Q. What size resistor would I need to operate these from a 15v DC Supply
A. Hi John, thank you for your question. The equation is to take the supply voltage from the typical forward voltage of the LED divided by the typical current of the LED in amps will give you the resistance in ohms of the required current limiting resistor. In this case - 15v-2.2V=12.8V/0.02A(20mA) = 640 ohms. In practice the preferred value of 680 ohms would be fine like 64-0068 per LED.