Real Power is often described as true power (or effective power) and is used by engineers to describe the actual amount of power present in a system, it is expressed in watts (W).

Engineers use three types of power to describe energy flow in a system:

Real power (P)

Apparent power (S)

Reactive power (Q)

In an alternating current circuit both the current and voltage are sinusoidal and power is expressed as the product of current and voltage. In a simple resistive circuit the voltage and current are in phase and the real power is equal to the apparent power.

However when the voltage and current pass through certain types of circuits (inductive or capacitive) the voltage and current become out of phase. This phase separation is usually called the Power factor and the current is said to lag or lead the voltage.

Understanding the relationship between these three quantities lies at the heart of understanding power engineering. The mathematical relationship between them is a vector and is typically expressed using complex numbers

S = P + jQ

As the quantities are vectors

S² = P² + Q²

The Real Power is the rms value of the voltage multiplied by the rms value of the current.