Jersey, W = V^2/R. So, if your max wattage is 15, and you're running a 1 ohm coil, your voltage would be Sqrt(15W * 1 ohm) = 3.9V (actually, a smear less than that.)
That being said, a "typical" starting point would be about 2V + Coil Resistance, so you should be starting around 9W = (2+1)^2/1. Then, as others have suggested, nudge up from there. Most of my coils are in the 1.8 ohm ballpark, and I run about 9-10W on about all my toppers/mods.
Just because you "can" doesn't mean you "should"
