Here we go, this is from the growfaq.
https://www.rollitup.org/view.php?pg=faq&cmd=article&id=584
How much light do I need?
A very general rule of thumb is that your garden needs 50 watts of HID lighting per square foot of illuminated area. This rule ignores the shape 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 entire grow area.. in other words each square foot/meter receives the same number of lumens. But in reality light diminishes rapidly the farther you go from the bulb (1/4 the intensity for each doubling of the distance). So each bulb has a limited range, beyond which you do not have good growth.
For example, a growspace that's 2 x 10 feet would require 1000 watts 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 - meaning the edges of your garden will be dark.. A better choice in this case would be three 400s or two 600s.
Another problem with WPSF is it assumes all bulbs have the same intensity. But 1000 watts of HPS is not the same as 1000 watts of fluorescents or (yuk) incandescents. Fluorescents have their lumens spread out over a long tube and are therefore dim.. incandescents have the wrong color spectrum and are also dim.
Nor is a 1000 HPS the same as four 250 HPSs.. 250s don't have anywhere near the intensity needed to penetrate thick canopy or tall, bushy plants. (Don't even think about growing meter-tall plants with a 250.)
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 and also reflective wall coverings. You can increase the figures a bit if using multiple bulbs, due to their overlapping effect. You can also increase 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. -~
Added on: Tuesday, May 1, 2007 Viewed: 5595 times
I know from growing with a 150 watt hps,I could only get one plant under her,maybe two if I had kept her small. But I let her get to almost 3 feet,because I'm just growing for myself. I think you could find some threads in the forum with examples of some grow cabinets,or plants growing under a 250 watter. Just remember she'll double or triple in size during flower,so allow for that. And if you're just growing for you,and you aren't planning on growing a bunch of different types at once, consider just a couple of plants,vegged for a good amount of time so they're big. You can try the screen of green method to maximize your yield and optimize the amount of light each bud gets. Smaller plants just mean less yield per plant, IMO, so if those are expensive seeds you have and you are just growing for personal use,consider storing the seeds properly and just concentrate on growing a small number of bigger plants.Then you can have many grows lined up, and you can realize the full potential of each plant you grow.Just a thought.
