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Added by: Bongaloid
Avery general rule of thumb is that your garden needs 50 watts of HIDlighting per square foot of illuminated area. This rule ignores theshape of your garden, so the following is really a better guide:
A 250 watt HID will illuminate a 2' x 2' garden.
A 400 watt HID will illuminate a 3' x 3' garden.
A 600 watt HID will illuminate a 3.5' x 3.5' garden.
A 1000 watt HID will illuminate a 4' x 4' garden.
From Jackerspackle, here's a different take on watts-per-square-foot/meter (WPSF).
WPSF assumes that the bulb's intensity is equal over the entiregrow area.. in other words each square foot/meter receives the samenumber of lumens. But in reality light diminishes rapidly the fartheryou go from the bulb (1/4 the intensity for each doubling of thedistance). So each bulb has a limited range, beyond which you do nothave good growth.
For example, a growspace that's 2 x 10 feet would require 1000watts if you go by the 50 WPSF guideline that's commonly mentioned..But a 1000 watt bulb only covers an area about 5 feet across - meaningthe edges of your garden will be dark.. A better choice in this casewould be three 400s or two 600s.
Another problem with WPSF is it assumes all bulbs have the sameintensity. But 1000 watts of HPS is not the same as 1000 watts offluorescents or (yuk) incandescents. Fluorescents have their lumensspread out over a long tube and are therefore dim.. incandescents havethe wrong color spectrum and are also dim.
Nor is a 1000 HPS the same as four 250 HPSs.. 250s don't haveanywhere near the intensity needed to penetrate thick canopy or tall,bushy plants. (Don't even think about growing meter-tall plants with a250.)
I could go on, but here's a basic guideline for lighting a growspace for good growth using common HIDs:
WATTAGE -- COVERAGE
1000 watt - 4 to 5 feet across (1.3 to 1.5 meters)
600 watt - 3.5 feet (1 meter)
400 watt - 2.5 possibly 3 feet (.8 to .9 m)
250 watt - 2 feet (.6-.7 m)
175 watt - small, less than 2 feet
These numbers assume you have a good reflector around your bulb andalso reflective wall coverings. You can increase the figures a bit ifusing multiple bulbs, due to their overlapping effect. You can alsoincrease coverage using a light mover...
I know this won't put to rest the old WPSF idea, but i hope it shed some light on the subject.
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