Active Power: It is the real power that is consumed by a load. It is the product of voltage and current and the cosine of the angle between them. It is measured in watts and denoted by the letter P. Useful work like heat dissipation in heaters, torque production lamps, light emission in lamps are all done by the active power. P=V*I*cos(phi).
Reactive Power: It is the useless (imaginary) power that moves between the source and load of a circuit. It is the product of voltage and current and the sine of the angle between them. It is measured in VAR and denoted by the letter Q. It is useful for calculating the power factor of the circuit. P=V*I*sin(phi).
Apparent Power: It is the vector sum of active power (P) and the reactive power (Q). It is denoted by S. S=P+jQ
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