Well, it's NOT simple. Because even the ones sold commercially don't seem to work.
I took an interest in this after seeing the indoor "Aerogrow" garden thing (which actually does work, using fluorescent grow lights) and researched LEDs because, well, technical fetish for LEDs. It's best I just admit that upfront.
I've talked to the "hydroponics" shops in town. Well, what most customers want to grow isn't lettuce. But, probably not a big, relevant difference. The thing is, they're very critical of hardware, and the LED products have made bold claims and never lived up to them.
Whether they don't have the radiant power right, don't have the right wavelengths, don't have the right "throw" to hit the leaves, or don't "mix" or sequence them properly, well, nobody's clear on that. But the bottom line is those particular products put on the market are neither cost-competitive nor energy-competitive with HID or even CFL for growing results (plant type not withstanding). 400W of HID can already grow a LOT of plants! CFL is less efficient, and less effective overall, but the initial investment is lower esp for a small-scale "toy" like the Aerogarden.
I'm not saying LEDs can't grow plants. But, the ones made commercially up to this date cannot do so effectively. The folks I spoke to seemed knowledgeable and results-oriented rather than repeating some sort of dogma to sell their own product. They were clear they'd sell LEDs if they did work- but they do not work, not in a way that was competitive with existing grow lights.
First, be aware that WHITE LEDs do not produce light more effectively. CFL grow light can do ~75-90 lumens/W. Some high-end white LEDs can exceed 100 lumens/W if well cooled and used at low power. The LED grow lights won't be. Even still, it'd only be a SMALL gain. And metal-halide HID- the standard for grow light- is like 125 lumens/W. LED won't beat it. Now it's debatable whether the single-color LED can cause photosynthesis more efficiently by hitting specific wavelengths the chlorophyll uses, and it's "interesting"- but again, field reports seem to be mixed.
The "UFO" LED device is the most popular and heavily marketed indoor horticulture lamp. The consensus among professionals seems to be that it just doesn't work. Growth is weak and abnormal, and it was fairly expensive. Can't rule out better results if they used several times more LED lamps than recommended, but the cost was already exceeding that of a known-effective HID system.
And I, too, am skeptical that they packed "600W" of LEDs into this. Because, as noted, it seems difficult to budget the cost of 600W emitters- regardless of type- construct a housing and cooling, and still make a profit even at a $659 price tag.
Well, I can imagine it if they took 200W nominal of emitters and ran them at triple the nominal power. There's no law that says you can't! But the lumen/W goes down a lot, so it's not really producing like a "600W" system, and the emitters won't last long.