=> P = V^2/R with this formula I get around 0.33 watts?
Is this not correct?
The power dissipated by the resistor is the voltage drop across the resistor squared divided by the resistance = 1.5^2 / 75 = 0.03 (or you can compute P = IV = 0.02 A * 1.5 V). The rest of the power is dissipated by the LED, mostly in the form of light. This power is computed by P = IV = 0.02 A * 3.5 V = 0.07 W, for a total resistor-LED power dissipation of 0.1 W.
- Ben