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 wattspersquarefoot/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 metertall 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. ~
