The PWR263S-20 is rated to 20W. The amount of power that it can dissipate without exceeding the maximum temperature of 155 °C depends on the thermal resistance of the resistor (6.5 °C/W) and the thermal resistance of the mounting surface. For instance, if the device is soldered onto an FR4 circuit board with 70µm of copper foil inside, the resistor can dissipate up to 2.8W of power. The higher the thermal resistance of the mounting surface then the lower the amount of power it can dissipate.