If you drive 15 watt, then calculation says current drawn will be ~83ma. Current drawn is very low and as the circuit seems to be, I hope it should work. Since it is less efficient, let consider its efficiency to be as less as 40%. Then the source should have the capacity of around 25W. (With 12v constant voltage, current = ~2.08 or 2A). So check your source is capable to give such current.
You can add one more diode to adjust the voltage drop by 15 diodes.
All rough calculations, so I advise not to go with this. Make your own calculation for better accuracy.