If those are really 19.5" in length that is HUGE. That's almost 2 ft for a panel only putting out 15w of light. Tbh, I think if you're going to invest money into LEDs, get the high power ones. The 5 or 10mm diodes are just cheap. They won't penetrate like you want. If you're using an area with 36 sq feet there is no way those diodes would cover all of it efficiently. What dimensions are your grow area?
Check
this site for your lumens calculator. When I was looking into those small LEDs I designed a panel that uses 90w and puts out 6000 lumens. Like other people have said though, lumens aren't accurate for LEDs. It's just not how they work. Luxeon, Cree, Seoul, Osram are some MFGs that produce high power LEDs. They're a LOT more efficient and give you more POWER for LESS watts.
And just so you know, it doesn't matter what kind of LED you use, they will ALL produce heat. Some more than others. Depends on the amps they run at mostly. If you insist on using those small LEDs since they're somewhat easy to work with, you'll need 10,000,000 MCD per square foot I'd say.
