I'm shockingly innumerate and was hoping someone could help me with something I'm puzzling over.
I've been reading about using CFL's in confined spaces. The guide I've been looking at advises that you want to achieve between 3000-10000 lumens per square foot. They then describe a simple formula, measure the length and height of the space and multiply the two to get the square area. Then they use a HPS as an example, saying that one 1000 watt lamp produces around 107,000 lumens, so if you had a 4'x4' space you'd have 16' square, divide the number of lumens to get a rate of 6687 lumens per square foot.
So I started looking around at a few CFL lamps to gauge a typical 125w lamp, seems 9000 lumens is typical. A few scribbles later and I figure 4'x5' space with 20' square means only 450 lumens per square foot, meaning a space that size would require ten lamps at a minimum!
So can anyone tell me what I'm doing wrong?
I'm thinking it's something to do with the fact you have to keep CFL's within inches of the plant, so the total area being lit is much less?
Depending on your bulb type you should work with watts per square foot rather than lumens (especially lumens quoted by the manufacturer, the figures of which are shockingly unreliable - you can guess which direction the error tends to fall - over inflated figures equals more sales). So unlesss you're going to get a light meter and specially constructed room, I would forget it. Rely on brand reputation - ask a great grower, or a number of great growers in your country which brand of bulb in your wattage range and bulb type which they would recommend.
Go by watts per square foot, rather than lumens per square foot. Trust me, it's a much better yardstick or rule of thumb to depend on.
The magic number for HID lighting is 50W/square foot. In an area of 16 square feet to get closest to the magic 50W/sq ft, an 800W HID is in order. Now of course they don't make them, or if they do they are rare and virtually unobtainable in mainstream grow shops. In terms of bulb types here are some options generally speaking - the 600W bulb which is the most efficient bulb there is, a bit more efficient than the 1kW, the 400W, less efficient than the 600W and the 250W bulb which although less efficient than the 600W gold standard it is more useful for small plants since it beats the equivalent in CFLs or even T5s (T5s are more efficient than CFLS). In a 4'x4 'area using a combination of a 250W and a 600W bulb is a total of 750W, giving you 47W/sq ft. The problem is, the 250W bulb would have to be much closer than the 600W, potentially blocking a lot of the 600W bulb's light. You could however buy a single 1kW HID bulb with a 1kW magnetic ballast, unless you can afford an electronic digital ballast. This would give you 62.5W/sq ft, an overshoot but not too much to cause an issue.
Now here's the important bit. The pros of having a 1kW bulb are much denser buds due to the light intensity and importantly, simplicity since there's only one light, one ballast and one contactor. With this wattage you need good ventilation and I would recommend mounting the bulb vertically - preferably with a parabolic reflector since they act a bit like a satellite dish in reverse, causing the heat to be dissipated very efficiently. Much easier to cope with than a horizontally mounted standard reflector that would beam all that radiant heat straight onto your plants, potentially burning them if the bulb got too close. As for the contactor, my advice would be to get a timer-contactor combo capable of handling 1kW. This saves you having to buy a contactor and a timer separately and it cuts down on the wires, making your grow tidier and safer.
Now the reason I'm going on about buying a 1kW bulb is because in a 5' x 4' (20 square foot) room, it's dead on 50W per square foot - bang on the ideal figure - and without looking it up would imagine a 1kW magnetic ballast would be cheaper than 10 125W CFLs. Relative to the HID counterparts, simply put, CFLs do not flower weed that well. Sure, it get the job done and is used to produce weed all the time for one's personal stash, but once you've used the bigger HID bulbs, there's just no comparison. It's like comparing a ferrari to a crappy old Ford Fiesta. My advice would be to keep costs down and forgo the more expensive metal halide bulb. Simply buy a 1kW HPS bulb and use from start to finish (except in the beginning stages when the plants are too small to handle the heat and light intensity. The whole thing about stretch being an issue when using HPS during veg is waay overstated. I've never really noticed a difference at all, but if there is one it's absolutely nothing to be concerned about.
high lumen 80w CFLs can be equivalent to a 250w HID
I hate to be the bearer of bad news but if a grow shop owner told you that it's not true. Honestly mate, even a 250W HID pisses over an 80W CFL. Please trust me on this one. I know a lot of growers that use a single 250W HPS to grow more than enough for one person in a closet. Works much better than CFLs.
Also, one word to the wise: an equivalent wattage of CFLs produces the same, if not more heat than HIDs. The only difference is the HID is a point source rather than more evenly distributed heat dissipation. As long as you have enough head room or are using a parabolic in particular, heat should not be an issue if your ventilation is up to the par. Remember, ventilation is as much for gas exchange as it is for moving out heat. Plants need a fresh supply of CO2 and removal of O2, which good ventilation fans are important for. This is an important one that many growers overlook. Don't be one of them.
), combined with being on the 2nd storey of a less than energy-efficient home (i.e. it gets frigggen hot up here) anything other than CFL simply isn't an option.