Formula for lighting

[Doubletoker]

Hi guys
New indoor grower here. My box is 42wx5hx5l I believe that's 90 sq ft. According to the threads I've been reading, isn't the formula for the total amount of lighting needed watts times square feet? I've got 20 cfl bulbs, mixed blue and red equal amounts of each, that are 100 watts per. That's 2000, times 90 square feet is 180,000.?
I am doing something wrong here, aren't I? Please help and tell me where I am going wrong.
Thanks in advance

 

[fanleaf]

42 inches I assume? 5 foot high I assume? 5 foot wide?

So if I deciphered that correctly your box is 42"X5'X5' high?

If that is correct measurements you have 17.5 square feet! 42" divided by 12" because that's how many inches are in a foot gives you 3.5 then take that 3.5 X Length in feet so 3.5X5=17.5.
Length X Width = Square feet.
Length X Width X Height= Cubic Feet which is 87.5 cubic feet

For lights you only need the square feet.
A decent LED watt per foot usually starts at 28-30 minimum and somewhere around 45-50 optimal. (This is actual watts used not advertised watts. Almost all 300W LED's for example will use around 175W so that's the number you work with.

MH/HPS 50 Watts per foot to 100 I would say is average. Some use much more than 100WPF

While this is not exact and peoples ideal WPF will vary it's a good starting point.

A 1000W HPS in your tent will give you 57 Watts per foot but you will need to buy the light and reflector style/type keeping temp control in mind.

An LED array that's actual usage watts that add up to 700+W will do great.

 

[fanleaf]

I've got 20 cfl bulbs, mixed blue and red equal amounts of each, that are 100 watts per. That's 2000, times 90 square feet is 180,000.?
I am doing something wrong here, aren't I? Please help and tell me where I am going wrong.
Thanks in advance

Yes, you are doing it wrong.

1. The CFL's you probably have are 100W equivalent not usage. 100W equivalent CFL's are 23W lights. If you wanted 35 Watts per square feet you need 27 of your CFL's. For 50W per square foot you need 38 of them.

for 30WPSF take your square footage which is 17.5 then times 30 WPF = 525 watts needed so divide 525 by 23W CFL's = 26 and some change.

I would use 3 2700k (red) color temp CFL's to 1 6500k (blue) color temp.

 

[Doubletoker]

Thank you fanleaf, I do appreciate your wisdom
And if I take your advice on the 3 2700k reds and 1 6500k blue, how do I figure the wattage used?
I read somewhere on here that optimum cfl is 100 watts per square foot, minimum is 42.5, up to, close to 100
I am new to the indoor part, have done it outside for a long time
Thanks again

 

[fanleaf]

Thank you fanleaf, I do appreciate your wisdom
And if I take your advice on the 3 2700k reds and 1 6500k blue, how do I figure the wattage used?
I read somewhere on here that optimum cfl is 100 watts per square foot, minimum is 42.5, up to, close to 100
I am new to the indoor part, have done it outside for a long time
Thanks again

Yes, for CFL 100W per square foot is great because you can use some of the CFL's to hang at the sides of the plant/s too for much better results since CFL does not penetrate the canopy very well at all.

If you are going to fill your 17.5 square feet at 100W per square foot then you need a lot of CFL's but are you going to fill the room with plants so that's really needed?

That's 1700 watts and 23W CFL's that means 74 of your CFL's! Yes, 74!

If you go with 3 to 1 red/blue take 74 divided by 3 and that gives you 24.6. So 25 blue and 50 red.

If you are only going to have say 2 plants then a rough estimate for 100W per square foot would be about 1200W. That's figuring a 6 square foot canopy per plant so 12 square feet at 100W each foot. 1200W divided by 23W each bulb means 52 CFL's. 52 divided by 3 is 17.3 so 18 blue (6500k) bulbs and 34 red (2700k)

Obviously there are more efficient ways to illuminate that space but I'm sure you have reasons for going all cfl on it.
It would be cheaper to run and probably buy a 1000W MH/HPS setup. LED would be a better option too but your call.

That many cfl's WILL generate a lot of heat to get rid of. Probably more than even HPS and way more than LED.

That many CFL's won't be cheap plus you have to buy equipment to control heat.

Honestly, if you wanna go all CFL on that area I would shoot for around 65W per square foot to attempt to keep things somewhat manageable.

That would be 50 of your CFL's. 17 blue and 33 red to cover the whole room or about 37 total CFL's for my 2 plant scenario. 13 blue and 24 red.

