Hello Psycrops,
I would use florescent
lights for growing lettuce and kale on small scale. HID (
metal halide) lights on large scale. Why? Because it's the most cost effective, and being cost effective is very important to me. You said you plan to have 3 raft systems but you didn't say how many Sq feet it would be. First I wouldn't grow lettuce and kale in the same system since kale needs a stronger
nutrient solution than lettuce.
Manufactures of
led lights like to promote their products as more energy efficient. But in reality you don't save much at all, and if you do manage to save some money in electricity, it will take between 5 and 10 years to break even for the much higher equipment costs. I'm not going to wait that long to start saving.
Two 4 foot twin bulb T5 florescent lights (216 watts) is enough to cover a 2x4 foot space (8 Sq feet), possibly even a 3x4 foot space (12 Sq feet). Or 6 four foot T8
bulbs (192 watts) to cover the same space. Since you will need between 25 and 35 watts of LED lighting per SQ foot, you won't be saving anything. In fact it will probably cost more to run LED lights.
For this example I will use the smaller Sq footage of 8 Sq feet
Florescent
T5's-216 watts covers 8 Sq feet
LED
8 Sq feet x35= 280 watts (64 watts more than T5 Florescent).
8 Sq feet x25= 200 watts (16 watts less than T5 Florescent).
Even if you can manage to do OK with using just 25 watts per Sq foot of LED lights you would only be saving 16 watts. Using the national average of 10 cents per kilowatt hr, those 16 watts only save you $0.86 cents a month running the lights for 18 hours a day.
$0.86 x 12= $10.32 a year, and $51.60 after 5 years, and $103.20 after 10 years.
Now when you run the numbers you have to take into consideration the cost of the lighting fixtures as well, not just the bulbs. By the time you add up the equipment cost for both types of lighting, and divide that by the little energy savings if any, it can easily take 10 years just to break even.
Not only do LED not really save you much at all in electricity, but LED lights aren't all the same. Manufactures sell blue, red, and white LED's. But there is a vast difference in both actual light wavelengths emitted, as well as intensity and concentration of those wavelengths from one LED bulb to another even if they are the same visible color. In other words, you won't get the same results from every LED light even if they are the same color spectrum.
What Wavelength Goes With a Color?
Light Absorption for Photosynthesis
Light and photosynthetic pigments
Which regions of the electromagnetic spectrum do plants use to drive photosynthesis?
What are these Pigment involved in Photosythesis?
As for your question about the importance of lumens for plants. There are many ways of measuring light intensity, lumens is just one of them and isn't really important to plants. Comparing lumens is only useful in comparing one bulb to another bulb of the same type. What is important is the actual wavelength of that light. Light is wavelengths. Plants use certain wavelengths in different processes, and the measurement of lumens doesn't measure the specific wavelengths plants use. That would be
PAR (Photosynthetically active radiation).
As I said lumins is only useful in comparing similar bulb types. As an example, if you were comparing Metal Halid Bulbs, and a 400 watt MH bulb put out 36,000 lumens, and the 200 watt bulb put out 18,000 lumens, and the 100 watt bulb put out 9,000 lumens, you can see that the lumen output is proportional to the wattage, and you can assume the par output will be proportional as well. In other words four 100 watt bulbs would give you the same lumen/par output as one 400 watt bulb. But since distance from the bulb is important, you could space the 4 lower wattage bulbs out and get better overall coverage from the 4 bulbs than the single higher wattage bulb, and for the same amount of electricity/wattage consumed.