Power dissipation of a resistor or any load is the amount of power (in watts) that is converted to heat, light, or other form of energy.
In a resistor, power dissipation is defined by Ohm's law
P =...
The power dissipated by a diode is P = Vf x I watts, where Vf is the forward voltage drop on diode (typically 0.5 volts for silicon diode) and I is the current.