 

[Doubletoker]

WOW.....Ok you have given me a lot to think about.........I had decided to hold off on purchasing those lights and am I glad I did, whew!
I am going to have to do a lot of rethinking.....I was thinking that every time I heard of someone getting busted, it was because they had high power lights, that is just an assumption tho. I am actually trying to have 15 plants in that space.....is that too many? I have that many square pots and they are 4 gallon, less than 10" across. I would still have close to 1 foot side to side and end to end. Might be pretty cramped in there. What are your thoughts?

 

[Big Sur]

Interesting read. I was looking at CFL and LED but decided to stick with my dual 400W MH/HPS setup. I have switchable ballasts, hoods and both types of lights. I never got busted growing before, and now it is legal to grow here in this state. But even with 400W, if I use a light intensity meter, even a foot away the light readings start to fall off. If I go outside on a sunny day (any day of the year) and always read that the sun's light intensity is far higher. But in overcast? Forget it. My problem is the heat that these MH/HPS lights put out. I generally grow outside in late spring/summer in greenhouses, but I use the lights indoors for starting seeds, rooting clones and keeping a few pet plants over winter.

 

[fanleaf]

WOW.....Ok you have given me a lot to think about.........I had decided to hold off on purchasing those lights and am I glad I did, whew!
I am going to have to do a lot of rethinking.....I was thinking that every time I heard of someone getting busted, it was because they had high power lights, that is just an assumption tho. I am actually trying to have 15 plants in that space.....is that too many? I have that many square pots and they are 4 gallon, less than 10" across. I would still have close to 1 foot side to side and end to end. Might be pretty cramped in there. What are your thoughts?

You will definitely need to fill the room with light then. Filling it with CFL's you will actually use much more power than even HPS! You have to consider that 100 bulbs will be far far less efficient than 1. At 60 Watts per foot it's cheaper for not only equipment but cheaper on the power bill to use HPS.

Cheaper yet for the power bill in a room that size is LED. 700 actual watts or about 40 per square foot of LED for the whole room and only use 700W instead of 1000 for HPS and 1300-1700 watts for full blown CFL's.

The LED's would cost about $600 up front but cheaper to run by far.

100 cfl's with power strips and sockets would probably cost more than $600

1000W HPS far cheaper but power hungry.

LED and CFL you could always buy it little at a time and grow in that part of the room until you have it filled.

 

[fanleaf]

Interesting read. I was looking at CFL and LED but decided to stick with my dual 400W MH/HPS setup. I have switchable ballasts, hoods and both types of lights. I never got busted growing before, and now it is legal to grow here in this state. But even with 400W, if I use a light intensity meter, even a foot away the light readings start to fall off. If I go outside on a sunny day (any day of the year) and always read that the sun's light intensity is far higher. But in overcast? Forget it. My problem is the heat that these MH/HPS lights put out. I generally grow outside in late spring/summer in greenhouses, but I use the lights indoors for starting seeds, rooting clones and keeping a few pet plants over winter.

I have MH and HPS but don't use em. For me in my space LED is by far better for me. Typical day my grow room runs at 78ish degrees using more than enough LED to fill the room. On the same day if I shut the LED off and fire up the 600W HPS the temps hit 89-90 in an hour or 2 and I have forced air intake and a 4" blower carbon air scrubber on high for the exhaust.

My HPS is not a vented sealed hood mind you but still. I believe my LED's fill the room much more evenly with higher PAR value.

 

[Doubletoker]

Hi fanleaf
I am in the process of researching and trying to find a good buy on leds, I am looking at some MarsHydro on ebay atm. If anyone has any suggestions, throw them out there. I did look at some MH/HPS, but you mentioned they are power hungry.

 

[Major PITA]

Hi fanleaf
I am in the process of researching and trying to find a good buy on leds, I am looking at some MarsHydro on ebay atm. If anyone has any suggestions, throw them out there. I did look at some MH/HPS, but you mentioned they are power hungry.

The closouts on older Mars lights are a great value, IMO. I got a couple for $0.47 per true watt.
I still think HPS gives you the most bang for the buck. You can grow a lot under $150 worth of HPS.

 

[Fuzzy Duck]

Ye never really got into the cfl wattage for growing ?

When right spectrum of light & i may suggest PAR out put is more of a key factor & really not seen much info on PAR of cfl to be honest !

But one most consider the the inverse square law of light effect when using different bulb types & with cfl ya need em with in a few inches of the plant tops for best effect.

It may include training methods to enhance yield tho with cfl & i may suggest scrog for best effect, than over a bush type of grow


Now i use a single 125w 6500k cfl for seedling, vegging for up to 8 weeks with in a soil/compost grow it is professional bulb than rather a domestic type in an area of 60cm by 60cm & i do just fine for vegging.

