There are many opinions on the question of “how much light is needed for growing marijuana indoors”.The info below, on lumens or watts per sq/ft, is only a general guideline to how much light you really need. The type of lighting used (High Pressure Sodium/Metal Halide, Compact Fluorescents, T-5, L.E.D.) as well as the stage of plant growth – seedlings: less light, vegetation: more light, flowering: most light. General rule of thumb is – Throw as much light on them as you can, with respect to heat, safety and affordability.
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.
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 but 50+ watts per square foot is optimal. You can determine the proper lighting for your area by using this formula:
50 watts (or the total watts you are using) x (your square footage).
Example: You have an area of 10 sq. ft. – 50w x 10 s.f. = 500watts/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 aminimum of 150 watts per square foot for fluorescent’s.
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