See Real power for detailed explanation

In alternating current power transmission and distribution, effective power (also true power or real power) is the actual power delivered to a system. This is to distinguish it from other definitions of power (apparent power, reactive power, complex power). It is usually denoted as P and expressed in watts. In fact, only effective power should be expressed in watts.

Effective power can be calculated as the product of the rms voltage and current (the apparent power), multiplied by the power factor (the cosine of the phase angle between the voltage and the current).

Based on Federal Standard 1037C