In Electrical engineering Real Power is described as that component of power flow that results on average in a net transfer of energy from a source to a load. 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 average product of the instantaneous current and voltage.