These will work, but to give you an idea on ratio, we've got 24 3W LED's on our tank (at 75% power). What types of corals you plan to keep will greatly determine how much light you need as well... if you're planning low-light requirement corals then you should be ok if your over all tank size doesn't exceed 30 gallons or so.
Keep in mind, in a 20" tall tank, you'll need to stack your rock pretty high and keep all light-needing corals right at the top to get the most light.
Good luck!