well the way I see it is that you want to either match or beat the 900 ppfd that de hps would give you...
and im a bit ocd when it comes to layout of the boards so I have in mind only a few diff configs.
per 4x4 tray (really 5x5 of canopy the way you described it)
4 boards would be 2 boards x 2 boards
6 boards would be 2 x 3
8 would be 2 x 4
9 would be 3 x 3
12 would be 3 x 4
for the least cost of materials you want to run the boards at max amperage, so...
4 boards @ 2800ma would be 608ppfd
6 boards @ 2800ma would be 912ppfd / 930w
8 boards @ 2800ma : 1216ppfd / 1240w
9 boards @ 2800ma : 1368ppfd / 1396w
12 boards @ 2800ma : 1824ppfd ( excessive)
if you want to increase your efficiency, you could run these configs:
8 boards @ 2100ma : 931ppfd / 914w
9 boards @ 2100ma : 1047ppfd / 1028w
12 boards @ 2100ma : 1397ppfd / 1370w
9 boards @ 1400ma : 716ppfd / 668w
12 boards @ 1400ma : 955ppfd / 890w
so IMHO, a minimum of 6 boards, with a max of 12,
could you do a config with 5 or 7 or 10 boards? sure but my ocd doesn't like it....
also after having built three setups with 12 boards for each 5x5 area, that's the most I would cram into a fixture because its quite heavy and awkward to carry/hang for one person. after the experience of building these i'd probably build no more than 9 boards per fixture from now on.
ps. all calculations were done with 25sq ft, and 90% ballast eff. no other losses included
my current config per 5'x5' area is 12 qb288v2 / 3 hlg-320h-c1400b, pulls 920w on 120v, not sure what it pulls on 240v prob ~900w. my goal in this room was to lower the wattage per tray compared to my gavitas so that I could add extra trays.
In the other room I plan to run a 9-board setup @ 2800ma. in this room my goal is to come close to max ppfd because I only have room for a certain number of trays
everyone tells me i'm going overboard trying to reach 1500ppfd or close to it, but id rather have the capability to reach these levels and just dim according to the ladies' response
