How much light do you really need?

Bignutes

Well-Known Member
This is a an exercise to figure out how many watts or what light is needed for your grow room to get your buds to their maximum potential. This may help to decrease your power bill and lower the cost of purchasing a light. It also help you to pick a light source based on its wattage and your flowering area.

Light follows something called the inverse square law, in layman’s terms it’s just that light intensity fades quickly with distance away from a light source.

Now let’s say the optimum lighting is one that covers your grow space well, like if you have a 2x4 space your light is 19”x46”, almost complete coverage. Do you want 45 w/sqft or 35, or 25?

If we have a set point that 35 W/sqft at 18” as our starting point, the question I had is what if I ran my lights at 30 W/sqft or even 25 W/sqft, at what height would I need to run to be the equivalent of 35 W/sqft.

Below as shown the first number is watts per square foot, calculated by taking the true wattage of your light divided by the length and width of your light, second number is the height at which you should hang your lights at to be equivalent to 35 W/sqft

At 45 it’s 20.4”
At 40 its 19.2”
At 35 it’s 18”
At 30 it’s 16.7”
At 25 it’s 15.2”
At 20 it’s 13.6”

What this tells me is that you can run a light at the lower wattage per square foot but it needs to be at a lower height to have the same performance as a light running at higher wattage per square foot.

By taking this into consideration, getting 1g/w at 35 W/sqft should increase to 1.4 g/w at 25 W/sqft considering all other variables are equal.
 

Markshomegrown

Well-Known Member
This is a an exercise to figure out how many watts or what light is needed for your grow room to get your buds to their maximum potential. This may help to decrease your power bill and lower the cost of purchasing a light. It also help you to pick a light source based on its wattage and your flowering area.

Light follows something called the inverse square law, in layman’s terms it’s just that light intensity fades quickly with distance away from a light source.

Now let’s say the optimum lighting is one that covers your grow space well, like if you have a 2x4 space your light is 19”x46”, almost complete coverage. Do you want 45 w/sqft or 35, or 25?

If we have a set point that 35 W/sqft at 18” as our starting point, the question I had is what if I ran my lights at 30 W/sqft or even 25 W/sqft, at what height would I need to run to be the equivalent of 35 W/sqft.

Below as shown the first number is watts per square foot, calculated by taking the true wattage of your light divided by the length and width of your light, second number is the height at which you should hang your lights at to be equivalent to 35 W/sqft

At 45 it’s 20.4”
At 40 its 19.2”
At 35 it’s 18”
At 30 it’s 16.7”
At 25 it’s 15.2”
At 20 it’s 13.6”

What this tells me is that you can run a light at the lower wattage per square foot but it needs to be at a lower height to have the same performance as a light running at higher wattage per square foot.

By taking this into consideration, getting 1g/w at 35 W/sqft should increase to 1.4 g/w at 25 W/sqft considering all other variables are equal.
You can build a full LED bulbs panel and run it at 30w ft2 no problem, very cheap to build and so are the bulbs.
you could even get it down to 26w ft2, very even light spread, (750par) over the whole canopy.
 

Markshomegrown

Well-Known Member
I agree, I think you could get down to even 20 W/sqft as long as your colas are less than 14” tall from the main canopy.
I have tried 12.5w ft2, the overlapping light was only 300-400 par in my veg room at 12" didn't work very well,
from some early tests, the lowest I could go was 18.9w ft2 (70 x 70cm flower room) bulbs spaced every 22cm and 11cm off the walls, grid of 3x3, but you can lose the center bulb(12.5w 4k, 8 bulbs), which gives you a very even light spread.

Reflective walls help to reflect the light back and keep the corners and sides even.
 

Bignutes

Well-Known Member
I have tried 12.5w ft2, the overlapping light was only 300-400 par in my veg room at 12" didn't work very well,
from some early tests, the lowest I could go was 18.9w ft2 (70 x 70cm flower room) bulbs spaced every 22cm and 11cm off the walls, grid of 3x3, but you can lose the center bulb(12.5w 4k, 8 bulbs), which gives you a very even light spread.

Reflective walls help to reflect the light back and keep the corners and sides even.
What par did you get with 18.9 and at what distance. You’ve done some interesting experiments!

Just to run the numbers. At 12.5 w/sqft you’d have to run it at 7.7” to get the same intensity as 30 W/sqft
 

Markshomegrown

Well-Known Member
What par did you get with 18.9 and at what distance. You’ve done some interesting experiments!

Just to run the numbers. At 12.5 w/sqft you’d have to run it at 7.7” to get the same intensity as 30 W/sqft
I made some home made reflective for each bulb, between the bulb and the canopy had to be 12"+ because the overlapping light, this gave me more par, the reading worked out between 500-600 par @12" ( 30k lux ).

Just added 4x cheap 15w bulbs(,£1.50p each) to my led panel
I had 24 x 12.5w leds and 4 x 13w 4k,
So I have 412w in a 1.1m x 1.1m
So now I am upto 412w (total over kill) hitting over 50k, some of the canopy hitting 950 par.
Adding more lights to warm the room up.
 

VincenzioVonHook

Well-Known Member
I like it! Doing the mostest with the leastest.
Was definately unexpected for sure. Crazy as the 2x4 tent beside it with 240w and two plants didn't yield that much more per plant. I got 1.5g per watt with the single plant and 1.36g per watt with the two plants. I managed to do worse with more light lol.
 

Bignutes

Well-Known Member
I’ve got a lot of respect for what you did, that would sure help out people who are constrained in their grow but still produce lots of bud.

I run mine at 30 watts/sqft but with complete coverage with the light. I’ve gone down to 27 watts / sqft, thinking I could even do 22-25 and rock the casbah
 

calvin.m16

Well-Known Member
Less intense/powerful lights tend to require a closer hanging distance to the plants. That being said, a weak light isn't going to make the same size crop yield more than a strong light at the same hanging distance and spread.

I'd look into manufacturer recommendations. Always oversize instead of undersize. The extra few bucks on the power bill is usually worth the extra few ounces in your stash.
 

jimihendrix1

Well-Known Member
Gavita 1700e is 645w and is made for a direct replacement for a 1000w HID. Which is 42.5w Sq/ft. They also dont claim it is better than a 1000w HID, only that its a direct 1:1 replacement. Its only better wherin it uses less electricity, and the LED last longer.
The 1700e is 1700umol, and a 1000w Hortilux HPS is 1600umole. HID in this case is 62w sq/ft. Both cover a 4 x 4 area

The 1000w Single Ended Hilux Ushio HPS is 2100umol. Same as a 1150w Gaita DE HID. Both will blast a 5 x 5 area.

I go by this formula. To equal HID one needs around 42.5w Sq/ft. HID is 62w sq/ft. I also start seedlings under these conditions, and all the way through flowering.


Full daylight sun at noon in the summer is around 2000 μmol/m2/s. How many Ppfd does the sun produce? At the equator, the sun's intensity gives us a Photosynthetic Photon Flux (PPF) of 2000 µmol/m 2 /sec of light , which is roughly equivalent to 10,200 foot candles or 108,000 lux.
 
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