or How Many Grow Lights do I need?
This is a common question for new growers, and goes hand in hand with
How many plants can I grow in this amount of space?
(which we will come to later).
Many people will talk about lumens per square foot but as lumens is a measurement of what the human eye perceives and not of the amount of light a plant receives and uses for PAR (photosynthetically active radiation) this is somewhat misleading.(Umol is the form of measurement used for PAR but is not given by many manufacturers). Novice growers often start off underestimating the amount of light they need to provide their plants because they mistake the brightness of the lights with what is necessary for a person to see by and not understand the quantity of light necessary for a plant to transform into energy and growth.
Grow room lighting has been established for a long enough period of time that we can discuss the number of lights needed in relation to the number of watts drawn over a physical area.
Using this concept of watts per metre the acceptable range to produce a quality product in terms of yield and strength is:
400 – 600 watts drawn per square meter
for a traditional High Intensity Discharge Grow Lighting (HID) set up. If you are using LED Grow Lights you can cut this by 25% so the average number of watts per meter ranges from 300 – 450 watts drawn/meter.
If you are using a 2.4m by 1.2m tent then the square meterage is 2.4 x 1.2 = 2.88m.
The minimum amount of lighting you want for this tent is:
2.88 x 400w = 1,152w so 3 x 400w or 2 x 600w HID lights will cover it nicely.
To maximise yield and resin production then look at
2.88 x 600w = 1752w = 3 x 600w HID Grow Lamps.
- If you’re using LEDs, then this size tent would want 2.88 x 300 = 864w drawn (you can probably get away with 2 x 400w draw lights) up to 1256w drawn (3 x 400w draw LED grow lights)
If you are growing in a larger space eg 3m by 4m the easiest way to work out the number of lights is to plot the shape of the room on squared paper and divide off meter width rows leaving access passages in between. The end of each access passage can be a grow space too. Divide each growing row into meter lengths then count the number of grow squares you have. Each square needs either a 400w or a 600w light covering it. If your room is an awkward size remember that a 600w light will happily cover up to 1.2m by 1.2m.
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There are some variables you ought to consider when planning your set-up:
- What stage in their growth the plants are at. The formula above is for plants in flower; during vegetative stage plants need less light.
- Seedling stage 200 watts per meter of grow space
- vegetative stage 300-400 watts per meter of grow space
- flowering stage 400-600 watts per meter of grow space
- If you have a separate grow chamber to flower room, the number of lights can be less even over the same space.
- If you are going to run a multiple light set-up you don’t need to switch on all your lights during veg.
- If you use dimmable ballasts you can reduce power consumption by turning down the light produced.
- Efficiency: somewhere around 450w/m2 the grams per watt figure decreases as you increase the light. Up to 650w/m2 trichome production continues to increase but this soon levels off as well.
- Quality of equipment. High end reflectors, ballasts and bulbs can modify the number of lights you need to cover an area and the yield from that space.
- Grower’s style. We all find a methodology of growing which works for us and the strains we enjoy cultivating. Inevitably this will have an effect on the set-up as it evolves.
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