More likely defeats the cfl wattage for growing so some thing is at question & yes i've being using cfl for veg etc for last few years...

 

[fanleaf]

Hello Fuzzy. Yea, I Have the 125W and 250w cfls kits with that same reflector. And to this point I'm very happy with them. I also agree that PAR is more useful too. It just seems that watts is what most use probably because the lack of PAR meters in everyones hands lol.

Light choice is definitely a choice that many factors need to be taken into consideration for. the biggest consideration of course is money and what overall results are satisfactory to you. For myself since I had the means to spend good money ( a few grand) on high-end LEDs and enough to totally fill the room with them that's the route I took. Other LEDs that don't cost near as much as mine may very well do about the same job I don't know but that was a choice I made for me and so far I'm very glad I did.

 

[JuanCarlos]

WOW.....Ok you have given me a lot to think about.........I had decided to hold off on purchasing those lights and am I glad I did, whew!
I am going to have to do a lot of rethinking.....I was thinking that every time I heard of someone getting busted, it was because they had high power lights, that is just an assumption tho. I am actually trying to have 15 plants in that space.....is that too many? I have that many square pots and they are 4 gallon, less than 10" across. I would still have close to 1 foot side to side and end to end. Might be pretty cramped in there. What are your thoughts?

When I have read in my area about people getting busted for growing it is always because they were growing outdoors. You can't really worry about high power lights, because whether you run CFL/HID/LED you are still going to require about the same power usage regardless. You will have LED/HID/CFL fans, but you have to find what works best for you. Mars Hydro is a forum sponsor here, so I suggest contacting SmokeSara and talking to her if you are considering LEDs. Reading on here the Mars Hydro guys really like their lights. The good thing about LED is the heat from the light is dispersed above the light where HID lights disperse their heat into the plant canopy. These are things that you need to consider depending on your grow area.

You are talking about growing in a sea of green method. It's where you grow many smaller plants versus a few larger plants. That all comes down to personal preference. If you are growing photoperiod plants you are going to cut down your veg time so your total grow time will be less as well.

 

[Big Sur]

Now i use a single 125w 6500k cfl for seedling, vegging for up to 8 weeks with in a soil/compost grow it is professional bulb than rather a domestic type in an area of 60cm by 60cm & i do just fine for vegging.

More likely defeats the cfl wattage for growing so some thing is at question & yes i've being using cfl for veg etc for last few years...

Good thing that you posted this. I completely forgot that I have a new 125W CFL grow light system with a reflector in a box around here someplace. I got it 5 years ago and never set it up. Doi! I am looking for a cooler grow method to start seeds and cuttings with, and I already have the solution. Now if I can only find it! ADD rulz here.

 

[fanleaf]

Good thing that you posted this. I completely forgot that I have a new 125W CFL grow light system with a reflector in a box around here someplace. I got it 5 years ago and never set it up. Doi! I am looking for a cooler grow method to start seeds and cuttings with, and I already have the solution. Now if I can only find it! ADD rulz here.

Hopefully it's 6500k so better for seedlings and veg for you! Cool.

 

[Big Sur]

Hopefully it's 6500k so better for seedlings and veg for you! Cool.

Yah, its 6500 Kelvin.

 

[Bobby Bill]

Correct me if I'm wrong, but if we are talking about light we should be talking lumens not watts.

 

[JuanCarlos]

Actually we should be talking PAR if you want to get down to it, but watts is the easiest spec to quote when comparing lights.

 

[Big Sur]

Correct me if I'm wrong, but if we are talking about light we should be talking lumens not watts.

Tomatoes, tomottos... Most meters for solar installations read watts/sq. meter. That is what I use to test my lights. Simple cheap meters. Newer CFL and LED lights are sold in units of lumens and watt "equivalents". Older bulbs and halide lights are sold in units of watts. You can convert them mathematically. So for your physics lesson of the Day:

The luminous flux ΦV in lumens (lm) is equal to the power P in watts (W) times the luminous efficacy η in lumens per watt (lm/W), giving us the formula: ΦV(lm) = P(W) × η(lm/W), or lm = W × (lm/W)

Luminous efficiency varies with the type of bulbs used. Here are typical values for bulb types used for growing weed:

Tungsten incandescent 12.5-17.5 lm/W
Fluoro tube/CFL lamp 50-75 lm/W
LED lamp 30-90 lm/W
MH lamp 75-100 lm/W
HPS lamp 85-150 lm/W

If you want to talk about indoor lights for growing MJ, you also need to talk about the color temperature (in degrees Kelvin). You also have to consider the intensity of the light at the distance that the plant is from the bulb. Light energy from a bulb falls off at the square of the distance.