What is true power?
1 Answer
In an ac circuit, there are 3 types of power:

Real or Active power is the power dissipated in the resistors and has no phase angle.
#P_T=v_Ti_T=i^2R_T# This is often called true power as it is actual useful power that can be used to do work in the circuit. 
Reactive or apparent power is the power dissipated in the reactive components, ie the capacitors and inductors in the circuit.
#Q_T=v_Ti_Tsintheta_T# . If the circuit is leading overall, then the power factor will be leading and the capacitor impedance is bigger than the inductor impedance. Otherwise lagging. This is the "wasted power" and does not contribute any real useful work in the circuit, and often power factor correction is necessary to reduce it. 
The apparent power is the phasor sum of the real power and reactive power.
#S_T=v_Ti_T=sqrt((P_T)^2+(Q_T)^2)#
These 3 powers may be represented on a power triangle as shown below (int he case of a lagging power factor where