I don't know why the manufacturers don't put any kind of information about the LED lights on the package. They show colors and a wave chart but no numbers. Kind of lame. If they did, then I would know.
I will try the pennywort. Thanks dougall
\The important numbers to look for with LEDs is the PAR value, and failing that the number of lumens output. (There are others, but not so easy to find) Watts are only really comparable to the same fixture or the same specific LED emitters used, unlike flourescent/cfl bulbs where the watts is comparable to the amount of light emitted about across the board.
Watts with LEDs is the amount of power consumed by the fixture overall, the amount of light produced will depend on the efficiency of the driver, the type of LED emitters used, and any other things that might use power (like an integrated timer with sunrise and sunset settings)
the colors and wave chart you see will show the spectrum of the bulb/fixture, which is important to how your tank looks but not so much to how stuff will grow; something with a lot of green light will make your plants/green stuff appear bright, but not so much the red or the blue fish.. as a high level example, this really controls how much light can reflect off things of a certain color/colors.
Variable fixtures will let you alter this to get something better for your use, but may still not have as much light created, especially when the different colors could all be increased to the max.. it might not be the color you are looking for and you're not going to get any more light.
lots of higher end manufacturers will give PAR values; lower end / cheaper ones will nto necessarily, but the community may have your back, if you do a google search for the fixture you're looking at, ass the acronym PAR at the end of it and search, after going through a few links, you'll likely find PAR values at certain depths (Likely on the planted tank from experience), this is the information you are looking for, being sure to note if the reading is taken through air or water (koolaid is unlikely...) so you can compare apples to apples; and then go from there.
anything else will be experience if they have that particular fixture, anecdotal if they have read about it, or just guesswork... or they may work/be sponsored by the manufacturer. so it's in your own best interest to get a good consensus by searching around.
Hope that helped a little