Lighting Formulas

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Lighting Formulas

Post by Weedguru Higher » Sat Sep 18, 2010 10:34 pm

Lighting Formulas
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To correctly determine the best lighting for your space there are several things that you have to know. At this point, a couple of definitions are in order.

Lumens - one lumen is equal to the amount of light emitted by one candle that falls on one square foot of surface one foot away.

Watts - A measure of the amount of electricity flowing through a wire. Watt hours measure the amount of watts used in one hour. A kilowatt/hour (KWH) is 1000 watt/hours.

To determine the cost of operating your light:

Find your KWH charge on your electric bill. Assume you have a 1000 watt light and your KWH charge is $.05/hour. A kilowatt equals 1000 watts, therefore it will cost you .05 cents per hour to run that light. Here's another example. Say you have a 400 watt light and your KWH charge is $.03. Since 400 watts is not a kilowatt, you must divide 400 by 1000 = .4 kilowatts x .03 (KWH rate from electric bill) = $0.012 cents per hour to run.

To determine how many lumens per square foot you have:

Find out the square footage of your space. Width x Depth = Square feet. Divide the lumens available by your square footage. This will give you lumens per square foot. Example: Say your space is 3 feet deep by 4 feet wide, 12 square feet. The total lumens available from your light(s) is 45000 lumens. 45000/12 = 3750 lumens per square foot.

Now for the big question. How much light do I need?

Technology has advanced so much in the last 15 years that we are constantly refining the process and updating what we know works best for growing. Current theory holds that the minimum amount of lighting needed to sustain growth is around 2000 lumens per square foot. Mid range is around 5000 lumens per square foot. Optimal is 7000-7500, or higher, lumens per square foot.

What if you want to determine how much light you need in watts?

The general rule of thumb for providing light for an area is a minimum of 30 watts per square foot. 50 watts per square foot is optimal. You can determine the proper lighting for your area by using this formula: 30 watts (or 50) x ?(your) square feet. Example: You have an area of 10 sq. ft. - 30w x 10 s.f. = 300watts/sq.ft minimum or 50 watts x 10 s.f. = 500 watts/sq. ft. (optimal). Also, remember that fluorescent's are weaker and emit less light than an HID. This means you will need 5 times the amount of wattage to equal the output of an HID. So, 30 watts of HID would equal 150 watts of fluorescent's. This is why it is advised to provide a minimum of 30 watts per square foot for HID lights and a minimum of 150 watts per square foot for fluorescent's.


This is all important because the light intensity will directly affect the quality and yield of your crop. If you have less than optimal lighting your yield and potency will be reduced and buds will not develop as dense. This point can not be stressed enough. You must have the right amount of light for your space to grow high quality bud. The question is often asked, "can I have too MUCH light?". The basic answer is no. According to the law of diminishing returns, you could theoretically reach a point when your plants just couldn't absorb any more light but it would be impossible to have that many lights in your space. Heat from the lights would become a problem long before you ever reached that point. So use as many lights as you want, just control the heat.

Experimentation is the only sure method to determine the best solution for each plant. If plants are not receiving enough light, they begin to grow tall and spindly as if stretching for the light and foliage becomes pale green. Or, if they need to be moved closer to the light, or given a longer light exposure period. Too much light may lead to bleaching of leaves and flowers, browning and shriveling. Leaves would become overly compact and curl under at the edges.

Light Size Estimates

Here is a good idea of what sized light you will need for your room. The wattage recommendation is just a rough estimate as different bulbs produce different amount of lumens, so go by the amount of lumens you need for your room. When purchasing your light all HID's should have a lumen output on the side of the box. Also, it is better to get a little more wattage than a little less.

2 x 2 or 4 sq ft - 150 watts or 16,000 lumens

2 x 3 or 6 sq ft - 250 watts or 28,500 lumens

3 x 3 to 3 x 4 or 9 to 12 sq ft - 400 watts or 50,000 lumens

4 x 4 or 4 x 5 or 16 to 20 sq ft - 600 watts or 90,000 lumens

5 x 5 or 5 x 6 or 25 to 30 sq ft - 1000 watts

6 x 6 or 36 sq ft - 1400 watts

6 x 9 or 54 sq ft - 1500 watts

Match the square footage with the wattage. Try and keep the room as square as possible, the longer the room the harder it is for the light to reach the sides on the long ends. Also, if the chart asks for 500 watts you could use one 500 watt HPS two 250 watt HPS and so on. It doesn’t matter how many lights you have as long as the wattage matches the footage you are fine.

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Re: Lighting Formulas

Post by johnny_144 » Thu Oct 27, 2011 8:43 pm

Here you say 100W per plant ... 88&t=26831 and here 150W/sq.ft. I am kinda confused though since most of the time i can't understand if the watts corresponding to a square foot are from HID or CFL(like this:250W/6 sq.ft in the above chart). Also,as a general rule should i consider 1W of HID =5W of CFL?

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