Above you can see a AC generator and a resistor of 50 ohm. A current will flow in the circuit and you can easy calculate the effect which will be consumed by the resistor. The effect will be transformed into heat. In the blue frame you can see how I calculate the power to 20mW. The red frame show how you can calculate the power into dBm, which which stand for decibels above 1 mW in 50 ohm, written dBm. In the green table at rigth I have calculated some power into PdBm.
Source : http://hem.passagen.se/communication/watt2.html
