannabis growers with one claiming to have a cannabis specific spectrum we are likely to see more white LED grow lights in response to forums like this


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## mauricem00 (Oct 25, 2017)

maximum autism said:


> I don't think that the coolness factor is a selling point outside of the low-end, like the cheap panel that OP linked, and these models are mostly 2010 surplus parts thrown together as a sort of low-info scam.
> As far as things being proven for effective growing, I'd say that CFL's and HPS have been proven to be less energy efficient and less cost effective in terms of PPW, which is the reason to make the switch to begin with. You can try to dissuade people from looking into LED's at all, it will fail as the post-2015 stock (including COB's) becomes more widespread. Enjoy your 1970s-tier setup.


HIDs and florescent may be less cost effective on a chock board but the only real test is how do they perform growing plants? the market has answered that question for us


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## salgeezi (May 10, 2019)

Rolla J said:


> 25 to 40 watts a sq ft
> Id go with 50 if you could. 25w the lowest id go. Be about 100watts per plant.
> Im running 43.75watts a sq foot right now


How high from the canopy do you keep the light? Thanks


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